switch, transliteration ( tr/// ) of $

whakka has asked for the wisdom of the Perl Monks concerning the following question:

Why doesn't this work?

switch ( $foo ) {
    case ( tr/I/I/ == 1 ) { $bar = 'low' }
    case ( tr/I/I/ == 2 ) { $bar = 'med' }
    case ( tr/I/I/ == 3 ) { $bar = 'high' }
    else { warn "Bad $_\n" }
}
[download]

Yields:

Use of uninitialized value $_ in transliteration (tr///)
[download]

(Update: Because switch doesn't set $_, it filters the source code - in this case "insanely".)

Here's a test script:

#!perl! -w
use strict;
use Switch;

my $foo = 'II';
print $foo =~ tr/I/I/, "\n";
my $bar;
switch ( $foo ) {
    case ( tr/I/I/ == 1 ) { $bar = 'low' }
    case ( tr/I/I/ == 2 ) { $bar = 'med' }
    case ( tr/I/I/ == 3 ) { $bar = 'high' }
    else { $bar = "drat" }
}

print "Foo: $foo Bar: $bar\n";

# This works:
switch ( $foo ) {
    case ( /I{3}/ ) { $bar = 'high' }
    case ( /I{2}/ ) { $bar = 'med' }
    case ( /I/ )    { $bar = 'low' }
    else { $bar = "drat" }
}

print "Foo: $foo Bar: $bar\n";
[download]

Thanks!

Update: Thanks kyle and jeffa, the lesson is learned - don't use Switch. (jeffa's solution indeed doesn't work - which surprised me)

Update2: moritz has it right, use the (5.010) given-when construct:

given ( $foo ) {
    when ( tr/I/I/ == 1 ) { $bar = 'low' }
    when ( tr/I/I/ == 2 ) { $bar = 'med' }
    when ( tr/I/I/ == 3 ) { $bar = 'high' }
    default { $bar = "drat" }
}
[download]

Comment on switch, transliteration ( tr/// ) of $_ issue Select or Download Code

Replies are listed 'Best First'.
Re: switch, transliteration ( tr/// ) of $_ issue by kyle (Abbot) on Nov 18, 2008 at 16:06 UTC
In Categorized Damian Modules, the author of Switch recommends against using it. It's a source filter, and it sometimes is confused by the code it's filtering. If you step through your script in the debugger, it might tell you what's going on.	[reply]
Re^2: switch, transliteration ( tr/// ) of $_ issue by moritz (Cardinal) on Nov 18, 2008 at 16:25 UTC
One should mention that newer versions of Perl do have a sane alternative, that's `use 5.010; given ($foo) { when ($condition) { code } ... }` [download] (described in perlsyn)	[reply] [d/l]
Re: switch, transliteration ( tr/// ) of $_ issue by trwww (Priest) on Nov 18, 2008 at 17:40 UTC
Hello, I don't have a reply that directly relates to your code... but my advice is to just use a data structure. Why write procedural code when you don't have to? my $foo = 'II'; my %table = ( III => 'high', II => 'med', I => 'low', default => 'drat', ); my $bar = $table{$foo} or $table{default}; print "Foo: $foo Bar: $bar\n"; Hope this helps, trwww	[reply]
Re: switch, transliteration ( tr/// ) of $_ issue by jeffa (Bishop) on Nov 18, 2008 at 16:07 UTC
Aren't you "switching" on $foo ... but checking the value of $_? Shouldn't it be this? (untested) `switch ( $foo ) { case ( ($foo ~= tr/I/I/) == 1 ) { $bar = 'low' } case ( ($foo ~= tr/I/I/) == 2 ) { $bar = 'med' } case ( ($foo ~= tr/I/I/) == 3 ) { $bar = 'high' } else { $bar = "drat" } }` [download] UPDATE: nope. That appears to be right. Yuck. I'm with kyle on this one -- don't use Switch. Use an if-else block or a dispatch table instead. jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply] [d/l] [select]
Re^2: switch, transliteration ( tr/// ) of $_ issue by Bloodnok (Vicar) on Nov 18, 2008 at 16:31 UTC
... or the poor mans' equivalent of a switch... `SWITCH: { $foo =~ tr/I/I/ == 1 && do { $bar = 'low'; last SWITCH; }; $foo =~ tr/I/I/ == 2 && do { $bar = 'med'; last SWITCH; }; $foo =~ tr/I/I/ == 3 && do { $bar = 'high'; last SWITCH; }; $bar = "drat"; }` [download] construct. Update Corrected typos... `~=` should, of course, be `=~` Extraneous `)` A user level that continues to overstate my experience :-))	[reply] [d/l] [select]
Re: switch, transliteration ( tr/// ) of $_ issue by Roy Johnson (Monsignor) on Nov 18, 2008 at 21:53 UTC
Or you could opt for the old-school approach, and just replace your switch with for, and your whens with if-elsifs: `for ( $foo ) { if ( tr/I/I/ == 1 ) { $bar = 'low' } elsif ( tr/I/I/ == 2 ) { $bar = 'med' } elsif ( tr/I/I/ == 3 ) { $bar = 'high' } else { $bar = "drat" } }` [download] Caution: Contents may have been coded under pressure.	[reply] [d/l]
Re: switch, transliteration ( tr/// ) of $_ issue by runrig (Abbot) on Nov 18, 2008 at 22:15 UTC
`my $bar = (0,'low','med','high')[$foo=~tr/I//] \|\| 'drat'; # or my $bar; $bar = (0,'low','med','high')[tr/I//] \|\| 'drat' for $foo;` [download]	[reply] [d/l]
Re^2: switch, transliteration ( tr/// ) of $_ issue by blazar (Canon) on Nov 19, 2008 at 11:40 UTC
No real advantage, but for completeness: `my ($bar) = map +(0,'low','med','high')[tr/I//] \|\| 'drat', $foo; # or my ($bar) = map qw(0 low med high)[tr/I//] \|\| 'drat', $foo;` [download] qw helps, and "0" is false... `--` ~~If you can't understand the incipit, then please check the IPB Campaign.~~	[reply] [d/l] [select]


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switch, transliteration ( tr/// ) of $_ issue