Question

I came to notice that executing a for/in operation in objective c on an initialized empty NSMutableArray was not working as expected.

Simplified code is :

+(void) convertArray: (NSMutableArray*)arrayIN {

    NSMutableArray *arrayOUT = [NSMutableArray array];

    NSLog(@"is nil %d - count %d", !arrayIN, [arrayIN count]);

    for(NSObject *o in arrayIN)
        [arrayOUT addObject:[o convertToAnotherClass]];

}

Actual code is :

+(BOOL) writeTasks: (NSArray*)tasksArray {

    NSMutableArray *arr = [NSMutableArray array];
    NSLog(@"is nil %d - count %d", !arr, [arr count]);
    for(Task *t in tasksArray)
        [arr addObject:[t getDictionary]];

    NSError *error; 
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:arr options:NSJSONWritingPrettyPrinted error:&error];

    if (! jsonData) {
        NSLog(@"Got an error: %@", error);
        return NO;
    } else {
        //NSString *jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
        [jsonData writeToFile:path options:NSDataWritingAtomic error:nil];
        return YES;
    }
}

the suprising thing is that executing [dummyClass convertArray:[NSMutableArray array]] is showing this :

2012-06-25 13:51:34.236 Planorama[740:707] is nil 0 - count 0
2012-06-25 13:51:34.239 Planorama[740:707] -[__NSArrayM convertToAnotherClass]: unrecognized selector sent to instance 0xde9b580
(lldb) 

Why ? arrayIN is empty, why is convertToAnotherClass even called ?

Answer 1

if you use the block based enumeration it will work the way you want.
Also, the output indicates that o is set to some instance of something, so you may have another problem.

Elegant way to get all objects of a specific type in an Objective-C array

Lastly, it looks like this is a static method, but your example calls it as an instance method.