Arrays in Objective-C

Our first example demonstrates how to work with arrays in Objective-C. Arrays are ordered collections of elements of a specific type.

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // Here we create an NSMutableArray 'a' that will hold NSNumber objects.
        // By default, the array is empty.
        NSMutableArray *a = [NSMutableArray arrayWithCapacity:5];
        for (int i = 0; i < 5; i++) {
            [a addObject:@0];
        }
        NSLog(@"emp: %@", a);

        // We can set a value at an index using the setObject:atIndex: method,
        // and get a value with objectAtIndex:.
        [a setObject:@100 atIndex:4];
        NSLog(@"set: %@", a);
        NSLog(@"get: %@", [a objectAtIndex:4]);

        // The count method returns the length of an array.
        NSLog(@"len: %lu", (unsigned long)[a count]);

        // Use this syntax to declare and initialize an array in one line.
        NSArray *b = @[@1, @2, @3, @4, @5];
        NSLog(@"dcl: %@", b);

        // In Objective-C, we can't directly create an array with a specific size and initial values.
        // However, we can create an array with repeated values like this:
        NSArray *c = [@[@100, @0, @0, @400, @500] mutableCopy];
        NSLog(@"idx: %@", c);

        // Array types are one-dimensional, but you can compose types to build multi-dimensional data structures.
        NSMutableArray *twoD = [NSMutableArray array];
        for (int i = 0; i < 2; i++) {
            NSMutableArray *row = [NSMutableArray array];
            for (int j = 0; j < 3; j++) {
                [row addObject:@(i + j)];
            }
            [twoD addObject:row];
        }
        NSLog(@"2d: %@", twoD);

        // You can create and initialize multi-dimensional arrays at once too.
        NSArray *twoD2 = @[
            @[@1, @2, @3],
            @[@1, @2, @3]
        ];
        NSLog(@"2d: %@", twoD2);
    }
    return 0;
}

When you run this program, you’ll see output similar to:

emp: (0, 0, 0, 0, 0)
set: (0, 0, 0, 0, 100)
get: 100
len: 5
dcl: (1, 2, 3, 4, 5)
idx: (100, 0, 0, 400, 500)
2d: ((0, 1, 2), (1, 2, 3))
2d: ((1, 2, 3), (1, 2, 3))

Note that arrays in Objective-C are actually objects (instances of NSArray or NSMutableArray), and they can only contain objects. For primitive types like int, we wrap them in NSNumber objects.

Also, Objective-C arrays are zero-indexed, meaning the first element is at index 0.

Objective-C provides two main types of arrays:

NSArray: An immutable array that can’t be changed after creation.
NSMutableArray: A mutable array that can be modified after creation.

In this example, we’ve used both types to demonstrate different array operations.

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Arrays in Objective-C