How do I select a random key from an NSDictionary?

问题

When I was using an NSArray, it was easy:

NSArray *array = ...
lastIndex = INT_MAX;
...
int randomIndex;
do {
  randomIndex = RANDOM_INT(0, [array count] - 1);
} while (randomIndex == lastIndex);
NSLog(@"%@", [array objectAtIndex:randomIndex]);
lastIndex = randomIndex;

I need to keep track of the lastIndex because I want the feeling of randomness. That is, I don't want to get the same element twice in a row. So it shouldn't be "true" randomness.

From what I can tell, NSDictionary doesn't have something like -objectAtIndex:. So how do I accomplish this?

回答1:

You can get an array of keys with allKeys (undefined order) or keysSortedByValueUsingSelector (if you want sorting by value). ~~One thing to keep in mind (regarding lastIndex) is that even with sorting, the same index may come to refer to a different key-value pair as the dictionary grows.~~

Either of these (but especially keysSortedByValueUsingSelector) will come with a performance penalty.

EDIT: Since the dictionary isn't mutable, you should just be able to call allKeys once, and then just pick random keys from that.

回答2:

You could use the code below:

- (YourObjectType *)getRandomObjectFromDictionary:(NSDictionary *)dictionary
{
    NSArray *keys = dictionary.allKeys;
    return dictionary[keys[arc4random_uniform((int)keys.count)]];
}

To make it more efficient, you can cache keys in an instance variable. Hope this helps.

来源：https://stackoverflow.com/questions/1112267/how-do-i-select-a-random-key-from-an-nsdictionary

标签

random

nsarray

element

nsdictionary