map and grep again...

[Marc Girod]

Hello,

I know this has been explained earlier... but why are map and grep
behaving differently here?

#!/usr/bin/perl -w

use strict;
use feature qw(say);

my @glb = qw(lbtype:FFF);
say $_ for map { s/^lbtype.*)(\@.*)?$/$1/ } @glb;
@glb = qw(lbtype:FFF);
say $_ for grep { s/^lbtype.*)(\@.*)?$/$1/ } @glb;

Running this gives:

$ /tmp/foo
1
FFF

Thanks,
Marc

 

[sreservoir]

Hello,

I know this has been explained earlier... but why are map and grep
behaving differently here?

#!/usr/bin/perl -w

use strict;
use feature qw(say);

my @glb = qw(lbtype:FFF);
say $_ for map { s/^lbtype.*)(\@.*)?$/$1/ } @glb;

map collect return values.

@glb = qw(lbtype:FFF);
say $_ for grep { s/^lbtype.*)(\@.*)?$/$1/ } @glb;

grep collects when return value is true.

Running this gives:

$ /tmp/foo
1
FFF

for (@glb) { s///; say }

of course. s/// return 1 or !1, depending whether something was matched
or replaced.

please don't use map and grep purely for side-effects unless you know
what you're doing.

and you don't have to specify the implicit argument.

 

[Marc Girod]

map collect return values.
grep collects when return value is true.

Thanks.

Marc

 

[sln]

Hello,

I know this has been explained earlier... but why are map and grep
behaving differently here?

#!/usr/bin/perl -w

use strict;
use feature qw(say);

my @glb = qw(lbtype:FFF);
say $_ for map { s/^lbtype.*)(\@.*)?$/$1/ } @glb;
@glb = qw(lbtype:FFF);
say $_ for grep { s/^lbtype.*)(\@.*)?$/$1/ } @glb;

Running this gives:

$ /tmp/foo
1
FFF

Thanks,
Marc

map {} .. is going to give you the result of the operation.
The line with map is creating a list with the result of the
regular expression.
Grep is is creating a list of sucessfull matches.

You confuse the issue when you when you stick map or grep
inside the for expression then put the default $_ inside its
body. Obviously each is say(ing) the itterative list values
AFTER the list is created.
So,
say $_ for map { s/^lbtype.*)(\@.*)?$/$1/ } @glb;
^^ is NOT { $_ =~ s/^lbtype.*)(\@.*)?$/$1/ }
^^

Nothing wrong with clarity.

print "----\n";
@glb = qw(lbtype:FFF);
@list = map { s/^lbtype.*)(\@.*)?$/$1/ } @glb;
for (@list) {
say $_;
}
@glb = qw(lbtype:FFF);
@list = map { s/^lbtype.*)(\@.*)?$/$1/; $_ } @glb;
for (@list) {
say $_;
}
print "----\n";
@glb = qw(lbtype:FFF);
@list = grep { s/^lbtype.*)(\@.*)?$/$1/ } @glb;
for (@list) {
say $_;
}

-sln