List value swapping: What's the correct order and why?












2















For a list of integers, such as A = [2, 10, -5], I get the error



Traceback (most recent call last):
File "so.py", line 6, in <module>
v, A[v-1] = A[v-1], v
IndexError: list assignment index out of range


Code:



for i, v in enumerate(A):
while 1<=v<=len(A) and v != A[v-1]:
v, A[v-1] = A[v-1], v


but this works:



for i, v in enumerate(A):
while 1<=v<=len(A) and v != A[v-1]:
A[v-1], v = v, A[v-1]


Why the order of the swapping elements matters here? v is always being checked to be in bound.



Weirdly enough cannot reproduce a smaller example. But,
A = [6, 5, 4, 3, 2]
becomes an infinite loop.










share|improve this question




















  • 4





    Can you provide A and n?

    – Ssein
    Nov 15 '18 at 17:29






  • 1





    You should refer to this post.

    – Dimeji Mudele
    Nov 15 '18 at 17:41











  • Don't reassign the iteration variable, v.

    – hpaulj
    Nov 15 '18 at 17:55






  • 1





    i is the index. v the value. A[i] makes sense. A[v] does not.

    – hpaulj
    Nov 15 '18 at 17:58











  • Note that your inner loop is a while, not an if. Your list [6, 5, 4, 3, 2] gets stuck in the while.

    – Prune
    Nov 15 '18 at 18:10
















2















For a list of integers, such as A = [2, 10, -5], I get the error



Traceback (most recent call last):
File "so.py", line 6, in <module>
v, A[v-1] = A[v-1], v
IndexError: list assignment index out of range


Code:



for i, v in enumerate(A):
while 1<=v<=len(A) and v != A[v-1]:
v, A[v-1] = A[v-1], v


but this works:



for i, v in enumerate(A):
while 1<=v<=len(A) and v != A[v-1]:
A[v-1], v = v, A[v-1]


Why the order of the swapping elements matters here? v is always being checked to be in bound.



Weirdly enough cannot reproduce a smaller example. But,
A = [6, 5, 4, 3, 2]
becomes an infinite loop.










share|improve this question




















  • 4





    Can you provide A and n?

    – Ssein
    Nov 15 '18 at 17:29






  • 1





    You should refer to this post.

    – Dimeji Mudele
    Nov 15 '18 at 17:41











  • Don't reassign the iteration variable, v.

    – hpaulj
    Nov 15 '18 at 17:55






  • 1





    i is the index. v the value. A[i] makes sense. A[v] does not.

    – hpaulj
    Nov 15 '18 at 17:58











  • Note that your inner loop is a while, not an if. Your list [6, 5, 4, 3, 2] gets stuck in the while.

    – Prune
    Nov 15 '18 at 18:10














2












2








2








For a list of integers, such as A = [2, 10, -5], I get the error



Traceback (most recent call last):
File "so.py", line 6, in <module>
v, A[v-1] = A[v-1], v
IndexError: list assignment index out of range


Code:



for i, v in enumerate(A):
while 1<=v<=len(A) and v != A[v-1]:
v, A[v-1] = A[v-1], v


but this works:



for i, v in enumerate(A):
while 1<=v<=len(A) and v != A[v-1]:
A[v-1], v = v, A[v-1]


Why the order of the swapping elements matters here? v is always being checked to be in bound.



Weirdly enough cannot reproduce a smaller example. But,
A = [6, 5, 4, 3, 2]
becomes an infinite loop.










share|improve this question
















For a list of integers, such as A = [2, 10, -5], I get the error



Traceback (most recent call last):
File "so.py", line 6, in <module>
v, A[v-1] = A[v-1], v
IndexError: list assignment index out of range


Code:



for i, v in enumerate(A):
while 1<=v<=len(A) and v != A[v-1]:
v, A[v-1] = A[v-1], v


but this works:



for i, v in enumerate(A):
while 1<=v<=len(A) and v != A[v-1]:
A[v-1], v = v, A[v-1]


Why the order of the swapping elements matters here? v is always being checked to be in bound.



