Using string as array index in Perl

user_2950065 0
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I have run into a strange behavior in Perl that I haven't been able to find documentation for. If I (by accident) use a string as an index in an array I get the first item of the array and not undef as I would have expected.

$index = "Some string";
@array = qw(one two three);
$item = $array[$index];
print "item: " . $item;

I expected to get item: as output, but instead I get item: one. I assume that because the string doesn't start with a number it's "translated" to 0 and hence giving me the first item in the array. If the string starts with a number that part of the string seems to be used as the index.

Is this to be expected, and is there any documentation describing how strings (e.g. "2strings") are interpreted as numbers in Perl?

perl
2 Answers

Array index imposes numeric context. The string "Some string" in numeric context is equal to 0.

Under warnings, Perl will complain

Argument "Some string" isn't numeric in array or hash lookup at ...
user_10306750

Array indexes must be integers, so non-integer values are converted to integers.

The string one produces the number 0, as well as the following warning:

Argument "Some string" isn't numeric in array or hash lookup

This concept is found throughout Perl. For the same reason, arguments to addition and multiplication will similarly be converted to numbers. And values used as hash keys will be converted to strings. Dereferencing undef scalars even produces the necessary value and reference in a process called autovivification!

$ perl -Mv5.10 -e'
   my $ref;
   say $ref // "[undef]";

   $ref->[1] = 123;
   say $ref // "[undef]";
'
[undef]
ARRAY(0x560b653ae4b8)

As you can see, an array and a reference to that array were spontaneously created in the above program because they were needed.

The lesson to take: Always use use strict; use warnings;.

user_5899240
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