Assign value if not exist in Objective-C

In Objective-C, you can assign a value only if it does not already exist in a dictionary, by using the setObject:forKey: method of NSMutableDictionary.

Here is an example:

NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
NSString *key = @"Key1";
NSString *value = @"Value1";
// Check if the key already exists in the dictionary

if ([dict objectForKey:key] == nil) {

    // If the key does not exist, add the value

    [dict setObject:value forKey:key];

}

In this example, we first create an empty dictionary called dict. We then define a key called Key1 and a value called Value1. We check if the Key1 already exists in the dict using the objectForKey: method. If it returns nil, we use the setObject:forKey: method to add the key-value pair to the dictionary. If the Key1 already exists in the dictionary, we do not do anything.