This is an Objective-C function that will parse a float from a string where the fractional part is expressed as numerator/denominator. So, for example, it will parse "1 1/2", or "3/4".

numberFromFractionalString

/**
 * Turn a string like "1 1/2" into 1.5.  If there is no fractional
 * part, then the whole number is returned.  If the number is not
 * well formatted, then nil is returned.
 */
+ (NSNumber *)numberFromFractionalString:(NSString *)string
{
    if (string == nil) {
        return nil;
    }
    
    NSArray *parts = [string componentsSeparatedByCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
    if (parts.count == 0) {
        return nil;
    }

    NSString *wholePart = [parts objectAtIndex:0];
    NSString *fractionalPart;
    if ([wholePart rangeOfString:@"/"].location != NSNotFound) {
        fractionalPart = wholePart;
        wholePart = nil;
    } else {
        if (parts.count == 2) {
            fractionalPart = [parts objectAtIndex:1];
        }
    }
    
    float value = 0.0;

    NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
    
    if (wholePart != nil) {
        NSNumber *number = [formatter numberFromString:wholePart];
        if (number == nil) {
            return nil;
        }
        
        value = [wholePart floatValue];
    }
    
    if (fractionalPart != nil) {
        // Fractional value: "1 1/2" or "2 3/4"
        NSArray *fractionParts = [fractionalPart componentsSeparatedByString:@"/"];
        if (fractionParts.count == 2) {
            NSNumber *numerator = [formatter numberFromString:[fractionParts objectAtIndex:0]];
            NSNumber *denominator = [formatter numberFromString:[fractionParts objectAtIndex:1]];
            if (numerator == nil || denominator == nil || denominator.integerValue == 0) {
                return nil;
            }
            value = value + (numerator.floatValue / denominator.floatValue);
        }
    }

    return [NSNumber numberWithFloat:value];
}