Why can I use a list index as an indexing variable in a for loop? The 2019 Stack Overflow Developer Survey Results Are InHow do I check if a list is empty?Finding the index of an item given a list containing it in PythonUsing global variables in a functionAccessing the index in 'for' loops?Understanding Python super() with __init__() methodsConvert bytes to a string?How do I sort a dictionary by value?How to make a flat list out of list of lists?How do I pass a variable by reference?How do I list all files of a directory?

What is this sharp, curved notch on my knife for?

What to do when moving next to a bird sanctuary with a loosely-domesticated cat?

"as much details as you can remember"

How can I add encounters in the Lost Mine of Phandelver campaign without giving PCs too much XP?

Accepted by European university, rejected by all American ones I applied to? Possible reasons?

Output the Arecibo Message

What does もの mean in this sentence?

Worn-tile Scrabble

Why didn't the Event Horizon Telescope team mention Sagittarius A*?

The difference between dialogue marks

Can we generate random numbers using irrational numbers like π and e?

Pokemon Turn Based battle (Python)

Is bread bad for ducks?

A word that means fill it to the required quantity

How much of the clove should I use when using big garlic heads?

How to support a colleague who finds meetings extremely tiring?

What do I do when my TA workload is more than expected?

How come people say “Would of”?

What is this business jet?

Short story: child made less intelligent and less attractive

Why isn't the circumferential light around the M87 black hole's event horizon symmetric?

Why can't devices on different VLANs, but on the same subnet, communicate?

Does adding complexity mean a more secure cipher?

Does HR tell a hiring manager about salary negotiations?



Why can I use a list index as an indexing variable in a for loop?



The 2019 Stack Overflow Developer Survey Results Are InHow do I check if a list is empty?Finding the index of an item given a list containing it in PythonUsing global variables in a functionAccessing the index in 'for' loops?Understanding Python super() with __init__() methodsConvert bytes to a string?How do I sort a dictionary by value?How to make a flat list out of list of lists?How do I pass a variable by reference?How do I list all files of a directory?



.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty height:90px;width:728px;box-sizing:border-box;








9















I have the following code:



a = [0,1,2,3]

for a[-1] in a:
print(a[-1])


The output is:



0,1,2,2


I'm confused about why a list index can be used as an indexing variable in a for a loop?









share









New contributor




Kundan Verma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.















  • 6





    Somehow this question looks like a bad and newbie question, but I don't get the logic either lol

    – gameon67
    32 mins ago






  • 3





    I don't know why you would ever want to do this, but now I know you can

    – Nathan
    30 mins ago











  • over the iteration only the last time a[-1] gets its value, all other times python treats as iteration variable.

    – Arun Augustine
    26 mins ago







  • 5





    This would be a great question for an awful coding interview

    – Nathan
    23 mins ago











  • @Nathan same idea, would be a great trivial question in interview

    – gameon67
    23 mins ago

















9















I have the following code:



a = [0,1,2,3]

for a[-1] in a:
print(a[-1])


The output is:



0,1,2,2


I'm confused about why a list index can be used as an indexing variable in a for a loop?









share









New contributor




Kundan Verma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.















  • 6





    Somehow this question looks like a bad and newbie question, but I don't get the logic either lol

    – gameon67
    32 mins ago






  • 3





    I don't know why you would ever want to do this, but now I know you can

    – Nathan
    30 mins ago











  • over the iteration only the last time a[-1] gets its value, all other times python treats as iteration variable.

    – Arun Augustine
    26 mins ago







  • 5





    This would be a great question for an awful coding interview

    – Nathan
    23 mins ago











  • @Nathan same idea, would be a great trivial question in interview

    – gameon67
    23 mins ago













9












9








9


6






I have the following code:



a = [0,1,2,3]

for a[-1] in a:
print(a[-1])


The output is:



0,1,2,2


I'm confused about why a list index can be used as an indexing variable in a for a loop?









share









New contributor




Kundan Verma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.












I have the following code:



a = [0,1,2,3]

for a[-1] in a:
print(a[-1])


The output is:



0,1,2,2


I'm confused about why a list index can be used as an indexing variable in a for a loop?







python





share









New contributor




Kundan Verma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.










share









New contributor




Kundan Verma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.








share



share








edited 1 min ago









Christian Dean

15.8k62859




15.8k62859






New contributor




Kundan Verma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









asked 42 mins ago









Kundan VermaKundan Verma

561




561




New contributor




Kundan Verma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.





New contributor





Kundan Verma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






Kundan Verma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







  • 6





    Somehow this question looks like a bad and newbie question, but I don't get the logic either lol

    – gameon67
    32 mins ago






  • 3





    I don't know why you would ever want to do this, but now I know you can

    – Nathan
    30 mins ago











  • over the iteration only the last time a[-1] gets its value, all other times python treats as iteration variable.

    – Arun Augustine
    26 mins ago







  • 5





    This would be a great question for an awful coding interview

    – Nathan
    23 mins ago











  • @Nathan same idea, would be a great trivial question in interview

    – gameon67
    23 mins ago












  • 6





    Somehow this question looks like a bad and newbie question, but I don't get the logic either lol

    – gameon67
    32 mins ago






  • 3





    I don't know why you would ever want to do this, but now I know you can

    – Nathan
    30 mins ago











  • over the iteration only the last time a[-1] gets its value, all other times python treats as iteration variable.

