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
转载于:https://stackoverflow.com/questions/974656/automatically-get-loop-index-in-foreach-loop-in-perl
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";
}
perldoc perlvar does not seem to suggest any such variable.
Not with foreach. If you definitely need the element cardinality in the array use a 'for' iterator.
for($i=0;$i<@x;++$i) {
print "Element at index $i is ",$x[$i],"\n";
}
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.
EDIT: Updated according to first ysth's comment.
Well there is this way:
use List::Rubyish;
$list = List::Rubyish->new( [ qw<a b c> ] );
$list->each_index( sub { say "\$_=$_" } );
see List::Rubyish
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 create with my). This lets you keep the $index variable in a smaller scope, but 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 = <a b c d e>;
for @a Z 0 .. Inf -> $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 -> $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).
You shouldn't need to know the index in most circumstances, you can do this
my @arr = (1, 2, 3);
foreach (@arr) {
$_++;
}
print join(", ", @arr);
In this case, the output would be 2, 3, 4 as foreach sets an alias to the actual element, not just a copy.
autobox::Core provides among many more things a handy for method:
use autobox::Core;
['a'..'z']->for( sub{
my ($index, $value) = @_;
say "$index => $value";
});
Alternatively have a look at an iterator module, for eg: Array::Iterator
use Array::Iterator;
my $iter = Array::Iterator->new( ['a'..'z'] );
while ($iter->hasNext) {
$iter->getNext;
say $iter->currentIndex . ' => ' . $iter->current;
}
Also see:
/I3az/
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 $count"; # just printing value before entering in loop
for (@array) { print "\n",$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 $count;
Yes. I have checked so many books and other blogs... conclusion is, there is no system variable for the loop counter. we have to make our own counter. correct me if i m wrong.
Perl version 5.14.4
Can be done with while loop (foreach doesn't support this)
my @arr = (1111, 2222, 3333);
while (my ($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
Please consider:
print "Element at index $_ is $x[$_]\n" for keys @x;