Re: Perl beginner's issue with hash

The reason of the weird result is the setting of this inconspicuous variable:

$/ = undef;

It enables the "slurp mode", i.e. <FILE> reads the whole file into $_. But you then check

if (/$i/g)

which only runs once.

You can fix it in several ways:

1. keep the $/ unchanged. It will read the file line by line and update the variable appropriately.
2. replace the if by while, it will run the increment for each match
3. create a regex from all the ids and match all of them at once:

   my $regex = join '|', map quotemeta, @ids;
   while (<FILE>) {
       $counts{$1}++ if /($regex)/;
   }
   

[Maire]

Thanks so much for this! Just as a quick follow-up, do you have any thoughts why I might be receiving an error when I change the 'if' to a 'while' statement? All I've changed is the 'if' for a 'while', but then the following appears when I try to run:

syntax error at step_two_redux.pl line 19, near "else"
syntax error step_two_redux.pl line 28, near "}"
Execution of step_two_redux.pl aborted due to compilation errors.

[AnomalousMonk]

All I've changed is the 'if' for a 'while' ...

If all you've done is change
    if (CONDITION) { ... } else { ... }
to
    while (CONDITION) { ... } else { ... }
then yes, this is a syntax error because there is no  while ... else ... statement block structure in Perl. (The only  if statement | statement block I can see in the OPed code is  if (/$id/g) { ... } — is this the one you're referring to?)

I'm not sure what you were trying to accomplish with this change, so I'm not sure what to advise as the proper change.