Weirdly enough cannot reproduce a smaller example. But,
A = [6, 5, 4, 3, 2]
becomes an infinite loop.







python






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Nov 15 '18 at 18:19









martineau

69.1k1092186




69.1k1092186










asked Nov 15 '18 at 17:22









kaiserasifkaiserasif

588




588








  • 4





    Can you provide A and n?

    – Ssein
    Nov 15 '18 at 17:29






  • 1





    You should refer to this post.

    – Dimeji Mudele
    Nov 15 '18 at 17:41











  • Don't reassign the iteration variable, v.

    – hpaulj
    Nov 15 '18 at 17:55






  • 1





    i is the index. v the value. A[i] makes sense. A[v] does not.

    – hpaulj
    Nov 15 '18 at 17:58











  • Note that your inner loop is a while, not an if. Your list [6, 5, 4, 3, 2] gets stuck in the while.

    – Prune
    Nov 15 '18 at 18:10














  • 4





    Can you provide A and n?

    – Ssein
    Nov 15 '18 at 17:29






  • 1





    You should refer to this post.

    – Dimeji Mudele
    Nov 15 '18 at 17:41











  • Don't reassign the iteration variable, v.

    – hpaulj
    Nov 15 '18 at 17:55






  • 1





    i is the index. v the value. A[i] makes sense. A[v] does not.

    – hpaulj
    Nov 15 '18 at 17:58











  • Note that your inner loop is a while, not an if. Your list [6, 5, 4, 3, 2] gets stuck in the while.

    – Prune
    Nov 15 '18 at 18:10








4




4





Can you provide A and n?

– Ssein
Nov 15 '18 at 17:29





Can you provide A and n?

– Ssein
Nov 15 '18 at 17:29




1




1





You should refer to this post.

– Dimeji Mudele
Nov 15 '18 at 17:41





You should refer to this post.

– Dimeji Mudele
Nov 15 '18 at 17:41













Don't reassign the iteration variable, v.

– hpaulj
Nov 15 '18 at 17:55





Don't reassign the iteration variable, v.

– hpaulj
Nov 15 '18 at 17:55




1




1





i is the index. v the value. A[i] makes sense. A[v] does not.

– hpaulj
Nov 15 '18 at 17:58





i is the index. v the value. A[i] makes sense. A[v] does not.

– hpaulj
Nov 15 '18 at 17:58













Note that your inner loop is a while, not an if. Your list [6, 5, 4, 3, 2] gets stuck in the while.

– Prune
Nov 15 '18 at 18:10





Note that your inner loop is a while, not an if. Your list [6, 5, 4, 3, 2] gets stuck in the while.

– Prune
Nov 15 '18 at 18:10












3 Answers
3






active

oldest

votes


















2














I reproduced this with [2, 10, -5].
The detailed sequence of operations is



i, v = 0, 2
1 <= 2 ? OK
2 != A[1] ? OK ... stay in loop
v, A[v-1] = 10, 2
# This assignment breaks into ...
v = 10
A[10-1] = 2
# ... and this second assignment is out of range.


If you switch the assignment order:



    A[v-1], v = 2, 10
# This assignment breaks into ...
A[2-1] = 2
v = 10


In this case, your while conditions have properly guarded a legal assignment.



Note that you are not swapping list values; v is a local variable; it is not a reference to A[i]. For instance, in the above example, you do get v=10, but this does not affect the value of A[0].






share|improve this answer

































    7














    Python swaps the variables in the order provided, so v is assigned the value at A[v-1], and then tries to reassign A[v-1] - but since v has been modified to be a list element, v-1 is out of range of A.






    share|improve this answer
























    • Then, why the opposite doesn't cause the problem, i.e. A[v-1] set to v first, then v still gets the previous A[v-1] value from before the update.

      – kaiserasif
      Nov 15 '18 at 17:44






    • 1





      Because you have an if statement ensuring that v is less than the length of the array, so A[v-1] is always set to a value that can index A in the next step.

