Objective-C: Randomly replace characters in string

Question

I'd like to have a function that removes a random set of characters from a string and replaces them with '_'. eg. to create a fill in the blanks type of situation. The way I have it now works, but its not smart. Also I don't want to replace spaces with blanks (as you can see in the while loop). Any suggestions on a more efficient way to do this?

blankItem = @"Remove Some Characters";
for(int j=0;j<totalRemove;j++)
{
    replaceLocation=arc4random() % blankItem.length;
    while ([blankItem characterAtIndex:replaceLocation] == '_' || [blankItem characterAtIndex:replaceLocation] == ' ') {
         replaceLocation=arc4random() % blankItem.length;
    }
   blankItem= [blankItem stringByReplacingCharactersInRange:NSMakeRange(replaceLocation, 1) withString:@"_"];
}

My issue is with the for and while loops in terms of efficiency. But, maybe efficiency isn't of the essence in something this small?

`blankItem= (NSMutableString *)[blankItem …` seems to show you believe casting magically changes the object's class. Fortunately you don't really use the fact that `blankItem` is a mutable string since you are allocating a new string at each iteration. — Julien, Aug 31 '13 at 20:44
`replaceLocation=arc4random() % blankItem.length;` -> `replaceLocation = arc4random_uniform(blankItem.length);` — , Aug 31 '13 at 20:46
Warning, a string of only _ and spaces will result in an infinite loop. — Pétur Ingi Egilsson, Aug 31 '13 at 21:13

score 1 · Accepted Answer · edited May 23 '17 at 10:25

If the number of characters to remove/replace is small compared to the length of the string, then your solution is good, because the probability of a "collision" in the while-loop is small. You can improve the method by using a single mutable string instead of allocating a new string in each step:

NSString *string = @"Remove Some Characters";
int totalRemove = 5;

NSMutableString *result = [string mutableCopy];
for (int j=0; j < totalRemove; j++) {
    int replaceLocation;
    do {
        replaceLocation = arc4random_uniform((int)[result length]);
    } while ([result characterAtIndex:replaceLocation] == '_' || [result characterAtIndex:replaceLocation] == ' ');
    [result replaceCharactersInRange:NSMakeRange(replaceLocation, 1) withString:@"_"];
}

If the number of characters to remove/replace is about the same magnitude as the length of the string, then a different algorithm might be better.

The following code uses the ideas from Unique random numbers in an integer array in the C programming language to replace characters at random positions with a single loop over all characters of the string.

An additional (first) pass is necessary because of your requirement that space characters are not replaced.

NSString *string = @"Remove Some Characters";
int totalRemove = 5;

// First pass: Determine number of non-space characters:
__block int count = 0;
[string enumerateSubstringsInRange:NSMakeRange(0, [string length])
                              options:NSStringEnumerationByComposedCharacterSequences
                           usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
    if (![substring isEqualToString:@" "]) {
        count++;
    }
}];

// Second pass: Replace characters at random positions:
__block int c = count; // Number of remaining non-space characters
__block int r = totalRemove; // Number of remaining characters to replace
NSMutableString *result = [string mutableCopy];
[result enumerateSubstringsInRange:NSMakeRange(0, [result length])
                              options:NSStringEnumerationByComposedCharacterSequences
                           usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
   if (![substring isEqualToString:@" "]) {
       // Replace this character with probability r/c:
       if (arc4random_uniform(c) < r) {
           [result replaceCharactersInRange:substringRange withString:@"_"];
           r--;
           if (r == 0) *stop = YES; // Stop enumeration, nothing more to do.
       }
       c--;
   }
}];

Another advantage of this solution is that it handles surrogate pairs (e.g. Emojis) and composed character sequences correctly, even if these are stores as two separate characters in the string.

Objective-C: Randomly replace characters in string

1 Answers1