Fastest way to manipulate strings in objective-c

Question

I'm relatively new to Objective C and I'm trying to figure out how to efficiently replace a character in a string in the case that I know the index where that character would be.

Basically if S is my string I would like to be able to do this s[i] = 'n' for some i

But this looks quite expensive to me:

NSRange range = NSMakeRange(0,1);
NSString *newString = [S stringByReplacingCharactersInRange:range withString:@"n"];

Is it not ??

Answer 1

Yes, for replacing one single character, copying the entire string is indeed an overkill. It's not like it would highly affect performance unless this is the bottleneck in your app, called a million times a second, but it still feels better to either use NSMutableString and replaceCharactersInRange:NSMakeRange(0, 1) withString:@"n", or simply fall back to using C strings if speed is essential.

Answer 2

H2CO3 is right. Use mutable arrays. You could also create a Category to support indexed access to your string:

NSMutableString+Index.h

@interface NSMutableString (Index)
- (void)setObject:(id)anObject atIndexedSubscript:(NSUInteger)idx;
@end

NSMutableString+Index.h

@implementation NSMutableString (Index)
- (void)setObject:(id)anObject atIndexedSubscript:(NSUInteger)idx {
    [self replaceCharactersInRange:NSMakeRange(idx, 1) withString:anObject];
}
@end

Somewhere else:

NSMutableString *str = [[NSMutableString alloc] initWithString:@"abcdef"];
str[2] = @"X";
NSLog(str);

Output:

abXdef

Note: Don't forget to import your Category, where you use the indexed syntax.