optimising perl hash appends

[columb]

I have a subroutine which returns a reference to a hash. Trimming out the excess the code looks like

my %fullhash;
foreach my $val ( @arr )
  {
  my $hashref = dosubroutine ( $val );
  %fullhash = ( %fullhash, %$hashref );
  }

However, I'm concerned that each time the line

%fullhash = ( %fullhash, %$hashref );

is run perl will need to effectively recreate %fullhash, which I expect to grow quite large. As an inexperienced perl coder this looks inefficient.

As an alterntive I'm looking at

my %fullhash;
foreach my $val ( @arr )
  {
  my $hashref = dosubroutine ( $val );
  foreach ( keys %$hashref )
    { $fullhash{$_} = $hashref->{$_}; }
  }

As my test data is much smaller than the live data at the moment I can't do any direct timings. I would welcome any comments on which way to go.

Thanks

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Columb Healy

 

[KevinADC]

I believe the second method is more efficient. But if there is a large amount of data the array (@arr) could consume a lot of system memory it self. You should maybe do something like:

my %fullhash;
foreach my $val ( @arr )
  {
  my $hashref = dosubroutine ( $val );
  foreach ( keys %$hashref )
    { $fullhash{$_} = $hashref->{$_}; }
  }
[b]@arr = ();[/b]

to free up the memory.

- Kevin, perl coder unexceptional!