C++ make_pair of std::map - 如何仅在未列出密钥时进行配对（否则更新密钥）？
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make_pair of std::map - how to make a pair only if the key is not listed (and update the key otherwise)?
提问by JAN
Consider the following code :
考虑以下代码：
std::map <string,string> myMap;
myMap.insert(std::make_pair("first_key" , "no_value" ));
myMap.insert(std::make_pair("first_key" , "first_value" ));
myMap.insert(std::make_pair("second_key" , "second_value" ));
typedef map<string, string>::const_iterator MapIterator;
for (MapIterator iter = myMap.begin(); iter != myMap.end(); iter++)
{
cout << "Key: " << iter->first << endl << "Values:" << iter->second << endl;
}
The output is :
输出是：
Key: first_key
Values:no_value
Key: second_key
Values:second_value
Meaning is that the second assignment :
意思是第二个任务：
myMap.insert(std::make_pair("first_key" , "first_value" ));
didn't take place .
没有发生。
How can I make a pair , only if the key is not already listed , and if is listed - change its value ?
仅当密钥尚未列出，如果已列出 - 更改其值时，我该如何制作一对？
Is there any generic method of std::map ?
有没有 std::map 的通用方法？
采纳答案by Haocheng
Add a condition before insert
插入前添加条件
if (myMap.find("first_key") == myMap.end()) {
myMap.insert(std::make_pair("first_key" , "first_value" ));
}
else {
myMap["first_key"] = "first_value";
}
回答by ForEveR
Use operator [], or use findand change value if key finded.
Will insert pair in map, if there is no such key and update value, if key exists.
如果找到键operator []，请使用，或使用find并更改值。将在地图中插入对，如果没有这样的键和更新值，如果键存在。
myMap["first_key"] = "first_value";
Or this:
或这个：
auto pos = myMap.find("first_key");
if (pos != myMap.end())
{
pos->second = "first_value";
}
else
{
// insert here.
}
回答by ddiepo
It's more efficient to avoid searching the map a second time when the value is present:
当值存在时，避免再次搜索地图会更有效：
const iterator i = myMap.find("first_key");
if (i == myMap.end()) {
myMap.insert(std::make_pair("first_key" , "first_value"));
} else {
i->second = "first_value";
}
