Subtracting 2 strings

Question

I have a string say for example @"012" and I have another string @"02". How can I extract the difference of the 2 strings in iPhone Objective-C. I just need to remove the existence of the characters in the 2nd string from the first string.The answer would be "1".

Answer 1

You're trying to do a set operation, so use sets.

{
    NSString* s1 = @"012";
    NSString* s2 = @"02";

    NSMutableSet* set1 = [NSMutableSet set];
    NSMutableSet* set2 = [NSMutableSet set];

    for(NSUInteger i = 0; i < [s1 length]; ++i)
    {
        [set1 addObject:[s1 substringWithRange:NSMakeRange(i, 1)]];
    }

    for(NSUInteger i = 0; i < [s2 length]; ++i)
    {
        [set2 addObject:[s2 substringWithRange:NSMakeRange(i, 1)]];
    }


    [set1 minusSet:set2];

    NSLog(@"set1: %@", set1);

    // To get a single NSString back from the set:
    NSMutableString* result = [NSMutableString string];
    for(NSString* piece in set1)
    {
        [result appendString:piece];
    }

    NSLog(@"result: %@", result);
}

Answer 2

You could do something like this;

NSString *s1 = @"012";
NSString *s2 = @"02";

NSCharacterSet *charactersToRemove;
charactersToRemove = [NSCharacterSet characterSetWithCharactersInString:s2];

NSMutableString *result = [NSMutableString stringWithCapacity:[s1 length]];
for (NSUInteger i = 0; i < [s1 length]; i++) {
    unichar c = [s1 characterAtIndex:i];
    if (![charactersToRemove characterIsMember:c]) {
        [result appendFormat:@"%C", c];
    }
}

// if memory is an issue:
result = [[result copy] autorelease];

Disclaimer: I typed this into the browser, and haven't tested any of this.

Answer 3

Even shorter:

NSString* s1 = @"012";
NSString* s2 = @"02";
NSCharacterSet * set = [NSCharacterSet characterSetWithCharactersInString:s2];
NSString * final = [[s1 componentsSeparatedByCharactersInSet:set] componentsJoinedByString:@""];
NSLog(@"Final: %@", final);

This preserves the order of the characters in the original string.