Question

Below is my code.

NSMutableArray *arr = [[NSMutableArray alloc] init];
[arr addObject:@"5"];
[arr addObject:@"7"];
[arr addObject:@"8"];
[arr enumerateObjectsUsingBlock:^(NSString *obj,NSUInteger idx,BOOL *stop) {
        [arr replaceObjectAtIndex:idx withObject:@"10"];
}];

The Exception log I got

 *** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSArrayM: 0x742a580> was mutated while being enumerated.'
*** First throw call stack:
(0x1596012 0x12a3e7e 0x161ecc5 0x158fe1b 0x158fa16 0x158f925 0x2ba4 0x1e87b7 0x1e8da7 0x1e9fab 0x1fb315 0x1fc24b 0x1edcf8 0x25f8df9 0x25f8ad0 0x150bbf5 0x150b962 0x153cbb6 0x153bf44 0x153be1b 0x1e97da 0x1eb65c 0x29fd 0x2925)
libc++abi.dylib: terminate called throwing an exception

The code is working fine while I am using for loop

for (int i = 0 ; i<  arr.count; i++) {
    [arr replaceObjectAtIndex:i withObject:@"8"];
}

So while I am using enumerateObjectsUsingBlock then I am getting exception. But both are enumerations. Right ? Then why upper code is giving was mutated while being enumerated exception?

Answer 1

Because your logic is flawed. It is not permited to mutate a collection during enumeration. And in the latter case, NSMutableArray doesn’t know you’re trying to emumerate it, only in the first case. And then it complains, since this is a semantic error. You should generally solve these kinds of problems by mutable copying the array and mutating the copy, then replacing the original one by the updated copy.