iOS Fastest method to extract a substring - ios

Question

I have several thousand strings in the form "verb-noun" for which I want to extract the noun portion. I am looking for the FASTEST way to do this. The verb and noun portions can be any length.
I have tried
NSString *noun = [[verb componentsSeparatedByString:#"-"] lastObject];
Which is slow... For my list of over 3000 entries it takes about 3 seconds.
Also tried
NSString *noun = [verb substringFromIndex:[verb rangeOfString:#"-"].location + 1];
which is MUCH faster, about a half second.
Anyone have suggestions for making this even faster?

Answer 1

If your work with these strings is thread-safe then one option is to use GCD to iterate over multiple verb values simultaneously, bringing multiple cores into action. Use dispatch_apply instead of whatever loop you're using, something like:
dispatch_apply([myWordArray count], queue, ^(size_t i) {
NSString *verb = [myWordArray objectAtIndex:i];
NSString *noun = [verb substringFromIndex:[verb rangeOfString:#"-"].location + 1];
// do something with noun...
});
Just keep in mind that this will do more than one pass simultaneously, so be very sure about threading issues.

Answer 2

Fastest way would probably to sort the most likely cases for where the hyphen is, and then check for those first without using a loop. For example, if the most likely cases for index of hyphen are 5, 4, 6, 7, 3, 2 in that order, you could do this:
NSString * verb = #"verb-noun";
NSString * noun = nil;
//use do...while(0) to avoid nested if else
do
{
if([verb characterAtIndex:5] == '-')
{
noun = [verb substringFromIndex:6];
break;
}
if([verb characterAtIndex:4] == '-')
{
noun = [verb substringFromIndex:5];
break;
}
if([verb characterAtIndex:6] == '-')
{
noun = [verb substringFromIndex:7];
break;
}
if([verb characterAtIndex:7] == '-')
{
noun = [verb substringFromIndex:8];
break;
}
if([verb characterAtIndex:3] == '-')
{
noun = [verb substringFromIndex:4];
break;
}
if([verb characterAtIndex:2] == '-')
{
noun = [verb substringFromIndex:4];
break;
}
} while(0);
//if not one of most likely cases, loop
if(!noun)
{
for(int j = 8; j < verb.length; j++)
{
if([verb characterAtIndex:j] == '-')
{
noun = [verb substringFromIndex:j + 1];
break;
}
}
}
if(noun)
{
//noun should never be nil
NSLog(#"found the noun");
}