    – Arun Augustine
    26 mins ago







  • 5





    This would be a great question for an awful coding interview

    – Nathan
    23 mins ago











  • @Nathan same idea, would be a great trivial question in interview

    – gameon67
    23 mins ago







6




6





Somehow this question looks like a bad and newbie question, but I don't get the logic either lol

– gameon67
32 mins ago





Somehow this question looks like a bad and newbie question, but I don't get the logic either lol

– gameon67
32 mins ago




3




3





I don't know why you would ever want to do this, but now I know you can

– Nathan
30 mins ago





I don't know why you would ever want to do this, but now I know you can

– Nathan
30 mins ago













over the iteration only the last time a[-1] gets its value, all other times python treats as iteration variable.

– Arun Augustine
26 mins ago






over the iteration only the last time a[-1] gets its value, all other times python treats as iteration variable.

– Arun Augustine
26 mins ago





5




5





This would be a great question for an awful coding interview

– Nathan
23 mins ago





This would be a great question for an awful coding interview

– Nathan
23 mins ago













@Nathan same idea, would be a great trivial question in interview

– gameon67
23 mins ago





@Nathan same idea, would be a great trivial question in interview

– gameon67
23 mins ago












5 Answers
5






active

oldest

votes


















8














To bring a language-lawyer aspect to the question, let's look at documentation:




for_stmt ::= "for" target_list "in" expression_list ":" suite


The expression list is evaluated once; it should yield an iterable object. An iterator is created for the result of the expression_list. The suite is then executed once for each item provided by the iterator, in the order returned by the iterator. Each item in turn is assigned to the target list using the standard rules for assignments (see Assignment statements), and then the suite is executed.




(emphasis not originally in docs)
The suite refers to the statements under the for-block. print(a[-1]) in OP's example.



Simply, on each iteration, the loop variable (target_list) gets assigned to the next item in the iterable (expression_list).



Let's extend the print statement:



a = [0, 1, 2, 3]
for a[-1] in a:
print(a, a[-1])


This gives the following output:



[0, 1, 2, 0] 0 # a[-1] assigned 0
[0, 1, 2, 1] 1 # a[-1] assigned 1
[0, 1, 2, 2] 2 # a[-1] assigned 2
[0, 1, 2, 2] 2 # a[-1] assigned itself (2)


As a[-1] is a valid form left-hand side, assignments to a[-1] will mutate a, modifying the list during iteration. In this particular example, a[-1] retains -2 from the previous evaluation. If we trace the assignment on each iteration, we have



a[-1] = 0 # a[0] # a[3] is 0 now
a[-1] = 1 # a[1] # a[3] is 1 now
a[-1] = 2 # a[2] # a[3] is 2 now
a[-1] = 2 # a[3] # a[3] is itself (2)





share|improve this answer




















  • 2





    Might be useful to bold Each item [...] is assigned to the target list using the standard rules for assignments.

    – Mateen Ulhaq
    18 mins ago







  • 2





    Essentially, the important thing to note here is that Python considers a[-1] to be a valid form of the left-hand side of an assignment statement (e.g. a[-1] = 1 is valid grammar). Thus a[-1] is a valid "variable" name, because as the documentation stated, it evaluates the binding variable(s) in a for loop declaration as it would the left-hand side of an assignment.

    – Christian Dean
    6 mins ago


















2














it is an interesting question, you can understand it by that:



for v in a:
a[-1] = v
print(a[-1])

print(a)


actually a becomes: [0, 1, 2, 2] after loop



output:



0
1
2
2
[0, 1, 2, 2]


Hope that helps you, and comment if you have further questions. : )






share|improve this answer






























    2














    (this is more of a long comment than an answer - there are a couple of good ones already, especially @TrebledJ's. But I had to think of it explicitly in terms of overwriting variables that already have values before it clicked for me.)



    If you had



    x = 0
    l = [1, 2, 3]
    for x in l:
    print(x)


    you wouldn't be surprised that x is overriden each time through the loop. Even though x existed before, its value isn't used (i.e. for 0 in l:, which would throw an error). Rather, we assign the values from l to x.



    When we do



    a = [0, 1, 2, 3]

    for a[-1] in a:
    print(a[-1])


    even though a[-1] already exists and has a value, we don't put that value in but rather assign to a[-1] each time through the loop.






    share|improve this answer

























    • Somehow I thought the variable used in for loop is immutable. +1 for pointing it out.

      – Amir A. Shabani
      10 mins ago


















    1














    The left expression of a for loop statement gets assigned with each item in the iterable on the right in each iteration, so



    for n in a:
    print(n)


    is just a fancy way of doing:



    for i in range(len(a)):
    n = a[i]
    print(n)


    Likewise,



    for a[-1] in a:
    print(a[-1])


    is just a fancy way of doing:



    for i in range(len(a)):
    a[-1] = a[i]
    print(a[-1])


    where in each iteration, the last item of a gets assigned with the next item in a, so when the iteration finally comes to the last item, its value got last assigned with the second-last item, 2.






    share|improve this answer






























      0














      a[-1] refers to the last element of a, in this case a[3]. The for loop is a bit unusual in that it is using this element as the loop variable. So it first gets set to 0, then 1, then 2. Finally, on the last iteration, the for loop retrieves a[3] which at that point is 2, so the list ends up as [0, 1, 2, 2].



