Question

// Declare index in Header.h

index=0;

- (IBAction)next {
    index++;
    // Set imageCount to as many images as are available
    int imageCount=2;
    if (index<=imageCount) {
        NSString* imageName=[NSString stringWithFormat:@"img%i", index];
        [picture setImage: [UIImage imageNamed: imageName]];
    }
}

Where do I declare index in my header file and how?

Answer 1

If index is used only within the -next method, you can define a static variable.

- (IBAction)next {
    static int index = 0;    // <-- here
    index++;
    // Set imageCount to as many images as are available
    int imageCount=2;
    if (index<=imageCount) {
        NSString* imageName=[NSString stringWithFormat:@"img%i", index];
        [picture setImage: [UIImage imageNamed: imageName]];
    }
}

Note that all instances will share the same index.

But I believe it’s better to make index as an ivar, e.g.

@interface ... {
   ...
   int index;
   ...
}

it is automatically initialized to 0 when the instance is constructed, and methods other than next can use the index. Also, each instance will have its own index.