Subroutines within if statements

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

Replies are listed 'Best First'.
Re: Subroutines within if statements by kyle (Abbot) on Feb 21, 2008 at 22:16 UTC
I think you mean: `if ($answer eq "add" \|\| $answer eq "a") { &addlinks; } elsif ($answer eq "remove" \|\| $answer eq "r") { &rmlinks; } else { exit 0; }` [download] You're probably better off calling those as `addlinks()` and `rmlinks()` until you understand how the `&sub` calls are special (see perlsub). Also, I recommend that you `use strict; use warnings;` until you know why you might not want strict and warnings.	[reply] [d/l] [select]
Re^2: Subroutines within if statements by lodin (Hermit) on Feb 21, 2008 at 23:48 UTC
Nowadays you can use `use 5.010; if ("$answer" ~~ [ 'add', 'a' ]) { ... }` [download] I explicitly stringify `$answer` because if `$answer` happens to be something else (like an object) the `~~` operator will act differently and I don't want that. On the contrary, I want to allow objects that may stringify to "add" or "a". lodin	[reply] [d/l] [select]
Re^2: Subroutines within if statements by wfsp (Abbot) on Feb 22, 2008 at 08:45 UTC
...until you know why you might not want strict and warnings. Why might I not want strict and warnings? I get twitchy seeing `no warnings` in a small block. When I see either commented out at the top of a script I hide behind the sofa. :-) What is it I don't know yet? Is there a secret handshake? :-)	[reply] [d/l]
Re^3: Subroutines within if statements by kyle (Abbot) on Feb 22, 2008 at 13:55 UTC
Why might I not want strict and warnings? I haven't written any significant code without them since I learned of them. (I write `perl -e` without them.) I don't have a case where I wouldn't want them. That said, I could imagine code that produces spurious warnings. I could further imagine some PHB who wants no error messages in a log somewhere. I've heard some say they turn off strict and warnings in production to avoid some overhead. I personally think this is a lousy way to gain performance. Like you, I don't like to see code without strict and warnings. I don't want to work on it. Nevertheless, other programmers have their own minds, and I want to respect that.	[reply] [d/l]
Re^2: Subroutines within if statements by muizelaar (Sexton) on Feb 22, 2008 at 11:03 UTC
That's great as soon as I saw your code I knew where I went wrong. :( Bit of along day :)	[reply]
Re: Subroutines within if statements by runrig (Abbot) on Feb 21, 2008 at 22:10 UTC
"a" is always true, so your code always calls addlinks().	[reply]
Re: Subroutines within if statements by Fletch (Bishop) on Feb 21, 2008 at 22:22 UTC
And the B::Deparse module can be handy when trying to figure out why when something's not behaving the way you expect. I show running over code given in a shell here-doc, but you can do the same thing to code in a file via `perl -MO=Deparse,-p my_file.plx`. `$ perl -MO=Deparse,-p <<'EOT' if ($answer eq "add" \|\| "a") { &addlinks; } elsif ($answer eq "remove" \|\| "r") { &rmlinks; } else {exit 0; } EOT if ((($answer eq 'add') or 'a')) { &addlinks; } elsif ((($answer eq 'remove') or 'r')) { &rmlinks; } else { exit(0); } - syntax OK` [download] In the explicitly parenthesized version you can see exactly what your comparisons really mean to Perl. The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l] [select]
Re: Subroutines within if statements by Joost (Canon) on Feb 21, 2008 at 22:17 UTC
In addition to the above (correct) answer, don't use the `&subname;` construct unless you know why I'm typing this. Use `subname;` or (and in general preferably) `subname();` to call subs without passing arguments. See perlsub. update: I see kyle beat me to it. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l] [select]
Re: Subroutines within if statements by johngg (Canon) on Feb 21, 2008 at 22:22 UTC
Another way to do it would be to use a regular expression match. `use strict; use warnings; print "Would you like to add or remove symbolic links?\n"; chomp( my $answer = <STDIN> ); if( $answer =~ m{^a(?:dd)?$} ) { addlinks(); } elsif( $answer =~ m{^r(?:emove)?$} ) { rmlinks() } else { exit( 0 ); }` [download] Cheers, JohnGG	[reply] [d/l]
Re^2: Subroutines within if statements by runrig (Abbot) on Feb 21, 2008 at 22:37 UTC
A simple and more flexible (for the user) way would be to reverse the args: `if ( "add" =~ /^\Q$answer/ ) { ... }` [download] Update: you can even precompile the regex for multiple uses: `my $ans = qr/^\Q$answer/; if ( "add" =~ /$ans/ ) { ... } elsif ( "remove" =~ /$ans/ ) { ... }` [download]	[reply] [d/l] [select]
Re^2: Subroutines within if statements by ikegami (Patriarch) on Feb 21, 2008 at 22:48 UTC
`chomp` isn't needed if you use "`$`" in your regexp. Conversely, "`\z`" is sufficient if `chomp` is used.	[reply] [d/l] [select]
Re: somewhat OT: Subroutines within if statements by ww (Archbishop) on Feb 21, 2008 at 22:40 UTC
OT, but, your `exit 0;` may leave users -- those who enter "rm" for example -- believing your script failed. Better to tell them what went wrong, now that above responders have provided direct answers to your OP. It may be well to insert something like line 2 (below): `else { print "Invalid response: use ONLY one of the following add, a, rem +ove, r\n"; exit 0; }` [download] Test your opens, regex captures and the like before using them; TELL your users (or at least, give them a hint) how they screwed up.	[reply] [d/l] [select]
Re^2: somewhat OT: Subroutines within if statements by Joost (Canon) on Feb 21, 2008 at 22:50 UTC
In fact you probably shouldn't use `exit 0` for an error at all. Better to use `die $message` instead. Giving a non-zero exit code (as die() does) on error makes combining scripts in shell scripts / one-liners a lot easier: `my-shell$ check-this-config-file.pl && then-stop-this-server && then-d +o-a-lot-of-stuff.pl && then-start-that-server` [download] And yes I do this a lot. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l] [select]
Re: Subroutines within if statements by toolic (Bishop) on Feb 21, 2008 at 22:19 UTC
Elaborating on runrig's answer, you probably meant to use: `if ($answer eq "add" \|\| $answer eq "a") {` [download] Same goes for your `elsif`.	[reply] [d/l] [select]