      A more typical for loop uses a simple local variable name as the loop variable, e.g. for x .... In that case, x is set to the next value upon each iteration. This case is no different, except that a[-1] is set to the next value upon each iteration. You don't see this very often, but it's consistent.






      share|improve this answer

























      • How does it actually gets set to 0, then 1, then 2 and finally 2? That's the confusing part!

        – Amir A. Shabani
        34 mins ago






      • 1





        Agree, I don't really understand by reading this answer

        – gameon67
        34 mins ago











      • I expected it would be like saying for 3 in a: print(a[-1]) (because a[-1] was 3 at the start of the loop) and give an error, but clearly that's not the case. You're obviously correct about what's happening, but I'm surprised this evaluates this way.

        – Nathan
        31 mins ago











      • It's set by the for loop. Each time through, it gets set to the next element of the list. What's the first element? Right, it's 0. What's the second? Right again, it's 1. etc.

        – Tom Karzes
        31 mins ago











      • I've expanded the answer to explain more precisely how this works. Hopefully people will understand it this time.

        – Tom Karzes
        22 mins ago











      Your Answer






      StackExchange.ifUsing("editor", function ()
      StackExchange.using("externalEditor", function ()
      StackExchange.using("snippets", function ()
      StackExchange.snippets.init();
      );
      );
      , "code-snippets");

      StackExchange.ready(function()
      var channelOptions =
      tags: "".split(" "),
      id: "1"
      ;
      initTagRenderer("".split(" "), "".split(" "), channelOptions);

      StackExchange.using("externalEditor", function()
      // Have to fire editor after snippets, if snippets enabled
      if (StackExchange.settings.snippets.snippetsEnabled)
      StackExchange.using("snippets", function()
      createEditor();
      );

      else
      createEditor();

      );

      function createEditor()
      StackExchange.prepareEditor(
      heartbeatType: 'answer',
      autoActivateHeartbeat: false,
      convertImagesToLinks: true,
      noModals: true,
      showLowRepImageUploadWarning: true,
      reputationToPostImages: 10,
      bindNavPrevention: true,
      postfix: "",
      imageUploader:
      brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
      contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
      allowUrls: true
      ,
      onDemand: true,
      discardSelector: ".discard-answer"
      ,immediatelyShowMarkdownHelp:true
      );



      );






      Kundan Verma is a new contributor. Be nice, and check out our Code of Conduct.









      draft saved

      draft discarded


















      StackExchange.ready(
      function ()
      StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55644201%2fwhy-can-i-use-a-list-index-as-an-indexing-variable-in-a-for-loop%23new-answer', 'question_page');

      );

      Post as a guest















      Required, but never shown

























      5 Answers
      5






      active

      oldest

      votes








      5 Answers
      5






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      8














      To bring a language-lawyer aspect to the question, let's look at documentation:




      for_stmt ::= "for" target_list "in" expression_list ":" suite


      The expression list is evaluated once; it should yield an iterable object. An iterator is created for the result of the expression_list. The suite is then executed once for each item provided by the iterator, in the order returned by the iterator. Each item in turn is assigned to the target list using the standard rules for assignments (see Assignment statements), and then the suite is executed.




      (emphasis not originally in docs)
      The suite refers to the statements under the for-block. print(a[-1]) in OP's example.



      Simply, on each iteration, the loop variable (target_list) gets assigned to the next item in the iterable (expression_list).



      Let's extend the print statement:



      a = [0, 1, 2, 3]
      for a[-1] in a:
      print(a, a[-1])


      This gives the following output:



      [0, 1, 2, 0] 0 # a[-1] assigned 0
      [0, 1, 2, 1] 1 # a[-1] assigned 1
      [0, 1, 2, 2] 2 # a[-1] assigned 2
      [0, 1, 2, 2] 2 # a[-1] assigned itself (2)


      As a[-1] is a valid form left-hand side, assignments to a[-1] will mutate a, modifying the list during iteration. In this particular example, a[-1] retains -2 from the previous evaluation. If we trace the assignment on each iteration, we have



      a[-1] = 0 # a[0] # a[3] is 0 now
      a[-1] = 1 # a[1] # a[3] is 1 now
      a[-1] = 2 # a[2] # a[3] is 2 now
      a[-1] = 2 # a[3] # a[3] is itself (2)





      share|improve this answer




















      • 2





        Might be useful to bold Each item [...] is assigned to the target list using the standard rules for assignments.

        – Mateen Ulhaq
        18 mins ago







      • 2





        Essentially, the important thing to note here is that Python considers a[-1] to be a valid form of the left-hand side of an assignment statement (e.g. a[-1] = 1 is valid grammar). Thus a[-1] is a valid "variable" name, because as the documentation stated, it evaluates the binding variable(s) in a for loop declaration as it would the left-hand side of an assignment.