      – Tim
      Nov 15 '18 at 17:57



















    0














    A = [6, 5, 4, 3, 2]

    for i, v in enumerate(A):
    while 1<=v<=len(A) and v != A[v-1]:
    v, A[v-1] = A[v-1], v


    You need to try running out your algorithm in your head:



    Start:



    i = 0, v = 6



    v(6) is not between 1 and 5: next iteration



    i = 1, v = 5, A[5-1] = 2



    5 is between 1 and 5; v is not equal to 2: swap ->
    v = 2; A[4] = 2



    i = 1, v = 2, A[2-1] = 5



    2 is between 1 and 5; v is not equal to 5: swap ->
    v = 5; A[1] = 5



    i = 1, v = 5, A[5-1] = 2



    5 is between 1 and 5; v is not equal to 2: swap ->
    v = 2; A[4] = 2



    i = 1, v = 2, A[2-1] = 5



    2 is between 1 and 5; v is not equal to 5: swap ->
    v = 5; A[1] = 5



    ... and on and on



    I don't think your algorithm makes sense. It's unclear why you are using the values in your list to index the list during your loop. I think this confusion about the index and values is at the root of your problem.






    share|improve this answer























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      3 Answers
      3






      active

      oldest

      votes








      3 Answers
      3






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      2














      I reproduced this with [2, 10, -5].
      The detailed sequence of operations is



      i, v = 0, 2
      1 <= 2 ? OK
      2 != A[1] ? OK ... stay in loop
      v, A[v-1] = 10, 2
      # This assignment breaks into ...
      v = 10
      A[10-1] = 2
      # ... and this second assignment is out of range.


      If you switch the assignment order:



          A[v-1], v = 2, 10
      # This assignment breaks into ...
      A[2-1] = 2
      v = 10


      In this case, your while conditions have properly guarded a legal assignment.



      Note that you are not swapping list values; v is a local variable; it is not a reference to A[i]. For instance, in the above example, you do get v=10, but this does not affect the value of A[0].






      share|improve this answer






























        2














        I reproduced this with [2, 10, -5].
        The detailed sequence of operations is



        i, v = 0, 2
        1 <= 2 ? OK
        2 != A[1] ? OK ... stay in loop
        v, A[v-1] = 10, 2
        # This assignment breaks into ...
        v = 10
        A[10-1] = 2
        # ... and this second assignment is out of range.


        If you switch the assignment order:



            A[v-1], v = 2, 10
        # This assignment breaks into ...
        A[2-1] = 2
        v = 10


        In this case, your while conditions have properly guarded a legal assignment.



        Note that you are not swapping list values; v is a local variable; it is not a reference to A[i]. For instance, in the above example, you do get v=10, but this does not affect the value of A[0].






        share|improve this answer




























          2












          2








          2







          I reproduced this with [2, 10, -5].
          The detailed sequence of operations is



          i, v = 0, 2
          1 <= 2 ? OK
          2 != A[1] ? OK ... stay in loop
          v, A[v-1] = 10, 2
          # This assignment breaks into ...
          v = 10
          A[10-1] = 2
          # ... and this second assignment is out of range.


          If you switch the assignment order:



              A[v-1], v = 2, 10
          # This assignment breaks into ...
          A[2-1] = 2
          v = 10


          In this case, your while conditions have properly guarded a legal assignment.



          Note that you are not swapping list values; v is a local variable; it is not a reference to A[i]. For instance, in the above example, you do get v=10, but this does not affect the value of A[0].






          share|improve this answer















          I reproduced this with [2, 10, -5].
          The detailed sequence of operations is



          i, v = 0, 2
          1 <= 2 ? OK
          2 != A[1] ? OK ... stay in loop
          v, A[v-1] = 10, 2
          # This assignment breaks into ...
          v = 10
          A[10-1] = 2
          # ... and this second assignment is out of range.


          If you switch the assignment order:



              A[v-1], v = 2, 10
          # This assignment breaks into ...
          A[2-1] = 2
          v = 10


          In this case, your while conditions have properly guarded a legal assignment.



          Note that you are not swapping list values; v is a local variable; it is not a reference to A[i]. For instance, in the above example, you do get v=10, but this does not affect the value of A[0].







