如何在unordered_map中设置一个值,并确定是否添加了新的密钥 [英] How to set a value in an unordered_map and find out if a new key was added
问题描述
我如何在 unordered_map
和中高效并惯用地设置一个值,以确定是否添加了新的密钥:
$ b
#include< unordered_map>
#include< string>
int main(){
auto map = std :: unordered_map< std :: string,int> {{foo,1},{bar,2}};
map [foo] = 3;
//如何找出是否添加了新密钥?
}
我不能使用 insert() code>,因为如果已经有一个值,并且
insert
没有这样做,我想覆盖该值。我不能直接使用 operator []
,因为它没有提供关于是否添加新密钥的信息。
我想在性能上避免在地图上进行两次搜索。
我在别处看到的一个技巧是获取引用并检查该值是否为默认构造:
auto& value = map [foo];
if(value == 0){
//正在插入一个新键
}
value = 3;
但我真的可以在我的地图中使用默认的构造值,所以默认的构造值并不是一个好的指示新密钥。
到目前为止我所能想到的最好的是:
auto size_before = map.size();
map [foo] = 3;
if(map.size()> size_before){
//正在插入一个新键
}
这看起来很丑,它假设获得 unordered_map
的大小很便宜(是吗?)。
它看起来像 unordered_map :: insert_or_assign
可能是我祈祷的答案,但遗憾的是它出现在C ++ 17中,所以我可能无法再使用它5年左右。这似乎是相当普遍的事情要这样做,我假设目前必须有一个合理的方式。
您可以使用 std :: unordered_map :: insert
和测试结果。
使用辅助函数:
模板< typename Map,typename T>
std :: pair< typename Map :: iterator,bool>
insert_or_assign(Map& m,const typename Map :: key_type& k,const T& t)
{
auto p = m.insert({k,t});
if(!p.second){
//覆盖以前的值
p.first-> second = t;
}
return p;
}
然后
auto p = insert_or_assign(map,foo,3);
if(p.second){
//插入
} else {
//分配
}
How can I efficiently and idiomatically set a value in an unordered_map
and find out if a new key was added:
#include <unordered_map>
#include <string>
int main() {
auto map = std::unordered_map<std::string, int>{{"foo", 1}, {"bar", 2}};
map["foo"] = 3;
// how to find out if a new key was added?
}
I can't use insert()
directly because I want to overwrite the value if there is one already and insert
does not do that. I can't use operator[]
directly because it provides no information about whether a new key was added.
I want to avoid doing two searches in the map for performance reasons.
One trick I've seen elsewhere is to get a reference and check if that value is default constructed:
auto& value = map["foo"];
if(value == 0) {
// am inserting a new key
}
value = 3;
But I can genuinely have default constructed values in my map so a default constructed value is not a good indication of new key.
The best I can come up with so far is:
auto size_before = map.size();
map["foo"] = 3;
if (map.size() > size_before) {
// am inserting a new key
}
which seems ugly and it assumes that getting the size of an unordered_map
is cheap (is it?).
It looks like unordered_map::insert_or_assign
might be the answer to my prayers but sadly it comes in C++17 so I probably wont be able to use it for another 5 years or so. It seems like quite a common thing to want to do so I assume there must be a reasonable way of doing it currently.
You may use std::unordered_map::insert
and testing result.
With helper function:
template<typename Map, typename T>
std::pair<typename Map::iterator, bool>
insert_or_assign(Map& m, const typename Map::key_type& k, const T& t)
{
auto p = m.insert({k, t});
if (!p.second) {
// overwrite previous value
p.first->second = t;
}
return p;
}
and then
auto p = insert_or_assign(map, "foo", 3);
if (p.second) {
// inserted
} else {
// assigned
}
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