Why does this random number sometimes get set to -1?

Question

Every now and again, the following comes out as -1

int randomIndex = random() % [directionsArray count] - 1;

The init method for this class is where directionsArray is created and srand() is called. It looks like this

- (id) initSprite
{
    if ( ( self = [super initSprite] ) ) {

        speed = 1;

        directionsArray = [[NSArray arrayWithObjects:STRING_CONSTANT, STRING_CONSTANT, nil] retain];
        srand(time(NULL));
        [self pickRandomDirection];


    }
    return self;
}

- (void) pickRandomDirection
{
    int randomIndex = random() % [directionsArray count] - 1; // This sometimes comes out as -1??
}

At the moment, I'm working around it by using abs(randomIndex), but that's cheating and I should probably know what's going on for future reference.

Answer 1

Don't use random

Use arc4random instead.

Now that issue is solved, you're subtracting 1 from the randomization expression, which, in the case of arc4random % len returning 0, you'd be subtracting 1 from that, which is -1.

Answer 2

I'd recommend arc4random. It seems to be the easiest to use and best rnd available in the SDK.

arc4random() % [directionsArray count];

Answer 3

Does % have precedence over -1? If it does then when the remainder is 0 minuses 1 will result in negative one.

Answer 4

Because 0 - 1 equals -1.

    random() % [directionsArray count] - 1;

What if random() returns 0 here ? You get 0%[directionsArray count] - 1; which is -1, since % has precedence over -

Perhaps you want random() % ([directionsArray count] - 1);

Answer 5

Know your operator precedence. If in doubt, use parentheses:

int randomIndex = random() % ([directionsArray count] - 1);

Answer 6

I don't know why nobody else has mentioned this, but the solution (in your case) is actually to not subtract at all.

As you're probably aware, indexes are zero-based. Remainders likewise start at 0, and they go up to the divisor minus one and then loop around. Consider, for example, the case of count == 3:

• 0 % 3 = 0
• 1 % 3 = 1
• 2 % 3 = 2
• 3 % 3 = 0

Doing the division first, as Stephen Canon and Jonathan Fischoff already told you, will cause you to subtract 1 from these valid indexes, causing the problem you saw.

Doing the division second is not much better: It solves the problem of coming up with an index of -1, but you're still cutting off the last possible index:

• 0 % (count - 1 = 2) = 0
• 1 % 2 = 1
• 2 % 2 = 0
• 3 % 2 = 1

Notice how index 3 never comes up.

Divide only; cut out the subtraction.

Also, indexes are normally NSUInteger (an unsigned integral type, unlike int, which is signed) in Cocoa and Cocoa Touch.