How to copy one NSMutablearray to another NSMutablearray?

[arr1 initWithObjects:@"1",@"2", nil];

makes no sense, what you probably want is to use the NSMutableArray:

NSArray *arr1 = [NSArray arrayWithObjects:@"1",@"2", nil];
NSMutableArray *arr2 = [arr1 mutableCopy];

NSLog(@"arr1: %p, %@", arr1, arr1);
NSLog(@"arr2: %p, %@", arr2, arr2);

NSLog output:

arr1: 0x7924c50, (
                 1,
                 2
                 )
arr2: 0x7924f00, (
                 1,
                 2
                 )

Your arr1 must be nil otherwise you would crash, but more to the point you are creating the NSMutableArray incorrectly:

You should not really do alloc and init on separate lines

NSMutableArray *myArray = [NSMutableArray alloc];

NSLog(@"%@", [myArray class]);

//=> __NSPlaceholderArray  

In this case the myArray pointer is pointing at a flyweight of type __NSPlaceholderArray which is most likely not what you want. You would prefer:

---------------------------------------

myArray = [[NSMutableArray alloc] init];

NSLog(@"%@", [myArray class]);

//=> __NSArrayM

You will also crash if you do have a legitimate NSMutableArray and you call initWithArray: with:

[NSMutableArray initWithCapacity:]: method only defined for abstract class.

This is because the method is most likely defined on __NSPlaceholderArray

Are you trying [arr2 objectAtIndex:0] in the same method where arr2 is being created?

Maybe the problem is that you are using autorelease on your arr2. So, in other methods it may have lost its values.

I tried the following and it works for me:

NSMutableArray * arr1 = [[NSMutableArray alloc]initWithObjects:@"1", @"2", nil];
NSMutableArray * arr2 = [[[NSMutableArray alloc] initWithArray:arr1 copyItems:YES] autorelease];
NSString * no = [arr2 objectAtIndex:0];
NSLog(@"%@", no);

In addition to tia's answer: you need to allocate it first.

The method 'mutableCopy' will create a copy of the array, not of the elements inside it. You could create a new immutable array with 'initWithArray:arr1 copyItems:YES' and subsequently create a mutable version of it by calling 'mutableCopy'.:

NSArray *arr1 = [[NSArray alloc] initWithObjects:@"1",@"2", nil];
NSMutableArray arr2 = [[[NSArray alloc] initWithArray: arr1 copyItems:YES] mutableCopy];