        – Christian Dean
        6 mins ago















      8














      To bring a language-lawyer aspect to the question, let's look at documentation:




      for_stmt ::= "for" target_list "in" expression_list ":" suite


      The expression list is evaluated once; it should yield an iterable object. An iterator is created for the result of the expression_list. The suite is then executed once for each item provided by the iterator, in the order returned by the iterator. Each item in turn is assigned to the target list using the standard rules for assignments (see Assignment statements), and then the suite is executed.




      (emphasis not originally in docs)
      The suite refers to the statements under the for-block. print(a[-1]) in OP's example.



      Simply, on each iteration, the loop variable (target_list) gets assigned to the next item in the iterable (expression_list).



      Let's extend the print statement:



      a = [0, 1, 2, 3]
      for a[-1] in a:
      print(a, a[-1])


      This gives the following output:



      [0, 1, 2, 0] 0 # a[-1] assigned 0
      [0, 1, 2, 1] 1 # a[-1] assigned 1
      [0, 1, 2, 2] 2 # a[-1] assigned 2
      [0, 1, 2, 2] 2 # a[-1] assigned itself (2)


      As a[-1] is a valid form left-hand side, assignments to a[-1] will mutate a, modifying the list during iteration. In this particular example, a[-1] retains -2 from the previous evaluation. If we trace the assignment on each iteration, we have



      a[-1] = 0 # a[0] # a[3] is 0 now
      a[-1] = 1 # a[1] # a[3] is 1 now
      a[-1] = 2 # a[2] # a[3] is 2 now
      a[-1] = 2 # a[3] # a[3] is itself (2)





      share|improve this answer




















      • 2





        Might be useful to bold Each item [...] is assigned to the target list using the standard rules for assignments.

        – Mateen Ulhaq
        18 mins ago







      • 2





        Essentially, the important thing to note here is that Python considers a[-1] to be a valid form of the left-hand side of an assignment statement (e.g. a[-1] = 1 is valid grammar). Thus a[-1] is a valid "variable" name, because as the documentation stated, it evaluates the binding variable(s) in a for loop declaration as it would the left-hand side of an assignment.

        – Christian Dean
        6 mins ago













      8












      8








      8







      To bring a language-lawyer aspect to the question, let's look at documentation:




      for_stmt ::= "for" target_list "in" expression_list ":" suite


      The expression list is evaluated once; it should yield an iterable object. An iterator is created for the result of the expression_list. The suite is then executed once for each item provided by the iterator, in the order returned by the iterator. Each item in turn is assigned to the target list using the standard rules for assignments (see Assignment statements), and then the suite is executed.




      (emphasis not originally in docs)
      The suite refers to the statements under the for-block. print(a[-1]) in OP's example.



      Simply, on each iteration, the loop variable (target_list) gets assigned to the next item in the iterable (expression_list).



      Let's extend the print statement:



      a = [0, 1, 2, 3]
      for a[-1] in a:
      print(a, a[-1])


      This gives the following output:



      [0, 1, 2, 0] 0 # a[-1] assigned 0
      [0, 1, 2, 1] 1 # a[-1] assigned 1
      [0, 1, 2, 2] 2 # a[-1] assigned 2
      [0, 1, 2, 2] 2 # a[-1] assigned itself (2)


      As a[-1] is a valid form left-hand side, assignments to a[-1] will mutate a, modifying the list during iteration. In this particular example, a[-1] retains -2 from the previous evaluation. If we trace the assignment on each iteration, we have



      a[-1] = 0 # a[0] # a[3] is 0 now
      a[-1] = 1 # a[1] # a[3] is 1 now
      a[-1] = 2 # a[2] # a[3] is 2 now
      a[-1] = 2 # a[3] # a[3] is itself (2)





      share|improve this answer















      To bring a language-lawyer aspect to the question, let's look at documentation:




      for_stmt ::= "for" target_list "in" expression_list ":" suite


      The expression list is evaluated once; it should yield an iterable object. An iterator is created for the result of the expression_list. The suite is then executed once for each item provided by the iterator, in the order returned by the iterator. Each item in turn is assigned to the target list using the standard rules for assignments (see Assignment statements), and then the suite is executed.




      (emphasis not originally in docs)
      The suite refers to the statements under the for-block. print(a[-1]) in OP's example.



      Simply, on each iteration, the loop variable (target_list) gets assigned to the next item in the iterable (expression_list).



      Let's extend the print statement:



      a = [0, 1, 2, 3]
      for a[-1] in a:
      print(a, a[-1])


      This gives the following output:



      [0, 1, 2, 0] 0 # a[-1] assigned 0
      [0, 1, 2, 1] 1 # a[-1] assigned 1
      [0, 1, 2, 2] 2 # a[-1] assigned 2
      [0, 1, 2, 2] 2 # a[-1] assigned itself (2)


      As a[-1] is a valid form left-hand side, assignments to a[-1] will mutate a, modifying the list during iteration. In this particular example, a[-1] retains -2 from the previous evaluation. If we trace the assignment on each iteration, we have



      a[-1] = 0 # a[0] # a[3] is 0 now
      a[-1] = 1 # a[1] # a[3] is 1 now
      a[-1] = 2 # a[2] # a[3] is 2 now
      a[-1] = 2 # a[3] # a[3] is itself (2)






      share|improve this answer














      share|improve this answer



      share|improve this answer








      edited 3 mins ago

























      answered 22 mins ago









      TrebledJTrebledJ

      3,74421328




      3,74421328







      • 2





        Might be useful to bold Each item [...] is assigned to the target list using the standard rules for assignments.

