hash{X}=array[X] results in two hash keys

[lucastas]

I coded the following myscript.pl …

#!/usr/bin/perl
$|=1;

my %arguments;

for( my $arg = 0 ; $arg <= $#ARGV ; $arg++ ) {
    print $ARGV[$arg]."\n";
    $arguments{ $arg } = $ARGV[$arg];
    foreach my $arg ( %arguments ) {
        print "key = >$arg< value = >$arguments{$arg}<\n";
    }
}

If I run the script like so…

myscript.pl arg1 arg2 agr3

I get the following output.

arg1
key = >0< value = >arg1<
key = >arg1< value = ><
arg2
key = >0< value = >arg1<
key = >arg1< value = ><
key = >1< value = >arg2<
key = >arg2< value = ><
arg3
key = >0< value = >arg1<
key = >arg1< value = ><
key = >1< value = >arg2<
key = >arg2< value = ><
key = >2< value = >arg3<
key = >arg3< value = ><

If I include use strict, I get the error "Use of uninitialized value in concatenation (.) or string at ./myscript.pl line 13." preceeding the print of the element that has the value as the key and no value.

I went a googling and did a search here at the Monistary but was unable to find an answer to the mystery. Can you help me to understand why the hash has a key/value pair of the real key and value, then a key/value pair with the real value as the key and no value to go with it?

[choroba]

The assignment is OK. The problem lies in the way how you iterate over the hash:

foreach my $arg ( %arguments ) {

You want to only iterate over the keys:

foreach my $arg ( keys %arguments ) {

Without keys , a hash behaves like a flattened list of key-value pairs, i.e.

key1, value1, key2, value2, ...

With warnings on, you'd have been warned about the uninitialized values for some "keys" (i.e. the real values):

Use of uninitialized value in concatenation (.) or string at ...

[kennethk]

If I include use strict, I get the error "Use of uninitialized value in concatenation (.) or string at ./myscript.pl line 13." preceeding the print of the element that has the value as the key and no value.

<pedantic>That comes from warnings not strict. Errors from strict are fatal, and the output you got above didn't kill the script</pedantic>