Substring out of bounds exception with NSMakeRange

Question

When looping through, making a substring of every letter in a NSString, this exception is thrown:

-[__NSCFString substringWithRange:]: Range or index out of bounds

Code:

- (void) analyzeContent:(NSString *)content
{
    content = [content uppercaseString];
    for (int i = 1; i < content.length; i++) {
        NSString *ch = [content substringWithRange:NSMakeRange(i-1, i)]; // <-- exception
    }
}

Called from here:

- (IBAction)analyze:(id)sender
{
    if (!self.text) {
        NSString *path = [[NSBundle mainBundle] pathForResource:CYPHERTEXT_FILE
                                                         ofType:@"txt"];
        self.text = [NSString stringWithContentsOfFile:path
                                              encoding:NSUTF8StringEncoding
                                                 error:NULL];
    }

    [self.progIndicator setHidden:NO];
    [self.progIndicator startAnimation:nil];
    dispatch_queue_t queue = dispatch_queue_create("queue", 0);
    dispatch_async(queue, ^{

        [self.analyzer analyzeContent:self.text];

        // When finished update UI
        dispatch_sync(dispatch_get_main_queue(), ^{
            [self.progIndicator setHidden:YES];
            [self.progIndicator stopAnimation:nil];
        });
    });
}

text-property declaration is: @property (copy) NSString *text;

The exception is thrown when i = 13172 and content.length = 26344.

The code is run in a background thread, but content is not accessed by any other thread.

Thank you for any help!

Answer 1

The problem was:

I though the NSMakeRange took the parameters index-from and index-to. It actually takes index-from and length of substring, so when the iteration passed the half way mark (see question), it goes out of bounds.

Silly...