Re: smart matching
On Thu, Oct 6, 2011 at 03:10, Shlomi Fish wrote:
> Furthermore, I think you can only put when inside given and not arbitrary
> code,
> and that "when()" operates on the datum that was given to given().
I don't think there's any restriction on what code can be inside the
'given' block -- at least, there's nothing in 'perlsyn' saying you
can't have other code in there.
You are correct that 'given()' sets up a (lexically scoped, I guess?)
$_ equal to the argument that it is given -- which is a copy, not an
alias -- and the smart matches in when() will then use that $_ -- but
if you have a 'when()' that doesn't check $_, then you're fine. See
sample code below.
Note: that is not say that any of this is *advisable* -- it's not,
unless you really really need to. (You don't really really need to, in
case you're wondering.) You're probably better off only having
'when()' blocks inside a 'given()' block, and you're probably better
off keeping the conditions in your 'when()' tests nice and simple.
You'll thank yourself down the road.
Sample code:
#! /usr/bin/env perl
use strict;
use warnings;
use feature 'switch';
$_ = 'outside';
my $foo = 'foo';
given( $foo ) {
when( 1 == 1 ) { print "INIT\n" ; continue }
when( 'bar' ) { print "BAR: $foo $_\n" }
$foo = 'bar';
when( 'foo' ) { print "FOO: $foo $_\n" }
$foo = 'baz';
when( 'outside' ) { print "OUT: $foo $_\n" }
default { print "DEF: $foo $_\n" }
}
print;
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Re: smart matching
Hi Chris,
I think you will need to work on your code to make it better or maybe this
is just a dirty codes for practice. If not, there are some stuff you may
have to weed out...some of which Shlomi Fish mentioned.
However, to make your code work as you suppose, please check this little
addition and you can take it up from there.
#!/usr/bin/perl
use warnings;
use strict;
use 5.010;
say "Checking the number <$ARGV[0]>";
my $favorite = 62;
given( $ARGV[0] ) {
when( ! /^\d+$/ ) { say "Not a number!" }
my @divisors = divisors( $ARGV[0] );
my $val=2; #added test value
when( @divisors ~~ /$val/) { # 2 is in @divisors ,/$val/ check it
say "$_ is even";
continue;
}
when( !( @divisors ~~/$val/ ) ) { # 2 isn't in @divisors /$val/ check
it
say "$_ is odd!";
continue;
}
when( @divisors ~~ /$favorite/ ) { # /$favorite/ check it
say "$_ is divisible by my favorite number";
continue;
}
when( /$favorite/ ) { # $_ ~~ $favorite
say "$_ is my favorite number";
continue;
}
my @empty;
when( @divisors ~~ @empty ) { say "Number is prime" }
default { say "$_ is divisible by @divisors" }
}
sub divisors {
my $number = shift;
my @divisors = ();
foreach my $divisor ( 2 .. ($ARGV[0]/2 + 1) ) {
push @divisors, $divisor unless $number % $divisor;
}
return @divisors;
}
Regards,
timothy
Re: smart matching
On Wed, 5 Oct 2011 23:44:21 -0500
Chris Stinemetz wrote:
> trying to learn smart matching in an exercise.
>
> Why does this program output "odd" when I input an even number?
>
A few comments on your code.
> Thank you,
>
> Chris
>
> #!/usr/bin/perl
>
> use warnings;
> use strict;
>
> use 5.010;
>
> say "Checking the number <$ARGV[0]>";
>
You're using $ARGV[0] several times (duplicate code) and it's a positional
argument:
http://perl-begin.org/tutorials/bad-elements/#subroutine-arguments
Better unpack it as:
my $number = shift(@ARGV);
> my $favorite = 62;
>
> given( $ARGV[0] ) {
> when( ! /^\d+$/ ) { say "Not a number!" }
>
> my @divisors = divisors( $ARGV[0] );
>
Returns arrays as references is preferable.
Furthermore, I think you can only put when inside given and not arbitrary code,
and that "when()" operates on the datum that was given to given().
> when( @divisors ~~ 2 ) { # 2 is in @divisors
> say "$_ is even";
> continue;
> }
>
> when( !( @divisors ~~ 2 ) ) { # 2 isn't in @divisors
> say "$_ is odd!";
> continue;
> }
>
> when( @divisors ~~ $favorite ) {
> say "$_ is divisible by my favorite number";
> continue;
> }
>
> when( $favorite ) { # $_ ~~ $favorite
> say "$_ is my favorite number";
> continue;
> }
>
> my @empty;
> when( @divisors ~~ @empty ) { say "Number is prime" }
>
> default { say "$_ is divisible by @divisors" }
> }
>
Your indentation is erratic.
> sub divisors {
> my $number = shift;
>
> my @divisors = ();
No need for the «= ()» here. An array is initialised as empty by default.
> foreach my $divisor ( 2 .. ($ARGV[0]/2 + 1) ) {
Do you want to operate on "$number" or on "$ARGV[0]"?
> push @divisors, $divisor unless $number % $divisor;
> }
>
I got a "WTF?" moment when I saw this unless construct. I'd write it as:
if ($number % $divisor == 0)
{
push @divisors, $divisor;
}
Regards,
Shlomi Fish
> return @divisors;
>
--
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Re: Smart matching
19.01.2011 0:43, Brian Fraser пишет: The smart match is no longer commutative - That was removed after 5.10.1, I think. http://www.learning-perl.com/?p=32 http://perldoc.perl.org/perlsyn.html#Smart-matching-in-detail Brian. Thank you! -- To unsubscribe, e-mail: beginners-unsubscr...@perl.org For additional commands, e-mail: beginners-h...@perl.org http://learn.perl.org/
Re: Smart matching
The smart match is no longer commutative - That was removed after 5.10.1, I think. http://www.learning-perl.com/?p=32 http://perldoc.perl.org/perlsyn.html#Smart-matching-in-detail Brian.
Re: Smart matching
19.01.2011 0:09, Uri Guttman пишет: but why don't you just call exists on the hash key? there is no win to using smart matching for that. it is included for consistancy but it isn't needed for hash keys. I just want to understand how does it work :) -- To unsubscribe, e-mail: beginners-unsubscr...@perl.org For additional commands, e-mail: beginners-h...@perl.org http://learn.perl.org/
Re: Smart matching
> "VDB" == Vladimir D Belousov writes: VDB> I'm trying to check whether the given key exists in the hash. smart matching is powerful and cool but why don't you just call exists on the hash key? there is no win to using smart matching for that. it is included for consistancy but it isn't needed for hash keys. uri -- Uri Guttman -- u...@stemsystems.com http://www.sysarch.com -- - Perl Code Review , Architecture, Development, Training, Support -- - Gourmet Hot Cocoa Mix http://bestfriendscocoa.com - -- To unsubscribe, e-mail: beginners-unsubscr...@perl.org For additional commands, e-mail: beginners-h...@perl.org http://learn.perl.org/
