Shuffled Array Uncaught Exception

=============================

In this article, we will explore a common issue in Objective-C programming: the NSRangeException error that occurs when trying to shuffle an array. We’ll break down the problem, discuss possible causes, and provide solutions.

Understanding the Error

The NSRangeException is raised when you try to access or manipulate an array index that is out of bounds. In this case, we’re dealing with a mutable array (NSMutableArray) and trying to shuffle its elements using the exchangeObjectAtIndex:withObjectAtIndex: method.

When we attempt to shuffle the array, we’re selecting two random indices, i and n, and swapping the elements at those indices. However, if we select an index that is beyond the bounds of the array, we’ll get an out-of-bounds error.

The Problem with the Original Code

Let’s take a closer look at the original code snippet:

words = [[NSMutableArray alloc] initWithObjects:@"1", @"2", @"3",@"4", nil];

NSUInteger count = [questar count];
for (NSUInteger i = 0; i < count; ++i) {
    // Select a random element between i and end of array to swap with.
    NSInteger nElements = count - i;
    NSInteger n = (arc4random() % nElements) + i;

    [words exchangeObjectAtIndex:i withObjectAtIndex:n];
}

The problem lies in the way we’re selecting the n index. We’re using (arc4random() % nElements) + i, which can result in an index greater than or equal to count. This is because arc4random() returns a random integer between 0 and nElements - 1, inclusive.

The Fix

To fix this issue, we need to ensure that the n index is always within the bounds of the array. One way to do this is to change the loop condition to:

for (NSUInteger i = 0; i < count; ++i) {
    // ...
}

However, this will not work because we’re still trying to access an element at n index that may be beyond the bounds of the array.

Instead, we need to use a different approach. One way is to select two random indices within the range [0, count - 1], and then swap the elements at those indices:

for (NSUInteger i = 0; i < count; ++i) {
    // Select two random indices between 0 and count - 1.
    NSInteger nElements = count - i;
    NSInteger n1 = arc4random() % nElements;
    NSInteger n2 = arc4random() % nElements;

    // Ensure that n1 is not equal to n2
    while (n1 == n2) {
        n2 = arc4random() % nElements;
    }

    [words exchangeObjectAtIndex:i withObjectAtIndex:n2];
}

This way, we’re guaranteed to select two random indices within the bounds of the array.

Alternating between questar and words

Another issue with the original code is that it uses questar as if it were an array. However, in the corrected version, we replace each element at index i with the current image object at index n.

So instead of this:

currentImage = [questar objectAtIndex:n];

[words replaceObjectAtIndex:i withObject:currentImage];

We should use this code:

currentImage = [questar objectAtIndex:n];

[words replaceObjectAtIndex:i withObject:currentImage];

Conclusion

Shuffling an array in Objective-C can be tricky, especially when dealing with mutable arrays and indices. By understanding the problem, identifying possible causes, and using the right approach, we can avoid the NSRangeException error and create a correctly shuffled array.

In this article, we’ve discussed the issue of shuffling an array, provided possible solutions, and highlighted best practices for working with arrays in Objective-C.

Last modified on 2025-01-11