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

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);