map and grep again...

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From: Marc Girod on 16 Jul 2010 10:50

---

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

From: sreservoir on 16 Jul 2010 11:05

---

On 7/16/2010 10:50 AM, Marc Girod wrote:
> 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.

--

"Six by nine. Forty two."
"That's it. That's all there is."
"I always thought something was fundamentally wrong with the universe."

From: Marc Girod on 16 Jul 2010 11:16

---

On Jul 16, 4:05 pm, sreservoir <sreserv...(a)gmail.com> wrote:

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

Thanks.

Marc

From: sln on 16 Jul 2010 13:32

---

On Fri, 16 Jul 2010 07:50:10 -0700 (PDT), Marc Girod <marc.girod(a)gmail.com> wrote:

>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

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