in reply to Using an array element as a loop iterator

I suppose you didn't write what you thought you were writing. First, square brackets are not used to make a list, like in python, but an arrayref, which is only one element. So @a contains only one element which is [1,2,3]. @b on the other is indeed an array.

Could you write what you were intending to in python? If I translate your code it would be something like: 

for each element in @b
   $a[1] = element
   print $a[1]
done

[download]

If you want to use the indexes in @a to go through @b you can either use a slice (EDIT: not splice ...) of @b : 

@a = (1,2,3);
@b = (2,4,6,7);
print for @b[@a]; # print for elements of @b with indexes in @a

[download]
Or loop through @a : 
@a = (1,2,3);
@b = (2,4,6,7);
for $i (@a)
{
   print $b[$i];
}

[download]

NB: As for the error, I guess in for VAR (LIST) VAR can only be a variable, not any lvalue.

Replies are listed 'Best First'.
Re^2: Using an array element as a loop iterator
by gurpreetsingh13 (Scribe) on Nov 08, 2013 at 03:56 UTC
    Could you write what you were intending to in python? If I translate your code it would be something like: 
    a = [1,2,3]
b = [10,11,12,13]
for a[1] in b:
    print a[1]

    [download]
[reply]
[d/l]

      That prints all four elements of b 

      10
11
12
13

      [download]
      and then a contains : [1, 13, 3]. So it does not correspond to your C-style for loop where you only print the first three elements, and the second value in the declaration of a is just thrown away.

      To achieve the same result in Perl you can do: 

      @a = 1..3;
@b = qw/2 4 7 9/;
for (@b) {$a[1] = $_; say; }

      [download]

[reply]
[d/l]
[select]

        Thanks. That is nice 

        for (@b){$a[1]=$_;say:}

        [download]

        But what I actually wanted to achieve was that I had several nested foreach loops and taking several variables, one for each iterator, is nearly horrible. I wanted to initialize one array and then use that as a container for all the iterators of nested arrays.

        This step $a[1]=$_or something similar will do that, but still that statement needs to be put as first step in loops of all 20 nested arrays. Isn't there any way through which we can avoid that and achieve the result via the foreach step only like something -- 

        my @a=();
foreach $a[0](`cat file`){
 foreach $a[1](@b){
  foreach $a[2](grep (/4/, @c)){
   <More statements>
}}}

        [download]

        And not using something like-- 

        my @a=();
foreach (`cat file`){
 $a[0]=$_;
 foreach $a[1](@b){
  $a[1]$_;
  foreach $a[2](grep (/4/, @c)){
   $a[2]=$_
   <More statements>
}}}

        [download]
        </code>
[reply]
[d/l]
[select]

In Section Seekers of Perl Wisdom