the if statement

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

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Re: the if statement by toolic (Bishop) on Sep 27, 2008 at 22:12 UTC
First, check for the existence of the hash key, then print the value of the hash key. Your "if" statement was performing an assignment and was probably always returning "true". Try this: `%hash = (one => 'uno', two => 'dos', three => 'tres'); $input = <STDIN>; chomp $input; if (exists $hash{$input}) { print $hash{$input}, "\n"; }` [download]	[reply] [d/l]
Re: the if statement by FunkyMonk (Chancellor) on Sep 27, 2008 at 22:06 UTC
You mean like this? `my %hash = (one => 'uno', two => 'dos', three => 'tres'); my $input = <STDIN>; chomp $input; if (my $ans = $hash{$input}) { print "$input => $ans\n"; }` [download] Note that I've declared all the variables with my because I have strict (and warnings) enabled. Unless I state otherwise, all my code runs with strict and warnings	[reply] [d/l]
Re: the if statement by broomduster (Priest) on Sep 27, 2008 at 22:11 UTC
This looks like homework, so here are some hints. `(keys %hash)` is a list of the keys for `%hash`. The scalar assignment `$input = (keys %hash)` puts the number of keys into `$input` (which is probably not what you want to do). ~~OTOH, to compare `$input` to each of the keys, you need to loop through the keys, comparing `$input` to each key. Since the keys (and the user input) are strings, you use `eq` to do the comparison.~~ Updated: Scrub the stupidity; no need to check everything; this is a hash, after all.	[reply] [d/l] [select]
Re^2: the if statement by eyepopslikeamosquito (Archbishop) on Sep 28, 2008 at 02:27 UTC
you need to loop through the keys, comparing $input to each key Doing linear scans over an associative array is like trying to club someone to death with a loaded Uzi. :-) -- Larry Wall in Authoritative list of Larry Wall quotes See also Clubbing someone to death with a loaded Uzi blog by Dominus.	[reply]
Re^2: the if statement by FunkyMonk (Chancellor) on Sep 27, 2008 at 22:50 UTC
you need to loop through the keys, comparing $input to each key That's what exists is for. In this case `if ($hash{$input}) { ... }` is probably good enough.	[reply] [d/l]
Re: the if statement by Wiggins (Hermit) on Sep 28, 2008 at 14:53 UTC
The comments from FunkyMonk and toolic bring up a (possible) subtle difference in the "pattern",that I have always puzzled about. --FunkyMonk 1 hash access, + possibly 1 scalar assignment `if (my $ans = $hash{$input}) { print "$input => $ans\n";` [download] --toolic 1 hash access + possibly 1 more hash access is the 1st result 'cached' and the second access reuses the 1st result? `if (exists $hash{$input}) { print $hash{$input}, "\n";` [download] ??? Is there any efficiency advantage in either approach? Does the hash size make a difference? How about when done 5000 times for different $input values?	[reply] [d/l] [select]
Re^2: the if statement by chrism01 (Friar) on Sep 29, 2008 at 08:17 UTC
In this particular case, the desired value is always 'true', but Funkymonk's soln would fail if it was 'false' eg 0 (zero) or undef. toolic's soln tells you there's a hash entry there, even if the associated value is 'false' or undef. :)	[reply]
Re^2: the if statement by blazar (Canon) on Sep 29, 2008 at 13:46 UTC
Is there any efficiency advantage in either approach? Does the hash size make a difference? How about when done 5000 times for different $input values? I personally believe that there's no significative advantage of one approach over the other but possibly in terms of personal tastes. As far as efficiency is concerned, what do you mean? Speed of execution? If so, then I wouldn't mind, since it's such a tiny difference, but you may answer your question(s) yourself with Benchmark.pm! I notoriously suck at benchmarks, doing continuous errors, (which are generally pointed out by others...) but here's my try anyway: Read more... (1199 Bytes) As I expected, as it is it doesn't show any noticeable difference. `kirk:~ [15:16:04]$ ./bm.pl Rate double assign double 29.4/s -- -1% assign 29.7/s 1% --` [download] Indeed I generally refrain from the temptation of doing benchmarks "like this" when someone suggests them, and even tend to slightly bash those who do: this time I was curious to see if at least a tiny systematic difference would have arisen, but that doesn't seem to be the case: feel free to modify it the way you like most though! `--` If you can't understand the incipit, then please check the IPB Campaign.	[reply] [d/l] [select]
Re^3: the if statement by Wiggins (Hermit) on Sep 29, 2008 at 18:37 UTC
I love your answer. Below I have put my result of running your benchmark in a Fedora9 VM on a Lenovo laptop. But what really intrigues me is how you built the test hash and keys to test with... 2 'map's in 2 lines of code. Figuring out 'genkeys' and the %hash, and @test values will take me the rest of the afternoon; thanks `### using 5.010 [~]# time perl hash_test_benchmark.pl Rate assign double assign 119/s -- -2% double 122/s 2% -- real 2m30.601s user 2m17.789s sys 0m6.317s ### without 5.010 [~]# time perl hash_test_benchmark.pl Rate double assign double 121/s -- -0% assign 122/s 0% -- real 2m32.598s user 2m19.745s sys 0m6.498s [~]#` [download]	[reply] [d/l]
Re^4: the if statement by blazar (Canon) on Sep 30, 2008 at 10:47 UTC
Re: the if statement by kwn (Novice) on Sep 27, 2008 at 22:34 UTC
Thanks guys, this helped me a lot.	[reply]


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