Question

I have NSString *string = @"Helo"; and NSString *editedString = @"Hello";. How find index for changed character or characters (for example here is @"l").

Answer 1

I’ve written a category on NSString that will do what you want. I’ve used my StackOverflow username as a postfix on the category method. This is to stop an unlikely potential future collision with a method of the same name. Feel free to change it.

First the interface definition NSString+Difference.h:

#import <Foundation/Foundation.h>

@interface NSString (Difference)

- (NSInteger)indexOfFirstDifferenceWithString_mttrb:(NSString *)string;

@end

and the implementation ‘NSString+Difference.m`:

#import "NSString+Difference.h"

@implementation NSString (Difference)

- (NSInteger)indexOfFirstDifferenceWithString_mttrb:(NSString *)string; {

    // Quickly check the strings aren't identical
    if ([self isEqualToString:string]) 
        return -1;

    // If we access the characterAtIndex off the end of a string
    // we'll generate an NSRangeException so we only want to iterate
    // over the length of the shortest string
    NSUInteger length = MIN([self length], [string length]);

    // Iterate over the characters, starting with the first
    // and return the index of the first occurence that is 
    // different
    for(NSUInteger idx = 0; idx < length; idx++) {
        if ([self characterAtIndex:idx] != [string characterAtIndex:idx]) {
            return idx;
        }
    }

    // We've got here so the beginning of the longer string matches
    // the short string but the longer string will differ at the next
    // character.  We already know the strings aren't identical as we
    // tested for equality above.  Therefore, the difference is at the
    // length of the shorter string.

    return length;        
}

@end

You would use the above as follows:

NSString *stringOne = @"Helo";
NSString *stringTwo = @"Hello";

NSLog(@"%ld", [stringOne indexOfFirstDifferenceWithString_mttrb:stringTwo]);