EXC_ARITHMETIC when accessing random elements of NSArray

Question

i'm trying to get the values of an array randomly but i'm getting an error here is my code so far:

NSMutableArray *validMoves = [[NSMutableArray alloc] init];

for (int i = 0; i < 100; i++){
    [validMoves removeAllObjects];

    for (TileClass *t in tiles ) {
        if ([self blankTile:t] != 0) {
            [validMoves addObject:t];
        }
    }

    NSInteger pick = arc4random() % validMoves.count;

    [self movePiece:(TileClass *)[validMoves objectAtIndex:pick] withAnimation:NO];
}

Answer 1

The error you're getting (an arithmetic exception) is because validMoves is empty and this leads to a division by zero when you perform the modulus operation.

You have to explicitly check for the case of an empty validMoves array. Also you should use arc4random_uniform for avoiding modulo bias.

if (validMoves.count > 0) {
    NSInteger pick = arc4random_uniform(validMoves.count);
    [self movePiece:(TileClass *)[validMoves objectAtIndex:pick] withAnimation:NO];
} else {
   // no valid moves, do something reasonable here...
}

As a final remark not that arc4random_uniform(0) returns 0, therefore such case should be avoided or you'll be trying to access the first element of an empty array, which of course will crash your application.