An Objective-C BinarySearch category on Array of which members are String. http://www.getappninja.com/blog/implementing-a-binary-search-in-ios

NSArray+BinarySearch.h

@interface NSArray (BinarySearch)

// Before calling this method, make sure this array is an array of string and it is sorted using sortedArrayUsingSelector:
// NSArray *sortedArray = [array sortedArrayUsingSelector:@selector(caseInsensitiveCompare:)];
- (unsigned long)indexOfStringUsingBinarySearch:(NSString *)searchString;
@end

NSArray+BinarySearch.m

#import "NSArray+BinarySearch.h"

@implementation NSArray (BinarySearch)

- (unsigned long)indexOfStringUsingBinarySearch:(NSString *)searchString {
    NSString *member;
    unsigned long mid;
    unsigned long min = 0;
    unsigned long max = [self count] - 1;

    while (min <= max) {
        mid = (min + max)/ 2;
        member = [self objectAtIndex:mid];

        if ([searchString caseInsensitiveCompare:member] == NSOrderedSame) {
            return mid;
        } else if ([searchString caseInsensitiveCompare:member] == NSOrderedAscending) {
            max = mid - 1;
        } else {
            min = mid + 1;
        }           
    }

    return NSNotFound;
}

@end