Perl, A hash of arrays: adding and removing keys, adding to an array, all in a while loop

addarrayshashperlwhile-loop

I have a hash which should contain certain keys which are linked to their own arrays. To be more specific, the hash keys are quality values and the arrays are sequence names. If there already is an array for that quality, I'd want to add the sequence name to the array that is linked to the quality in question. If there isn't one, I want to create one and add the sequence name to it. All this is done in a while loop, going through all the sequences one by one.

I've tried to do things like in Perl How do I retrieve an array from a hash of arrays? but I can't seem to get it right.

I just get these error messages:
Scalar value @{hash{$q} better written as ${hash{$q} at asdasd.pl line 69.
Global symbol "@q" requires explicit package name asdasd.pl line 58.
And some others, too.

Here is an example of what I've tried:

my %hash;
while (reading the sequences) {
    my $q = "the value the sequence has";
    my $seq = "the name of the sequence";

    if (exists $hash{$q}) {
        push (@{$hash{$q}}, $seq);
    } else {
        $hash{$q} = \@q;
        $hash{$q} = [$seq];
        next;
    }
}

This obviously shouldn't be a very complicated problem but I'm new to perl and this kind of a problem feels difficult. I've googled this from various places but there seems to be something I just don't realize, and it might be really obvious, too.

Best Answer

You can use what perl calls autovivification to make this quite easy. Your code doesn't need that central if-statement. You can boil it down to:

    push @{ $hash{$q} }, $seq;

If the particular key doesn't yet exist in the hash, perl will autoviv it, since it can infer that you wanted an array reference here.

You can find further resources on autovivification by Googling it. It's a unique enough word that the vast majority of the hits seem relevant. :-)

Related Solutions

Perl – What’s the safest way to iterate through the keys of a Perl hash

The rule of thumb is to use the function most suited to your needs.

If you just want the keys and do not plan to ever read any of the values, use keys():

foreach my $key (keys %hash) { ... }

If you just want the values, use values():

foreach my $val (values %hash) { ... }

If you need the keys and the values, use each():

keys %hash; # reset the internal iterator so a prior each() doesn't affect the loop
while(my($k, $v) = each %hash) { ... }

If you plan to change the keys of the hash in any way except for deleting the current key during the iteration, then you must not use each(). For example, this code to create a new set of uppercase keys with doubled values works fine using keys():

%h = (a => 1, b => 2);

foreach my $k (keys %h)
{
  $h{uc $k} = $h{$k} * 2;
}

producing the expected resulting hash:

(a => 1, A => 2, b => 2, B => 4)

But using each() to do the same thing:

%h = (a => 1, b => 2);

keys %h;
while(my($k, $v) = each %h)
{
  $h{uc $k} = $h{$k} * 2; # BAD IDEA!
}

produces incorrect results in hard-to-predict ways. For example:

(a => 1, A => 2, b => 2, B => 8)

This, however, is safe:

keys %h;
while(my($k, $v) = each %h)
{
  if(...)
  {
    delete $h{$k}; # This is safe
  }
}

All of this is described in the perl documentation:

% perldoc -f keys
% perldoc -f each

Javascript – What’s the difference between “Array()” and “[]” while declaring a JavaScript array

There is a difference, but there is no difference in that example.

Using the more verbose method: new Array() does have one extra option in the parameters: if you pass a number to the constructor, you will get an array of that length:

x = new Array(5);
alert(x.length); // 5

To illustrate the different ways to create an array:

var a = [],            // these are the same
    b = new Array(),   // a and b are arrays with length 0

    c = ['foo', 'bar'],           // these are the same
    d = new Array('foo', 'bar'),  // c and d are arrays with 2 strings

    // these are different:
    e = [3]             // e.length == 1, e[0] == 3
    f = new Array(3),   // f.length == 3, f[0] == undefined

;

Another difference is that when using new Array() you're able to set the size of the array, which affects the stack size. This can be useful if you're getting stack overflows (Performance of Array.push vs Array.unshift) which is what happens when the size of the array exceeds the size of the stack, and it has to be re-created. So there can actually, depending on the use case, be a performance increase when using new Array() because you can prevent the overflow from happening.

As pointed out in this answer, new Array(5) will not actually add five undefined items to the array. It simply adds space for five items. Be aware that using Array this way makes it difficult to rely on array.length for calculations.

Best Answer

Related Solutions

Perl – What’s the safest way to iterate through the keys of a Perl hash

Javascript – What’s the difference between “Array()” and “[]” while declaring a JavaScript array

Related Topic