          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited Nov 15 '18 at 18:06

























          answered Nov 15 '18 at 17:56









          PrunePrune

          45.2k143559




          45.2k143559

























              7














              Python swaps the variables in the order provided, so v is assigned the value at A[v-1], and then tries to reassign A[v-1] - but since v has been modified to be a list element, v-1 is out of range of A.






              share|improve this answer
























              • Then, why the opposite doesn't cause the problem, i.e. A[v-1] set to v first, then v still gets the previous A[v-1] value from before the update.

                – kaiserasif
                Nov 15 '18 at 17:44






              • 1





                Because you have an if statement ensuring that v is less than the length of the array, so A[v-1] is always set to a value that can index A in the next step.

                – Tim
                Nov 15 '18 at 17:57
















              7














              Python swaps the variables in the order provided, so v is assigned the value at A[v-1], and then tries to reassign A[v-1] - but since v has been modified to be a list element, v-1 is out of range of A.






              share|improve this answer
























              • Then, why the opposite doesn't cause the problem, i.e. A[v-1] set to v first, then v still gets the previous A[v-1] value from before the update.

                – kaiserasif
                Nov 15 '18 at 17:44






              • 1





                Because you have an if statement ensuring that v is less than the length of the array, so A[v-1] is always set to a value that can index A in the next step.

                – Tim
                Nov 15 '18 at 17:57














              7












              7








              7







              Python swaps the variables in the order provided, so v is assigned the value at A[v-1], and then tries to reassign A[v-1] - but since v has been modified to be a list element, v-1 is out of range of A.






              share|improve this answer













              Python swaps the variables in the order provided, so v is assigned the value at A[v-1], and then tries to reassign A[v-1] - but since v has been modified to be a list element, v-1 is out of range of A.







              share|improve this answer












              share|improve this answer



              share|improve this answer










              answered Nov 15 '18 at 17:35









              TimTim

              1,772621




              1,772621













              • Then, why the opposite doesn't cause the problem, i.e. A[v-1] set to v first, then v still gets the previous A[v-1] value from before the update.

                – kaiserasif
                Nov 15 '18 at 17:44






              • 1





                Because you have an if statement ensuring that v is less than the length of the array, so A[v-1] is always set to a value that can index A in the next step.

                – Tim
                Nov 15 '18 at 17:57



















              • Then, why the opposite doesn't cause the problem, i.e. A[v-1] set to v first, then v still gets the previous A[v-1] value from before the update.

                – kaiserasif
                Nov 15 '18 at 17:44






              • 1





                Because you have an if statement ensuring that v is less than the length of the array, so A[v-1] is always set to a value that can index A in the next step.

                – Tim
                Nov 15 '18 at 17:57

















              Then, why the opposite doesn't cause the problem, i.e. A[v-1] set to v first, then v still gets the previous A[v-1] value from before the update.

              – kaiserasif
              Nov 15 '18 at 17:44





              Then, why the opposite doesn't cause the problem, i.e. A[v-1] set to v first, then v still gets the previous A[v-1] value from before the update.

              – kaiserasif
              Nov 15 '18 at 17:44




              1




              1





              Because you have an if statement ensuring that v is less than the length of the array, so A[v-1] is always set to a value that can index A in the next step.

              – Tim
              Nov 15 '18 at 17:57





              Because you have an if statement ensuring that v is less than the length of the array, so A[v-1] is always set to a value that can index A in the next step.

              – Tim
              Nov 15 '18 at 17:57











              0














              A = [6, 5, 4, 3, 2]

              for i, v in enumerate(A):
              while 1<=v<=len(A) and v != A[v-1]:
              v, A[v-1] = A[v-1], v


              You need to try running out your algorithm in your head:



              Start:



              i = 0, v = 6



              v(6) is not between 1 and 5: next iteration



              i = 1, v = 5, A[5-1] = 2



              5 is between 1 and 5; v is not equal to 2: swap ->
              v = 2; A[4] = 2



              i = 1, v = 2, A[2-1] = 5



              2 is between 1 and 5; v is not equal to 5: swap ->
              v = 5; A[1] = 5



              i = 1, v = 5, A[5-1] = 2



              5 is between 1 and 5; v is not equal to 2: swap ->
              v = 2; A[4] = 2



              i = 1, v = 2, A[2-1] = 5



              2 is between 1 and 5; v is not equal to 5: swap ->
              v = 5; A[1] = 5



              ... and on and on



              I don't think your algorithm makes sense. It's unclear why you are using the values in your list to index the list during your loop. I think this confusion about the index and values is at the root of your problem.






