Efficient way to generate a random alphabet string? - ios

Question

I want a string of all the characters of the alphabet randomized. Right now, I create a mutable array of the 26 characters, shuffle them with the exchangeObjectAtIndex: method and then add each character to a string that I return.
There has to be a better way to do this. Here is my code:
- (NSString *)shuffledAlphabet {
NSMutableArray * shuffledAlphabet = [NSMutableArray arrayWithArray:#[#"A",#"B",#"C",#"D",#"E",#"F",#"G",#"H",#"I",#"J",#"K",#"L",#"M",#"N",#"O",#"P",#"Q",#"R",#"S",#"T",#"U",#"V",#"W",#"X",#"Y",#"Z"]];
for (NSUInteger i = 0; i < [shuffledAlphabet count]; ++i) {
// Select a random element between i and end of array to swap with.
int nElements = [shuffledAlphabet count] - i;
int n = (random() % nElements) + i;
[shuffledAlphabet exchangeObjectAtIndex:i withObjectAtIndex:n];
}
NSString *string = [[NSString alloc] init];
for (NSString *letter in shuffledAlphabet) {
string = [NSString stringWithFormat:#"%#%#",string,letter];
}
return string;
}

Answer 1

Here's an efficient Fisher-Yates shuffle, adapted to your use case:
- (NSString *)shuffledAlphabet {
NSString *alphabet = #"ABCDEFGHIJKLMNOPQRSTUVWXYZ";
// Get the characters into a C array for efficient shuffling
NSUInteger numberOfCharacters = [alphabet length];
unichar *characters = calloc(numberOfCharacters, sizeof(unichar));
[alphabet getCharacters:characters range:NSMakeRange(0, numberOfCharacters)];
// Perform a Fisher-Yates shuffle
for (NSUInteger i = 0; i < numberOfCharacters; ++i) {
NSUInteger j = (arc4random_uniform(numberOfCharacters - i) + i);
unichar c = characters[i];
characters[i] = characters[j];
characters[j] = c;
}
// Turn the result back into a string
NSString *result = [NSString stringWithCharacters:characters length:numberOfCharacters];
free(characters);
return result;
}

Answer 2

This is the more efficient way to perform a correctly shuffled alphabet generation.
- (NSString *)shuffledAlphabet
{
const NSUInteger length = 'Z' - 'A' + 1;
unichar alphabet[length];
alphabet[0] = 'A';
for ( NSUInteger i = 1; i < length; i++ )
{
NSUInteger j = arc4random_uniform((uint32_t)i + 1);
alphabet[i] = alphabet[j];
alphabet[j] = 'A' + i;
}
return [NSString stringWithCharacters:alphabet length:length];
}
It uses the "inside-out" version of the Fischer Yates shuffle and avoids modula bias by generating the pseudorandom numbers with arc4random_uniform. Also, it requires a single allocation as all the permutations are performed in a temporary buffer.

Answer 3

Generating random numbers in Objective-C does this help?
*generate random number
*divide by 26 and take reminder
*index array[reminder]

Answer 4

You could pick random elements from the (remaining) alphabet while you build your string instead of shuffling it first:
NSMutableArray *alphabet = [NSMutableArray arrayWithObjects:#"A",#"B",#"C",#"D",#"E",#"F",#"G",#"H",#"I",#"J",#"K",#"L",#"M",#"N",#"O",#"P",#"Q",#"R",#"S",#"T",#"U",#"V",#"W",#"X",#"Y",#"Z", nil];
NSMutableString *result = [NSMutableString string];
NSUInteger numberOfLetters = alphabet.count;
for (NSUInteger i = 0; i < numberOfLetters; i++) {
int n = arc4random() % alphabet.count;
[result appendString:[alphabet objectAtIndex:n]];
[alphabet removeObjectAtIndex:n];
}
NSLog(#"%#", result);
This makes the code a bit shorter. Note also that using NSMutableString is more efficient than creating a new NSString each time a letter is added.