NSArray index beyond bounds error - ios

Question

I have created a simple image browser with two buttons to view the next and the previous images.
Here's the code.
#import "ViewController.h"
#interface ViewController ()
#property (weak, nonatomic) IBOutlet UIImageView *imageView;
#property (strong, nonatomic) NSArray *images;
#property (assign, nonatomic) NSInteger imageIndex;
#property (strong, nonatomic) UIBarButtonItem *nextButton;
#property (strong, nonatomic) UIBarButtonItem *prevButon;
#end
#implementation ViewController
- (void)viewDidLoad
{
[super viewDidLoad];
// Adding Next and Previous buttons
self.nextButton = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:#"next"] style:UIBarButtonItemStylePlain target:self action:#selector(nextButtonPressed)];
self.prevButon = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:#"previous"] style:UIBarButtonItemStylePlain target:self action:#selector(previousButtonPressed)];
self.navigationItem.rightBarButtonItems = #[self.nextButton, self.prevButon];
// Loading images
self.images = #[
[UIImage imageNamed:#"1"],
[UIImage imageNamed:#"2"],
[UIImage imageNamed:#"3"],
[UIImage imageNamed:#"4"],
[UIImage imageNamed:#"5"],
[UIImage imageNamed:#"6"],
[UIImage imageNamed:#"7"],
[UIImage imageNamed:#"8"],
[UIImage imageNamed:#"9"]
];
self.imageIndex = 0;
self.imageView.image = self.images[self.imageIndex];
}
- (void)nextButtonPressed
{
self.imageIndex++;
self.imageView.image = self.images[self.imageIndex];
}
- (void)previousButtonPressed
{
self.imageIndex--;
self.imageView.image = self.images[self.imageIndex];
}
#end
My problem is say if I tap next button after the last image or previous button when the first image, it'll throw a index beyond bounds error.
How can I stop this from happening?
Thank you.

Answer 1

You have to check whether the index is present in your index.
You can make the verification in your nextButtonPressed and previousButtonPressed methods:
- (void)nextButtonPressed
{
if (imageIndex < ([self.images count] - 1))
{
self.imageIndex++;
self.imageView.image = self.images[self.imageIndex];
}
}
- (void)previousButtonPressed
{
if (imageIndex >= 1)
{
self.imageIndex--;
self.imageView.image = self.images[self.imageIndex];
}
}
For information the NSArray raises an NSRangeException if the index is beyond the end of the array (see the documentation).

Answer 2

You are keeping track of the current image index, which is good, because you can use that to check if there is a next image or a previous by using the image arrays count property.
- (void)nextButtonPressed
{
if ( self.imageIndex < self.images.count - 1) {
self.imageIndex++;
self.imageView.image = self.images[self.imageIndex];
}
}
- (void)previousButtonPressed
{
if ( self.imageIndex > 0 ) {
self.imageIndex--;
self.imageView.image = self.images[self.imageIndex];
}
}
Another alternative is starting over when you reach the last image.
- (void)nextButtonPressed
{
if ( self.imageIndex < self.images.count - 1) {
self.imageIndex++;
} else {
self.imageIndex = 0
}
self.imageView.image = self.images[self.imageIndex];
}
- (void)previousButtonPressed
{
if ( self.imageIndex > 0 ) {
self.imageIndex--;
} else {
self.imageIndex = self.images.count - 1;
}
self.imageView.image = self.images[self.imageIndex];
}

Answer 3

You can use modulus like this - so it wraps around on itself.
This way you never need to disable buttons and it also makes it easier for the user to get from one end of the list to the other more efficiently.
replace the following line
self.imageIndex++;
with
self.imageIndex = (self.imageIndex + 1) % [self.images count];
and where you have the following line
self.imageIndex--;
replace with
self.imageIndex = (self.imageIndex + [self.images count] - 1) % [self.images count];

Answer 4

A good UI habit will be to disable the next (or previous) button once you reach the last image (and of course prevent the overflow in the array):
- (void)nextButtonPressed
{
self.imageIndex++;
self.imageView.image = self.images[self.imageIndex];
[self.previousButton setEnabled:YES];
if (self.imageIndex == [self.images count] - 1) {
[self.nextButton setEnabled:NO];
return ;
}
}
- (void)previousButtonPressed
{
self.imageIndex--;
self.imageView.image = self.images[self.imageIndex];
[self.nextButton setEnabled:YES];
if (self.imageIndex == 0) {
[self.previousButton setEnabled:NO];
return ;
}
}
Since you start with index = 0 you should disable the previous button in viewDidLoad method (as you do not have more previous images until you press next) or do it directly in IB if possible