iOS - limiting the arrayIndex between 0 and 98

Question

in my project i have an array of images. The images are displayed on an imageView, i can move between the images with swipes. But when i get to zero and swipe left (aka previous photo, which decreases the arrayIndex by one) i get an error (SIGABRT). here's my code that's responsible for moving between images:

-(IBAction)nextPhoto:(id)sender{
arrayIndex++;
NSLog(@"rigth! at index %lu", arrayIndex);
if (arrayIndex <= 98){
displayImage.image = [UIImage imageNamed:[imageArray objectAtIndex:arrayIndex]];
} else {
    displayImage.image = [UIImage imageNamed:[imageArray objectAtIndex:0]];
}

}
-(IBAction)previousPhoto:(id)sender{
 arrayIndex--;
 NSLog(@"Left! at index %lu", arrayIndex);
 if (arrayIndex >= 0){
 displayImage.image = [UIImage imageNamed:[imageArray  objectAtIndex:arrayIndex]];
} else {
   displayImage.image = [UIImage imageNamed:[imageArray objectAtIndex:98]];
}

}

Answer 1

You didn't show us the declaration of arrayIndex, but I guess it's unsigned long, since you print it with %lu.

Consider what happens when arrayIndex == 0:

// arrayIndex == 0
arrayIndex--;
// Now arrayIndex == ULONG_MAX.
// The following condition is always true for unsigned longs.
if (arrayIndex >= 0){
    // This branch is always taken.
    // The following objectAtIndex: fails for ULONG_MAX.
    displayImage.image = [UIImage imageNamed:[imageArray  objectAtIndex:arrayIndex]];

You need to check for arrayIndex == 0 before you decrement it, like this:

-(IBAction)nextPhoto:(id)sender {
    ++arrayIndex;
    if (arrayIndex >= imageArray.count) {
        arrayIndex = 0;
    }
    displayImage.image = [UIImage imageNamed:imageArray[arrayIndex]];
}

-(IBAction)previousPhoto:(id)sender {
    if (arrayIndex == 0) {
        arrayIndex = imageArray.count - 1;
    } else {
        --arrayIndex;
    }
    displayImage.image = [UIImage imageNamed:imageArray[arrayIndex]];
}