        – Mateen Ulhaq
        18 mins ago







      • 2





        Essentially, the important thing to note here is that Python considers a[-1] to be a valid form of the left-hand side of an assignment statement (e.g. a[-1] = 1 is valid grammar). Thus a[-1] is a valid "variable" name, because as the documentation stated, it evaluates the binding variable(s) in a for loop declaration as it would the left-hand side of an assignment.

        – Christian Dean
        6 mins ago












      • 2





        Might be useful to bold Each item [...] is assigned to the target list using the standard rules for assignments.

        – Mateen Ulhaq
        18 mins ago







      • 2





        Essentially, the important thing to note here is that Python considers a[-1] to be a valid form of the left-hand side of an assignment statement (e.g. a[-1] = 1 is valid grammar). Thus a[-1] is a valid "variable" name, because as the documentation stated, it evaluates the binding variable(s) in a for loop declaration as it would the left-hand side of an assignment.

        – Christian Dean
        6 mins ago







      2




      2





      Might be useful to bold Each item [...] is assigned to the target list using the standard rules for assignments.

      – Mateen Ulhaq
      18 mins ago






      Might be useful to bold Each item [...] is assigned to the target list using the standard rules for assignments.

      – Mateen Ulhaq
      18 mins ago





      2




      2





      Essentially, the important thing to note here is that Python considers a[-1] to be a valid form of the left-hand side of an assignment statement (e.g. a[-1] = 1 is valid grammar). Thus a[-1] is a valid "variable" name, because as the documentation stated, it evaluates the binding variable(s) in a for loop declaration as it would the left-hand side of an assignment.

      – Christian Dean
      6 mins ago





      Essentially, the important thing to note here is that Python considers a[-1] to be a valid form of the left-hand side of an assignment statement (e.g. a[-1] = 1 is valid grammar). Thus a[-1] is a valid "variable" name, because as the documentation stated, it evaluates the binding variable(s) in a for loop declaration as it would the left-hand side of an assignment.

      – Christian Dean
      6 mins ago













      2














      it is an interesting question, you can understand it by that:



      for v in a:
      a[-1] = v
      print(a[-1])

      print(a)


      actually a becomes: [0, 1, 2, 2] after loop



      output:



      0
      1
      2
      2
      [0, 1, 2, 2]


      Hope that helps you, and comment if you have further questions. : )






      share|improve this answer



























        2














        it is an interesting question, you can understand it by that:



        for v in a:
        a[-1] = v
        print(a[-1])

        print(a)


        actually a becomes: [0, 1, 2, 2] after loop



        output:



        0
        1
        2
        2
        [0, 1, 2, 2]


        Hope that helps you, and comment if you have further questions. : )






        share|improve this answer

























          2












          2








          2







          it is an interesting question, you can understand it by that:



          for v in a:
          a[-1] = v
          print(a[-1])

          print(a)


          actually a becomes: [0, 1, 2, 2] after loop



          output:



          0
          1
          2
          2
          [0, 1, 2, 2]


          Hope that helps you, and comment if you have further questions. : )






          share|improve this answer













          it is an interesting question, you can understand it by that:



          for v in a:
          a[-1] = v
          print(a[-1])

          print(a)


          actually a becomes: [0, 1, 2, 2] after loop



          output:



          0
          1
          2
          2
          [0, 1, 2, 2]


          Hope that helps you, and comment if you have further questions. : )







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 24 mins ago









          recnacrecnac

          1,183123




          1,183123





















              2














              (this is more of a long comment than an answer - there are a couple of good ones already, especially @TrebledJ's. But I had to think of it explicitly in terms of overwriting variables that already have values before it clicked for me.)



              If you had



              x = 0
              l = [1, 2, 3]
              for x in l:
              print(x)


              you wouldn't be surprised that x is overriden each time through the loop. Even though x existed before, its value isn't used (i.e. for 0 in l:, which would throw an error). Rather, we assign the values from l to x.



              When we do



              a = [0, 1, 2, 3]

              for a[-1] in a:
              print(a[-1])


              even though a[-1] already exists and has a value, we don't put that value in but rather assign to a[-1] each time through the loop.






              share|improve this answer

























              • Somehow I thought the variable used in for loop is immutable. +1 for pointing it out.

                – Amir A. Shabani
                10 mins ago















              2














              (this is more of a long comment than an answer - there are a couple of good ones already, especially @TrebledJ's. But I had to think of it explicitly in terms of overwriting variables that already have values before it clicked for me.)



              If you had



              x = 0
              l = [1, 2, 3]
              for x in l:
              print(x)


              you wouldn't be surprised that x is overriden each time through the loop. Even though x existed before, its value isn't used (i.e. for 0 in l:, which would throw an error). Rather, we assign the values from l to x.



