What is the quickest way of inserting/updating std::unordered_map elements without using an if?

Question

I currently have lots of code which looks like this:

std::unordered_map<int,int> my_dict;
.
.
.
// If the key does exist in the dictionary
if(my_dict.count(key) == 1){
    my_dict[key] = value;
}

// If its a new key
else{
    my_dict.insert(std::make_pair(key,value));
}

Is there any way I can speed this up by just overwriting the value every time?

Answer 1

You just do (for map and unordered_map)

mydict[key]=value;

Answer 2

I think it might be fastest like this:

auto it = my_dict.find(key);
if( it != my_dict.end() ) {
    it->second = value;
}
else {
    my_dict.insert(std::make_pair(key,value));
}

that way you don't modify the structure of the unordered_map if the key already exists and you only have one lookup.

---

Another option in case you don't need/access value afterwards:

my_dict[key] = std::move(value);

This might be better in cases when the assignment of value is expensive and benefits from move-semantics.

Answer 3

To update for C++17, you can use:

std::unordered_map::insert_or_assign()

http://en.cppreference.com/w/cpp/container/unordered_map/insert_or_assign