              share|improve this answer




























                0














                A = [6, 5, 4, 3, 2]

                for i, v in enumerate(A):
                while 1<=v<=len(A) and v != A[v-1]:
                v, A[v-1] = A[v-1], v


                You need to try running out your algorithm in your head:



                Start:



                i = 0, v = 6



                v(6) is not between 1 and 5: next iteration



                i = 1, v = 5, A[5-1] = 2



                5 is between 1 and 5; v is not equal to 2: swap ->
                v = 2; A[4] = 2



                i = 1, v = 2, A[2-1] = 5



                2 is between 1 and 5; v is not equal to 5: swap ->
                v = 5; A[1] = 5



                i = 1, v = 5, A[5-1] = 2



                5 is between 1 and 5; v is not equal to 2: swap ->
                v = 2; A[4] = 2



                i = 1, v = 2, A[2-1] = 5



                2 is between 1 and 5; v is not equal to 5: swap ->
                v = 5; A[1] = 5



                ... and on and on



                I don't think your algorithm makes sense. It's unclear why you are using the values in your list to index the list during your loop. I think this confusion about the index and values is at the root of your problem.






                share|improve this answer


























                  0












                  0








                  0







                  A = [6, 5, 4, 3, 2]

                  for i, v in enumerate(A):
                  while 1<=v<=len(A) and v != A[v-1]:
                  v, A[v-1] = A[v-1], v


                  You need to try running out your algorithm in your head:



                  Start:



                  i = 0, v = 6



                  v(6) is not between 1 and 5: next iteration



                  i = 1, v = 5, A[5-1] = 2



                  5 is between 1 and 5; v is not equal to 2: swap ->
                  v = 2; A[4] = 2



                  i = 1, v = 2, A[2-1] = 5



                  2 is between 1 and 5; v is not equal to 5: swap ->
                  v = 5; A[1] = 5



                  i = 1, v = 5, A[5-1] = 2



                  5 is between 1 and 5; v is not equal to 2: swap ->
                  v = 2; A[4] = 2



                  i = 1, v = 2, A[2-1] = 5



                  2 is between 1 and 5; v is not equal to 5: swap ->
                  v = 5; A[1] = 5



                  ... and on and on



                  I don't think your algorithm makes sense. It's unclear why you are using the values in your list to index the list during your loop. I think this confusion about the index and values is at the root of your problem.






                  share|improve this answer













                  A = [6, 5, 4, 3, 2]

                  for i, v in enumerate(A):
                  while 1<=v<=len(A) and v != A[v-1]:
                  v, A[v-1] = A[v-1], v


                  You need to try running out your algorithm in your head:



                  Start:



                  i = 0, v = 6



                  v(6) is not between 1 and 5: next iteration



                  i = 1, v = 5, A[5-1] = 2



                  5 is between 1 and 5; v is not equal to 2: swap ->
                  v = 2; A[4] = 2



                  i = 1, v = 2, A[2-1] = 5



                  2 is between 1 and 5; v is not equal to 5: swap ->
                  v = 5; A[1] = 5



                  i = 1, v = 5, A[5-1] = 2



                  5 is between 1 and 5; v is not equal to 2: swap ->
                  v = 2; A[4] = 2



                  i = 1, v = 2, A[2-1] = 5



                  2 is between 1 and 5; v is not equal to 5: swap ->
                  v = 5; A[1] = 5



                  ... and on and on



                  I don't think your algorithm makes sense. It's unclear why you are using the values in your list to index the list during your loop. I think this confusion about the index and values is at the root of your problem.







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered Nov 15 '18 at 18:07









                  vaer-kvaer-k

                  3,94852335




                  3,94852335






























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