              When we do



              a = [0, 1, 2, 3]

              for a[-1] in a:
              print(a[-1])


              even though a[-1] already exists and has a value, we don't put that value in but rather assign to a[-1] each time through the loop.






              share|improve this answer

























              • Somehow I thought the variable used in for loop is immutable. +1 for pointing it out.

                – Amir A. Shabani
                10 mins ago













              2












              2








              2







              (this is more of a long comment than an answer - there are a couple of good ones already, especially @TrebledJ's. But I had to think of it explicitly in terms of overwriting variables that already have values before it clicked for me.)



              If you had



              x = 0
              l = [1, 2, 3]
              for x in l:
              print(x)


              you wouldn't be surprised that x is overriden each time through the loop. Even though x existed before, its value isn't used (i.e. for 0 in l:, which would throw an error). Rather, we assign the values from l to x.



              When we do



              a = [0, 1, 2, 3]

              for a[-1] in a:
              print(a[-1])


              even though a[-1] already exists and has a value, we don't put that value in but rather assign to a[-1] each time through the loop.






              share|improve this answer















              (this is more of a long comment than an answer - there are a couple of good ones already, especially @TrebledJ's. But I had to think of it explicitly in terms of overwriting variables that already have values before it clicked for me.)



              If you had



              x = 0
              l = [1, 2, 3]
              for x in l:
              print(x)


              you wouldn't be surprised that x is overriden each time through the loop. Even though x existed before, its value isn't used (i.e. for 0 in l:, which would throw an error). Rather, we assign the values from l to x.



              When we do



              a = [0, 1, 2, 3]

              for a[-1] in a:
              print(a[-1])


              even though a[-1] already exists and has a value, we don't put that value in but rather assign to a[-1] each time through the loop.







              share|improve this answer














              share|improve this answer



              share|improve this answer








              edited 6 mins ago









              Amir A. Shabani

              455616




              455616










              answered 13 mins ago









              NathanNathan

              2,02511326




              2,02511326












              • Somehow I thought the variable used in for loop is immutable. +1 for pointing it out.

                – Amir A. Shabani
                10 mins ago

















              • Somehow I thought the variable used in for loop is immutable. +1 for pointing it out.

                – Amir A. Shabani
                10 mins ago
















              Somehow I thought the variable used in for loop is immutable. +1 for pointing it out.

              – Amir A. Shabani
              10 mins ago





              Somehow I thought the variable used in for loop is immutable. +1 for pointing it out.

              – Amir A. Shabani
              10 mins ago











              1














              The left expression of a for loop statement gets assigned with each item in the iterable on the right in each iteration, so



              for n in a:
              print(n)


              is just a fancy way of doing:



              for i in range(len(a)):
              n = a[i]
              print(n)


              Likewise,



              for a[-1] in a:
              print(a[-1])


              is just a fancy way of doing:



              for i in range(len(a)):
              a[-1] = a[i]
              print(a[-1])


              where in each iteration, the last item of a gets assigned with the next item in a, so when the iteration finally comes to the last item, its value got last assigned with the second-last item, 2.






              share|improve this answer



























                1














                The left expression of a for loop statement gets assigned with each item in the iterable on the right in each iteration, so



                for n in a:
                print(n)


                is just a fancy way of doing:



                for i in range(len(a)):
                n = a[i]
                print(n)


                Likewise,



                for a[-1] in a:
                print(a[-1])


                is just a fancy way of doing:



                for i in range(len(a)):
                a[-1] = a[i]
                print(a[-1])


                where in each iteration, the last item of a gets assigned with the next item in a, so when the iteration finally comes to the last item, its value got last assigned with the second-last item, 2.






                share|improve this answer

























                  1












                  1








                  1







                  The left expression of a for loop statement gets assigned with each item in the iterable on the right in each iteration, so



                  for n in a:
                  print(n)


                  is just a fancy way of doing:



                  for i in range(len(a)):
                  n = a[i]
                  print(n)


                  Likewise,



                  for a[-1] in a:
                  print(a[-1])


                  is just a fancy way of doing:



                  for i in range(len(a)):
                  a[-1] = a[i]
                  print(a[-1])


                  where in each iteration, the last item of a gets assigned with the next item in a, so when the iteration finally comes to the last item, its value got last assigned with the second-last item, 2.






                  share|improve this answer













                  The left expression of a for loop statement gets assigned with each item in the iterable on the right in each iteration, so



                  for n in a:
                  print(n)


                  is just a fancy way of doing:



                  for i in range(len(a)):
                  n = a[i]
                  print(n)


                  Likewise,



                  for a[-1] in a:
                  print(a[-1])


                  is just a fancy way of doing:



                  for i in range(len(a)):
                  a[-1] = a[i]
                  print(a[-1])


                  where in each iteration, the last item of a gets assigned with the next item in a, so when the iteration finally comes to the last item, its value got last assigned with the second-last item, 2.







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered 19 mins ago









                  blhsingblhsing

                  43.5k41744




                  43.5k41744





















                      0














                      a[-1] refers to the last element of a, in this case a[3]. The for loop is a bit unusual in that it is using this element as the loop variable. So it first gets set to 0, then 1, then 2. Finally, on the last iteration, the for loop retrieves a[3] which at that point is 2, so the list ends up as [0, 1, 2, 2].



