old C array may help
id array[anotherArray.count];
id __strong *arrayPtr = array;
[anotherArray enumerateObjectsWithOptions:NSEnumerationConcurrent
usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
// do some calculation and generate an object
arrayPtr[idx] = anObject;
}];
NSMutableArray *myArray = [NSMutableArray arrayWithObjects:array count:anotherArray.count];