Using the end of the list as an indexing variable in a for loop





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9















Please help me out understanding this piece of python code?



Python:



a = [0,1,2,3]

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


Output:



0,1,2,2


I'm confused how it's coming out.










share|improve this question









New contributor




Kundan Verma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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  • 6





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

    – gameon67
    28 mins ago






  • 3





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

    – Nathan
    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
    22 mins ago








  • 5





    This would be a great question for an awful coding interview

    – Nathan
    19 mins ago











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

    – gameon67
    19 mins ago


















9















Please help me out understanding this piece of python code?



Python:



a = [0,1,2,3]

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


Output:



0,1,2,2


I'm confused how it's coming out.










share|improve this question









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
    28 mins ago






  • 3





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

    – Nathan
    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
    22 mins ago








  • 5





    This would be a great question for an awful coding interview

    – Nathan
    19 mins ago











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

    – gameon67
    19 mins ago














9












9








9


6






Please help me out understanding this piece of python code?



Python:



a = [0,1,2,3]

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


Output:



0,1,2,2


I'm confused how it's coming out.










share|improve this question









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.












Please help me out understanding this piece of python code?



Python:



a = [0,1,2,3]

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


Output:



0,1,2,2


I'm confused how it's coming out.







python






share|improve this question









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|improve this question









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|improve this question




share|improve this question








edited 30 mins ago









TrebledJ

3,74421328




3,74421328






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 38 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
    28 mins ago






  • 3





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

    – Nathan
    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
    22 mins ago








  • 5





    This would be a great question for an awful coding interview

    – Nathan
    19 mins ago











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

    – gameon67
    19 mins ago














  • 6





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

    – gameon67
    28 mins ago






  • 3





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

    – Nathan
    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
    22 mins ago








  • 5





    This would be a great question for an awful coding interview

    – Nathan
    19 mins ago











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

    – gameon67
    19 mins ago








6




6





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

– gameon67
28 mins ago





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

– gameon67
28 mins ago




3




3





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

– Nathan
26 mins ago





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

– Nathan
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
22 mins ago







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

– Arun Augustine
22 mins ago






5




5





This would be a great question for an awful coding interview

– Nathan
19 mins ago





This would be a great question for an awful coding interview

– Nathan
19 mins ago













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

– gameon67
19 mins ago





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

– gameon67
19 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 see extend the print statement:



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


In OP's example, a[-1] retains -2 from the previous evaluation. We initially 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
    14 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
    2 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


























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

      – Amir A. Shabani
      6 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
        30 mins ago






      • 1





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

        – gameon67
        30 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
        27 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
        27 mins ago











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

        – Tom Karzes
        18 mins ago












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      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 see extend the print statement:



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


      In OP's example, a[-1] retains -2 from the previous evaluation. We initially 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
        14 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
        2 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 see extend the print statement:



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


      In OP's example, a[-1] retains -2 from the previous evaluation. We initially 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
        14 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
        2 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 see extend the print statement:



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


      In OP's example, a[-1] retains -2 from the previous evaluation. We initially 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 see extend the print statement:



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


      In OP's example, a[-1] retains -2 from the previous evaluation. We initially 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 5 mins ago

























      answered 18 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
        14 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
        2 mins ago














      • 2





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

        – Mateen Ulhaq
        14 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
        2 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
      14 mins ago







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

      – Mateen Ulhaq
      14 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
      2 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
      2 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 20 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


























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

                – Amir A. Shabani
                6 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


























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

                – Amir A. Shabani
                6 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















              (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













              share


              share








              edited 2 mins ago









              Amir A. Shabani

              455616




              455616










              answered 9 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
                6 mins ago



















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

                – Amir A. Shabani
                6 mins ago

















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

              – Amir A. Shabani
              6 mins ago





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

              – Amir A. Shabani
              6 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 15 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
                        30 mins ago






                      • 1





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

                        – gameon67
                        30 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
                        27 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
                        27 mins ago











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

                        – Tom Karzes
                        18 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
                        30 mins ago






                      • 1





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

                        – gameon67
                        30 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
                        27 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
                        27 mins ago











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

                        – Tom Karzes
                        18 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 19 mins ago

























                      answered 32 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
                        30 mins ago






                      • 1





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

                        – gameon67
                        30 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
                        27 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
                        27 mins ago











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

                        – Tom Karzes
                        18 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
                        30 mins ago






                      • 1





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

                        – gameon67
                        30 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
                        27 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
                        27 mins ago











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

                        – Tom Karzes
                        18 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
                      30 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
                      30 mins ago




                      1




                      1





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

                      – gameon67
                      30 mins ago





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

                      – gameon67
                      30 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
                      27 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
                      27 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
                      27 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
                      27 mins ago













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

                      – Tom Karzes
                      18 mins ago





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

                      – Tom Karzes
                      18 mins ago










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










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                      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.
















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