C++ 在不使用 if 的情况下插入/更新 std::unordered_map 元素的最快方法是什么？
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What is the quickest way of inserting/updating std::unordered_map elements without using an if?
提问by user997112
I currently have lots of code which looks like this:
我目前有很多看起来像这样的代码：
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?
有什么办法可以通过每次都覆盖该值来加快速度吗？
回答by Joe
You just do (for mapand unordered_map)
你只是做（为map和unordered_map）
mydict[key]=value;
回答by Daniel Frey
I think it might be fastest like this:
我认为它可能是最快的：
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_mapif the keyalready exists and you only have one lookup.
这样，unordered_map如果key已经存在，您就不会修改 的结构，并且您只有一次查找。
Another option in case you don't need/access valueafterwards:
如果您value之后不需要/访问，另一种选择：
my_dict[key] = std::move(value);
This might be better in cases when the assignment of valueis expensive and benefits from move-semantics.
在分配value昂贵且受益于移动语义的情况下，这可能会更好。
回答by htmlboss
To update for C++17, you can use:
要更新 C++17，您可以使用：
std::unordered_map::insert_or_assign()
http://en.cppreference.com/w/cpp/container/unordered_map/insert_or_assign
http://en.cppreference.com/w/cpp/container/unordered_map/insert_or_assign
回答by Serge Rogatch
Usually you avoid extra typing by means of defining a function that saves you from typing the same again.
If you don't have access to C++17's insert_or_assign(), you can implement something like this yourself:
通常您通过定义一个函数来避免额外的输入，从而避免再次输入相同的内容。如果您无权访问 C++17's insert_or_assign()，您可以自己实现如下内容：
bool InsertOrAssign(std::unordered_map& m, int key, int value) {
// Your code or one of the suggested answers goes here
}
