mutiple {} in perl script

Question

I have this syntax in a perl script and I don't know what it means: $string.= "$$info{$id}{free}\t";.

I am very new to pearl (first time reading a perl script) and I didn't find any useful information for this.

Answer 1

$info is a hash reference. This refers to a multi level data structure, with the first level key set by another variable $id. Your structure might look something like this:

my $info = {
  1 => { 
    free => 2,
    used => 3,
  },
  2 => {
    free => 1,
    used => 0,
  },
};

The $id in this example would be 1 or 2.

You can read about this in more detail in perlreftut.

Your code takes the value from inside the data structure and appends it to a variable $string followed by a tab character. It looks like it's building a tab separated file, similar to a CSV file.

Answer 2

As per Perl Dereferencing Syntax, $$info{ $id } can also be written as $info->{ $id }. (I'm not alone in finding the latter much clearer.)

$info->{ $id }{ free }

is short for

$info->{ $id }->{ "free" }

This is just just following two hash element dereferences chained into a single expression:

my $anon = $info->{ $id };  $anon->{ "free" }

HASHREF->{ KEY } is used to get the value of a hash element given a reference to a hash and the key of the element.

This means that $info is expected to be a reference to a hash. $info->{ $id } gets the value of the element with the value of $id for key.

Similarly, $info->{ $id }/$anon is expected to be a reference to a hash. $anon->{ "free" } gets the value of the element with key free.

For example,

my $info = {
   machine01 => { 
      free => 100,
      used => 200,
   },
   machine02 => {
      free =>  50,
      used => 450,
   },
};

my $id = "machine01";

say $info->{ $id }{ free };  # 100

See perlreftut