如何在目标c中迭代NSString？

Question

How can iterate through an NSString object in Objective c whiling maintaining an index for the character I am currently at?

I want to increment the ASCII value of every third character by 3, and then print this incremented character in a label in my user interface.

Answer 1

Wasn't clear whether you just wanted to print the incremented characters or all. If the former, here's is how you would do it:

NSString *myString = @"myString";
NSMutableString *newString = [NSMutableString string];
for (int i = 0; i < [myString length]; i++) 
{
    int ascii = [myString characterAtIndex:i];
    if (i % 3 == 0) 
    {
        ascii++;
        [newString appendFormat:@"%c",ascii];
    }
}
myLabel.text = newString;

Answer 2

Will this do the trick?

NSString *incrementString(NSString *input)
{
    const char *inputUTF8 = [input UTF8String]; // notice we get the buffers so that we don't have to deal with the overhead of making many message calls.
    char *outputUTF8 = calloc(input.length + 1, sizeof(*outputUTF8));

    for (int i = 0; i < input.length; i++)
    {
        outputUTF8[i] = i % 3 == 0 ? inputUTF8[i] + 3 : inputUTF8[i];
    }

    NSString *ret = [NSString stringWithUTF8String:outputUTF8];
    free(outputUTF8); // remember to free the buffer when done!
    return ret;
}