IF Statement Testing

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

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Re: IF Statement Testing by Zaxo (Archbishop) on May 14, 2005 at 06:02 UTC
How about this? `die "Don't tamper with me" if $expire >= 1 and $expire <= 14;` If you need it to be an integer you can throw in `$expire == int $expire` After Compline, Zaxo	[reply] [d/l] [select]
Re: IF Statement Testing by davido (Cardinal) on May 14, 2005 at 05:53 UTC
Several problems. First, <=> is a three-state operator. It has different return values for less than comparisons, greater than, and equal to. It's primarily used for numeric comparisons in sort routines. You should be using != The next problem is that you cannot chain 1\|\|2\|\|3\|\|4\|\|5 like that. That's like saying "If $expire is not equal to 1, or if two, or if three, or if four, etc." In other words, the comparison only binds to $expire != 1. To write that correctly, the way you're trying to test, it would be: `if( $expire != 1 or $expire != 2 or $expire != 3...... )` [download] ...and so on. But how about this instead? `if( $expire != int $expire or $expire < 1 or $expire > 14 ) { die "Don't tamper with me!\n"; }` [download] You could also do it like this, using a hash slice: `my %test; @test{ 1 .. 14 } = (); unless( exists $test{$expire} ) { die "Don't tamper with me!\n"; }` [download] In all my examples, I've assumed that you want to reject non-integers. Dave	[reply] [d/l] [select]
Re^2: IF Statement Testing by holli (Abbot) on May 14, 2005 at 10:38 UTC
You flipped the logic here. `if( $expire != 1 or $expire != 2 or $expire != 3...... )` [download] should be `if( $expire != 1 and $expire != 2 and $expire != 3...... )` [download] OTOH, this can also be solved by a hash lookup holli, /regexed monk/	[reply] [d/l] [select]
Re: IF Statement Testing by !1 (Hermit) on May 14, 2005 at 06:02 UTC
`if ($expire < 1 \|\| 14 < $expire \|\| int $expire != $expire) { die "Don't tamper with me!"; }` or `my %check; undef @check{1..14}; if (! exists $check{$expire}) { die "Don't tamper with me!"; }` [download] or `if ($expire !~ /^(?:[1-9]\|1[0-4])$/) { die "Don't tamper with me!"; }` [download] or `use Quantum::Superpositions; if ($expire != any(1..14)) { die "Don't tamper with me!"; }` [download] and many other ways.	[reply] [d/l] [select]
Re: IF Statement Testing by gaal (Parson) on May 14, 2005 at 06:13 UTC
In Perl6 (works with current Pugs): `if $expire ~~ none(1 .. 14) { ... }`	[reply] [d/l]
Re^2: IF Statement Testing by rg0now (Chaplain) on May 14, 2005 at 09:27 UTC
But what I find more interesting is that the OP totally intuitively discovered junctions, and almost perfectly found the right syntax as well. Look at this variant: `if ($expire !~ any(1\|2\|3\|4\|5\|6\|7\|8\|9\|10\|11\|12\|13\|14)){...}` [download] Update: thanks gaal for lighting me up, although, I can not entirely grasp, what is the problem above...:-). Intuitively, `$x ~~ none(...)` should give the same result as `$x !~ any(...)`, or at least, my intuition suggests this... Anyway, here is an updated syntax that even more resembles to the OP's idea, and it actually works right in Pugs: `if (!($expire ~~ 1\|2\|3\|4\|5\|6\|7\|8\|9\|10\|11\|12\|13\|14)) {...}` [download] Interestingly, Pugs requires parens around the `if` condition. Is this a bug? rg0now	[reply] [d/l] [select]
Re^3: IF Statement Testing by gaal (Parson) on May 14, 2005 at 09:32 UTC
That unfortunately doesn't work, because there always exists an element in the junction so that $expire !~ it. The condition is always true.	[reply]
Re^4: IF Statement Testing by TimToady (Parson) on May 14, 2005 at 16:49 UTC
Re: IF Statement Testing by gopalr (Priest) on May 14, 2005 at 05:44 UTC
Change your code as follows `if ($expire !~m#^(1\|2\|3\|4\|5\|6\|7\|8\|9\|10\|11\|12\|13\|14)$#) { die "Don't tamper with me!"; }` [download] Update:: Remove doubel "\|" to single	[reply] [d/l]
Re^2: IF Statement Testing by ikegami (Patriarch) on May 14, 2005 at 05:49 UTC
Except it should be single "\|"s in your regexp, not "\|\|"s But why use a regexp at all? `if ($expire < 1 \|\| $expire > 14) { die "Don't tamper with me!"; }` [download]	[reply] [d/l]
Re: IF Statement Testing by prasadbabu (Prior) on May 14, 2005 at 05:42 UTC
<=> is wrong. you can use regular expression !~ `$expire !~m/^(1\|2\|3\|4\|5\|6\|7\|8\|9\|10\|11\|12\|13\|14)$/` updated Prasad	[reply] [d/l]
Re: IF Statement Testing by northwind (Hermit) on May 14, 2005 at 10:41 UTC
If you're looking to exclude specific values (not necessarily contiguous), then here is another way to solve your problem: `if (scalar grep { $expire ne $_ } @values) { die "Don't tamper with m +e!"; }` [download] Where `@values` is filled with the values you want to check for. Also, if you are doing only numbers you can change the `ne` test to a `!=` test. Update: Added italics to "another" to emphasize TMTOWTDI. For further insight into TMTOWTDI (and why algorithms, unlike men, are not all created equal), might I suggest reading the chain of replies resulting from this post.	[reply] [d/l]
Re^2: IF Statement Testing by davido (Cardinal) on May 14, 2005 at 15:41 UTC
Yes, you can use grep, but that means doing a linear search that only exits after stepping through all elements, even if the first element found satisfies the criteria. If this conditional is to be tested repeatedly, doing a hashtable lookup is more elegant and more efficient...and depending on who you ask, easier on the eyes. Dave	[reply]
Re^3: IF Statement Testing by northwind (Hermit) on May 14, 2005 at 18:37 UTC
Yes, I agree using a hash is the best implementation. Dark, wierd, and frought with perils would be the situation where `grep` would be the best choice. TMTOWTDI doesn't mean all algorithms are equal... :)	[reply]