Question

I’m trying to develop a simple application where i can encrypt a message. The algorithm is Caesar’s algorithm and for example, for ‘Hello World’ it prints ‘KHOOR ZRUOG’ if the increment is 3 (standard).

My problem is how to take each single character and increment it…

I’ve tried this:

NSString *text = @"hello";
int q, increment = 3;
NSString *string;

for (q = 0; q < [text length]; q++) {

    string = [text substringWithRange:NSMakeRange(q, 1)];

    const char *c = [string UTF8String] + increment;
    NSLog(@"%@", [NSString stringWithUTF8String:c]);
}

very simple but it doesn’t work.. My theory was: take each single character, transform into c string and increment it, then return to NSString and print it, but xcode print nothing, also if i print the char ‘c’ i can’t see the result in console. Where is the problem?

Answer 1

First of all, incrementing byte by byte only works for ASCII strings. If you use UTF-8, you will get garbage for glyphs that have multi-byte representations.

With that in mind, this should work (and work faster than characterAtIndex: and similar methods):

NSString *foo = @"FOOBAR";
int increment = 3;

NSUInteger bufferSize = [foo length] + 1;
char *buffer = (char *)calloc(bufferSize, sizeof(char));

if ([foo getCString:buffer maxLength:bufferSize encoding:NSASCIIStringEncoding]) {
    int bufferLen = strlen(buffer);
    for (int i = 0; i < bufferLen; i++) {
        buffer[i] += increment;
        if (buffer[i] > 'Z') {
            buffer[i] -= 26;
        }
    }

    NSString *encoded = [NSString stringWithCString:buffer 
                                           encoding:NSASCIIStringEncoding];
}
free(buffer);