Re^3: Hash assignments using map

njcodewarrior,
You're right in that the map function acts like a loop. Where is gets tricky is when you assign the output of the map function to a hash, because the hash assignment operation evaluates two elements at a time. So Perl does the equivalent of this:

my $i=0;
while ($i<$#group) {
    $hash{$group[$i]}=$group[$i+1];
    $i+=2;
    }
[download]

As other monkers already pointed out the "odd number of elements" warning will be generated when you feed the hash with a group that has an odd number of elements.

Now if we look at the map function itself in your code

@destination = map {$_++} @source
[download]

The effect of the above code is that the elements are copied one by one. At the same time the element in the source (!) group is changed/incremented. That´s probably not what you wanted, right?

If the destination of the map function is a hash then commonly the map is used to produce 2 elements at a time in a list fashion:

.... = map { ("key$_" , "value$_") } .....
[download]

For clarity purpose the comma operator is typically replaced by a '=>' operator to indicate that we mean to produce something for a hash.

.... = map { ("key$_" => "value$_") } .....
[download]

I'm not sure what you hoped to achieve through your code but hopefully this shows why your code behaved as it did

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Re^4: Hash assignments using map by njcodewarrior (Pilgrim) on Feb 24, 2007 at 20:14 UTC
Thanks varian. This makes sense. I want to quickly create a hash with keys that are the elements of the list. All I want access to is the keys...I don't care what the values are. I can then use `exists` to see if those keys either do or do not exist in another hash. Thanks again!	[reply] [d/l]
Re^5: Hash assignments using map by shmem (Chancellor) on Feb 24, 2007 at 20:32 UTC
All I want access to is the keys... ok, but a hash has a key and a value. So your code should be: `my @keys = qw{ a b c d }; my %hash = map { $_,1 } @keys;` [download] if you want your keys unaltered. If you want a "string increment" (`$_="a";$_++; # $_ is b now`) its ok to use `$_++`. In the loop `my %hash; foreach ( @keys ) { $hash{$_}++; }` [download] you are also using two variables - in a shortcut: you create a new hash slot with key `$_` you increment (the previously undefined) value for that key So, it's the same thing with keys and values here. With map iterating over an array of keys, you must provide a value to have pairs to be stored as (key,value) in a hash. --shmem _($_=" "x(1<<5)."?\n".q·/)Oo. G°\ / /\_¯/(q / ---------------------------- \__(m.====·.(_("always off the crowd"))."· ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}	[reply] [d/l] [select]


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