Automatically get loop index in foreach loop in Perl

Question

If I have the following array in Perl    x   qw a b c     and I iterate over it with foreach  then    will refer to the current element in the array   foreach   x        print      will print   abc   Is there a similar way to get the index of the current element  without manually updating a counter  Something such as   foreach   x        print  index      where  index is updated like    to yield the output   012

User · Answer

Yes  I have checked so many books and other blogs    The conclusion is  there isn t any system variable for the loop counter  We have to make our own counter  Correct me if I m wrong

User · Answer

Please consider    print  Element at index    is  x     n  for keys  x

User · Answer

autobox  Core provides  among many more things  a handy for method  use autobox  Core     a    z  - gt for  sub      my   index   value            say  quot  index   gt   value quot        Alternatively  have a look at an iterator module  for example  Array  Iterator use Array  Iterator   my  iter   Array  Iterator- gt new    a    z      while   iter- gt hasNext         iter- gt getNext      say  iter- gt currentIndex       gt       iter- gt current     Also see   each to their own  autobox  perl5i

User · Answer

Well  there is this way   use List  Rubyish    list   List  Rubyish- gt new    qw lt a b c gt        list- gt each index  sub   say                 See List  Rubyish

User · Answer

It can be done with a while loop  foreach doesn t support this    my  arr    1111  2222  3333    while  my   index   element    each  arr          You may need to  use feature  say       say  Index   index  Element   element       Output   Index  0  Element  1111 Index  1  Element  2222 Index  2  Element  3333   Perl version  5 14 4

User · Answer

Not with foreach   If you definitely need the element cardinality in the array  use a  for  iterator   for   i 0   i lt  x     i      print  Element at index  i is      x  i      n

User · Answer

perldoc perlvar does not seem to suggest any such variable

User · Answer

Oh yes  you can   sort of  but you shouldn t   each  array  in a scalar context gives you the current index of the array    a    a  z   for   a       print each  a      t           n        Here each  a  is in a scalar context and returns only the index  not the value at that index  Since we re in a for loop  we have the value in    already  The same mechanism is often used in a while-each loop  Same problem   The problem comes if you do for  a  again  The index isn t back to 0 like you d expect  it s undef followed by 0 1 2    one count off  The perldoc of each   says to avoid this issue  Use a for loop to track the index     each  Basically   for my  i 0   i lt    a   i        print  The Element at  i is  a  i  n       I m a fan of the alternate method   my  index 0  for   a      print  The Element at  index is  a  index  n      index

User · Answer

You shouldn t need to know the index in most circumstances  You can do this   my  arr    1  2  3   foreach   arr                print join        arr     In this case  the output would be 2  3  4 as foreach sets an alias to the actual element  not just a copy

User · Answer

I have tried like       array   qw  tomato banana papaya potato                 Example array my  count                                                Local variable initial value will be 0  print   nBefore For loop value of counter is  count      Just printing value before entering the loop   for   array    print   n   count                         String and variable seperated by comma to                                                          execute the value and print  undef  count                                             Undefining so that later parts again it will                                                          be reset to 0   print   nAfter for loop value of counter is  count       Checking the counter value after for loop    In short      array   qw  a b c d   my  count  for   array    print   n   count            undef  count

User · Answer

In Perl prior to 5 10  you can say     usr bin perl  use strict  use warnings   my  a   qw a b c d e    my  index  for my  elem   a        print  At index     index       I saw  elem n       or  for my  index  0      a        print  At index  index I saw  a  elem  n       In Perl 5 10  you use state to declare a variable that never gets reinitialized  unlike ones created with my   This lets you keep the  index variable in a smaller scope  but it can lead to bugs  if you enter the loop a second time it will still have the last value       usr bin perl  use 5 010  use strict  use warnings   my  a   qw a b c d e    for my  elem   a        state  index      say  At index     index       I saw  elem       In Perl 5 12 you can say     usr bin perl  use 5 012    This enables strict use warnings   my  a   qw a b c d e    while  my   index   elem    each  a        say  At index  index I saw  elem       But be warned  you there are restrictions to what you are allowed to do with  a while iterating over it with each   It won t help you now  but in Perl 6 you will be able to say     usr bin perl6  my  a    lt a b c d e gt   for  a Z 0    Inf - gt   elem   index       say  at index  index  I saw  elem      The Z operator zips the two lists together  i e  it takes one element from the first list  then one element from the second  then one element from the first  and so on    The second list is a lazy list that contains every integer from 0 to infinity  at least theoretically    The - gt   elem   index says that we are taking two values at a time from the result of the zip   The rest should look normal to you  unless you are not familiar with the say function from 5 10 yet

User · Answer

No  you must make your own counter  Yet another example   my  index  foreach   x        print  index        when used for indexing  my  index  foreach   x        print  x  index   y  index        index        And of course you can use local  index  instead my  index  and so and so

User · Answer

Like codehead said  you d have to iterate over the array indices instead of its elements  I prefer this variant over the C-style for loop   for my  i  0      x        print   i   x  i  n

[perl] Automatically get loop index in foreach loop in Perl

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