                      A more typical for loop uses a simple local variable name as the loop variable, e.g. for x .... In that case, x is set to the next value upon each iteration. This case is no different, except that a[-1] is set to the next value upon each iteration. You don't see this very often, but it's consistent.






                      share|improve this answer

























                      • How does it actually gets set to 0, then 1, then 2 and finally 2? That's the confusing part!

                        – Amir A. Shabani
                        34 mins ago






                      • 1





                        Agree, I don't really understand by reading this answer

                        – gameon67
                        34 mins ago











                      • I expected it would be like saying for 3 in a: print(a[-1]) (because a[-1] was 3 at the start of the loop) and give an error, but clearly that's not the case. You're obviously correct about what's happening, but I'm surprised this evaluates this way.

                        – Nathan
                        31 mins ago











                      • It's set by the for loop. Each time through, it gets set to the next element of the list. What's the first element? Right, it's 0. What's the second? Right again, it's 1. etc.

                        – Tom Karzes
                        31 mins ago











                      • I've expanded the answer to explain more precisely how this works. Hopefully people will understand it this time.

                        – Tom Karzes
                        22 mins ago















                      0














                      a[-1] refers to the last element of a, in this case a[3]. The for loop is a bit unusual in that it is using this element as the loop variable. So it first gets set to 0, then 1, then 2. Finally, on the last iteration, the for loop retrieves a[3] which at that point is 2, so the list ends up as [0, 1, 2, 2].



                      A more typical for loop uses a simple local variable name as the loop variable, e.g. for x .... In that case, x is set to the next value upon each iteration. This case is no different, except that a[-1] is set to the next value upon each iteration. You don't see this very often, but it's consistent.






                      share|improve this answer

























                      • How does it actually gets set to 0, then 1, then 2 and finally 2? That's the confusing part!

                        – Amir A. Shabani
                        34 mins ago






                      • 1





                        Agree, I don't really understand by reading this answer

                        – gameon67
                        34 mins ago











                      • I expected it would be like saying for 3 in a: print(a[-1]) (because a[-1] was 3 at the start of the loop) and give an error, but clearly that's not the case. You're obviously correct about what's happening, but I'm surprised this evaluates this way.

                        – Nathan
                        31 mins ago











                      • It's set by the for loop. Each time through, it gets set to the next element of the list. What's the first element? Right, it's 0. What's the second? Right again, it's 1. etc.

                        – Tom Karzes
                        31 mins ago











                      • I've expanded the answer to explain more precisely how this works. Hopefully people will understand it this time.

                        – Tom Karzes
                        22 mins ago













                      0












                      0








                      0







                      a[-1] refers to the last element of a, in this case a[3]. The for loop is a bit unusual in that it is using this element as the loop variable. So it first gets set to 0, then 1, then 2. Finally, on the last iteration, the for loop retrieves a[3] which at that point is 2, so the list ends up as [0, 1, 2, 2].



                      A more typical for loop uses a simple local variable name as the loop variable, e.g. for x .... In that case, x is set to the next value upon each iteration. This case is no different, except that a[-1] is set to the next value upon each iteration. You don't see this very often, but it's consistent.






                      share|improve this answer















                      a[-1] refers to the last element of a, in this case a[3]. The for loop is a bit unusual in that it is using this element as the loop variable. So it first gets set to 0, then 1, then 2. Finally, on the last iteration, the for loop retrieves a[3] which at that point is 2, so the list ends up as [0, 1, 2, 2].



                      A more typical for loop uses a simple local variable name as the loop variable, e.g. for x .... In that case, x is set to the next value upon each iteration. This case is no different, except that a[-1] is set to the next value upon each iteration. You don't see this very often, but it's consistent.







                      share|improve this answer














                      share|improve this answer



                      share|improve this answer








                      edited 23 mins ago

























                      answered 36 mins ago









                      Tom KarzesTom Karzes

                      11.2k1926




                      11.2k1926












                      • How does it actually gets set to 0, then 1, then 2 and finally 2? That's the confusing part!

                        – Amir A. Shabani
                        34 mins ago






                      • 1





                        Agree, I don't really understand by reading this answer

                        – gameon67
                        34 mins ago











                      • I expected it would be like saying for 3 in a: print(a[-1]) (because a[-1] was 3 at the start of the loop) and give an error, but clearly that's not the case. You're obviously correct about what's happening, but I'm surprised this evaluates this way.

                        – Nathan
                        31 mins ago











                      • It's set by the for loop. Each time through, it gets set to the next element of the list. What's the first element? Right, it's 0. What's the second? Right again, it's 1. etc.

                        – Tom Karzes
                        31 mins ago











                      • I've expanded the answer to explain more precisely how this works. Hopefully people will understand it this time.

                        – Tom Karzes
                        22 mins ago

















                      • How does it actually gets set to 0, then 1, then 2 and finally 2? That's the confusing part!

                        – Amir A. Shabani
                        34 mins ago






                      • 1





                        Agree, I don't really understand by reading this answer

                        – gameon67
                        34 mins ago











                      • I expected it would be like saying for 3 in a: print(a[-1]) (because a[-1] was 3 at the start of the loop) and give an error, but clearly that's not the case. You're obviously correct about what's happening, but I'm surprised this evaluates this way.

