Question

I have a random number generator method in iOS that I am currently using. While it is working fine, I would like to modify it such that in its output, it ALWAYS includes at least one number. How would I do this? Here is the random number method that I am using:

- (NSString *)generateRandString {

    NSString *alphabet  = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXZY0123456789";
    NSMutableString *s = [NSMutableString stringWithCapacity:5];
    for (NSUInteger i = 0U; i < 5; i++) {

        u_int32_t r = arc4random() % [alphabet length];
        unichar c = [alphabet characterAtIndex:r];
        [s appendFormat:@"%C", c];

    }

    return s;
}

Thanks in advance to all who reply.

Answer 1

This is my edited and checked solution:

- (NSString *)generateRandString {

         NSString *alphabet  = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXZY0123456789";
         NSMutableString *s;
         NSCharacterSet *set = [NSCharacterSet characterSetWithCharactersInString:@"0123456789"];

         do
        {
            s = [NSMutableString stringWithCapacity:5];
           for (NSUInteger i = 0U; i < 5; i++)
           {

              u_int32_t r = arc4random() % [alphabet length];
              unichar c = [alphabet characterAtIndex:r];
              [s appendFormat:@"%C", c];
            }
       }
       while ([s rangeOfCharacterFromSet:set].location == NSNotFound);  

      return s;
 }

I’ve also implemented the solution suggested by Dan Dyer (and it’s obviously better than mine). My implementation is probably not perfect ( i use the same functionality few times instead of creating a separate function for it, but i believe it’s not difficult to implement for you) but it works exactly as Dan has proposed.

  NSString *alphabet  = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXZY0123456789";
NSMutableString *s  = [NSMutableString string];
NSCharacterSet *set = [NSCharacterSet decimalDigitCharacterSet];
NSString *digits    = @"0123456789";

for (NSUInteger i = 0U; i < 4; i++)
{
    u_int32_t r = arc4random() % [alphabet length];
    unichar c = [alphabet characterAtIndex:r];
    [s appendFormat:@"%C", c];
}

if ([s rangeOfCharacterFromSet:set].location == NSNotFound)
{
    u_int32_t r = arc4random() % [digits length];
    unichar c = [digits characterAtIndex:r];
    [s appendFormat:@"%C", c];
}
else
{
    u_int32_t r = arc4random() % [alphabet length];
    unichar c = [alphabet characterAtIndex:r];
    [s insertString:[NSString stringWithFormat:@"%C", c] atIndex:arc4random() % s.length];
}