grep and map differ in the return values. The former will always returns a subset of the original list (the entire list is a subset as well). An element of the original list will be returned if the expression (or block) returned a true value when that element was processed. The return value of map is a list consisting of the actual return values of the expression or block.

Abigail

I swear on life, I have read every perldoc and Monastery Example on map (and sometimes grep). Pages and pages of intructions, and just have since avoided using it.
But now I get it!
One little paragraph, thank you so much! Maybe you should re-write all the perldocs ;-)

SMiTZ

my @foo_bar;

foreach my $foo (@fooz)
{
     if ($foo > 12)
     {
          push(@foo_bar, $foo - 1);
     }
}
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my @foo_bar = map { $_ - 1 } grep { $_ > 12 } @fooz;

There are many, many uses. Here are a few examples:

From the perldoc on readdir():

opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
closedir DIR;
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Here's one I use often: I might have an array of values, that, at some point, I need to do a series of checks to see if a value is in the array. It's inefficient to loop through the array multiple times. Likewise, we mostly need this data as an array, not a hash, this is just the exception. So we use map() to copy it to a hash:

  my @array = qw(One Two Five Nine);
  my %check_hash = map {$_ => 1} @array;
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Let's say you're doing a database update, but you have a variable number of fields. In fact, you have the fields as @fields. Let's also assume you have a CGI object in $q that has all the needed fields there (and taint-checked)

  @fields = qw(Name ID Age);
  #assuming $dbh and $q exist, and primary key in $key
  my $statement = 'update my_table '.join(",", map {"set $_ = ?"} @fie
+lds)." where Key=?";
  my $sth = $dbh->prepare($statement);
  $sth->execute(map({$q->param($_)} @fields), $key);
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What if we didn't have $key defined in the above? I mean, what if $key was the name of the field that was the primary key, and the value was taken from the passed parameters? A slight modification that would use grep():

  @fields = qw(Name ID Age Key);
  #assuming $dbh and $q exist, and primary key name in $key
  #we don't want to change the value of the key!
  my $statement = 'update my_table '.join(",", map {"set $_ = ?"} grep
+(!/^$key$/, @fields))." where $key=?";
  my $sth = $dbh->prepare($statement);
  $sth->execute(map({$q->param($_)} @fields));
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$sth->execute(map({$q->param($_} grep(!/^$key$/, @fields)), $q->param(
+$key));
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Hope that helps, there are many more examples out there. In general, use grep when you need part of an array as an array, and use map when you need an array based off another list. If you need to actually execute code based on the contents of a list, you're better off using a loop.