Keith Devens .com - Weblog: For loop idioms

Friday, July 2, 2004

For loop idioms

Everyone knows the standard for loop idiom:

for($n=0; $n<@arr; $n++) #...

It turns out there are a few more idioms that people use. First, usually you want to cache the upper bound to avoid having to calculate it every time. That's not as important for a language like Perl or PHP where scalar(@arr) or count($arr) or strlen($str) is a quick operation, but it's very important for something like strlen(str) in C:

for(int n=0,int c=strlen(str); n<c; n++) //...

Another one is the following, which works nicely if you want to count backwards:

for($n=@arr-1; $n--;) #...

Permalink | 2 Comments | Tags: Programming | id 4987

10:57 (utc-4)
edited: 11:05 (utc-4)

Comments

Aggelos wrote:

It is important for PHP too. Say, give it a 1000000 loops without pre-calculation and with pre-calculation. And compare the results. Of course there is no difference if you have 10 or 100 loops. But for big numbers, the difference is quite noticeable.

∴ Aggelos | 2-Jul-2004 11:42am est | #4898

Jon Bentley has a nice column on profileers, with the running example of calculating prime numbers. Amusingly when I translated the code to Ada the problem with recalculating the sqrt each iteration disappeared...

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