gistfile1.m

#import <Foundation/Foundation.h>

int main() {
    NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];

    NSMutableArray* array = [NSMutableArray array];
    [array addObject:[NSArray array]];
    [array addObject:[NSNull null]];
    [array addObject:[NSArray array]];
    [array addObject:[NSNull null]];

    NSLog(@"%@", array);
    NSLog(@"%d", [array indexOfObject:[NSNull null]]);

    [array replaceObjectAtIndex:[array indexOfObject:[NSNull null]] withObject:[NSArray array]];
    NSLog(@"%@", array);
    NSLog(@"%d", [array indexOfObject:[NSNull null]]);

    [pool release];

    return 0;
}