Q:

Given an array of NSDictionary object how to get all the keys? 

If you have an array of dictionaries, how do I create a new array containing all the keys present for each dictionary in the array ?  
NSArray *array = @[@{@"key1" : @"value 1"},
                   @{@"key2" : @"value 2"},
                   @{@"key3" : @"value 3"} ];

// how to achieve this? 
NSArray *allKeys = @{@"key1", @"key2", @"key3"};

A:

plz use this code, I think it helps you
 NSArray *array = @[@{@"key1" : @"value 1"},
                       @{@"key2" : @"value 2"},
                       @{@"key3" : @"value 3"} ];

    NSMutableArray *key_array=[NSMutableArray array];

    for (NSDictionary *dictionary in array) {

        NSArray *key_dictionary=[dictionary allKeys];

        for (NSString *string_key in key_dictionary) {
            [key_array addObject:string_key];
        }

    }
    NSLog(@"%@",key_array);