                        – Nathan
                        31 mins ago











                      • It's set by the for loop. Each time through, it gets set to the next element of the list. What's the first element? Right, it's 0. What's the second? Right again, it's 1. etc.

                        – Tom Karzes
                        31 mins ago











                      • I've expanded the answer to explain more precisely how this works. Hopefully people will understand it this time.

                        – Tom Karzes
                        22 mins ago
















                      How does it actually gets set to 0, then 1, then 2 and finally 2? That's the confusing part!

                      – Amir A. Shabani
                      34 mins ago





                      How does it actually gets set to 0, then 1, then 2 and finally 2? That's the confusing part!

                      – Amir A. Shabani
                      34 mins ago




                      1




                      1





                      Agree, I don't really understand by reading this answer

                      – gameon67
                      34 mins ago





                      Agree, I don't really understand by reading this answer

                      – gameon67
                      34 mins ago













                      I expected it would be like saying for 3 in a: print(a[-1]) (because a[-1] was 3 at the start of the loop) and give an error, but clearly that's not the case. You're obviously correct about what's happening, but I'm surprised this evaluates this way.

                      – Nathan
                      31 mins ago





                      I expected it would be like saying for 3 in a: print(a[-1]) (because a[-1] was 3 at the start of the loop) and give an error, but clearly that's not the case. You're obviously correct about what's happening, but I'm surprised this evaluates this way.

                      – Nathan
                      31 mins ago













                      It's set by the for loop. Each time through, it gets set to the next element of the list. What's the first element? Right, it's 0. What's the second? Right again, it's 1. etc.

                      – Tom Karzes
                      31 mins ago





                      It's set by the for loop. Each time through, it gets set to the next element of the list. What's the first element? Right, it's 0. What's the second? Right again, it's 1. etc.

                      – Tom Karzes
                      31 mins ago













                      I've expanded the answer to explain more precisely how this works. Hopefully people will understand it this time.

                      – Tom Karzes
                      22 mins ago





                      I've expanded the answer to explain more precisely how this works. Hopefully people will understand it this time.

                      – Tom Karzes
                      22 mins ago










                      Kundan Verma is a new contributor. Be nice, and check out our Code of Conduct.









                      draft saved

                      draft discarded


















                      Kundan Verma is a new contributor. Be nice, and check out our Code of Conduct.












                      Kundan Verma is a new contributor. Be nice, and check out our Code of Conduct.











                      Kundan Verma is a new contributor. Be nice, and check out our Code of Conduct.














                      Thanks for contributing an answer to Stack Overflow!


                      • Please be sure to answer the question. Provide details and share your research!

                      But avoid


                      • Asking for help, clarification, or responding to other answers.

                      • Making statements based on opinion; back them up with references or personal experience.

                      To learn more, see our tips on writing great answers.




                      draft saved


                      draft discarded














                      StackExchange.ready(
                      function ()
                      StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55644201%2fwhy-can-i-use-a-list-index-as-an-indexing-variable-in-a-for-loop%23new-answer', 'question_page');

                      );

                      Post as a guest















                      Required, but never shown





















































                      Required, but never shown














                      Required, but never shown












                      Required, but never shown







                      Required, but never shown

































                      Required, but never shown














                      Required, but never shown












                      Required, but never shown







                      Required, but never shown







                      Popular posts from this blog

                      Möglingen Índice Localización Historia Demografía Referencias Enlaces externos Menú de navegación48°53′18″N 9°07′45″E / 48.888333333333, 9.129166666666748°53′18″N 9°07′45″E / 48.888333333333, 9.1291666666667Sitio web oficial Mapa de Möglingen«Gemeinden in Deutschland nach Fläche, Bevölkerung und Postleitzahl am 30.09.2016»Möglingen

                      Virtualbox - Configuration error: Querying “UUID” failed (VERR_CFGM_VALUE_NOT_FOUND)“VERR_SUPLIB_WORLD_WRITABLE” error when trying to installing OS in virtualboxVirtual Box Kernel errorFailed to open a seesion for the virtual machineFailed to open a session for the virtual machineUbuntu 14.04 LTS Virtualbox errorcan't use VM VirtualBoxusing virtualboxI can't run Linux-64 Bit on VirtualBoxUnable to insert the virtual optical disk (VBoxguestaddition) in virtual machine for ubuntu server in win 10VirtuaBox in Ubuntu 18.04 Issues with Win10.ISO Installation

                      Torre de la Isleta Índice Véase también Referencias Bibliografía Enlaces externos Menú de navegación38°25′58″N 0°23′02″O / 38.43277778, -0.3838888938°25′58″N 0°23′02″O / 38.43277778, -0.38388889Torre de la Illeta de l’Horta o Torre Saleta. Base de datos de bienes inmuebles. Patrimonio Cultural. Secretaría de Estado de CulturaFicha BIC Torre de la Illeta de l’Horta. Dirección General de Patrimonio Cultural. Generalitat ValencianaLugares de interés. Ayuntamiento del CampelloTorre de la Isleta en CastillosNet.org