C++ 使用find方法后如何更新std::map？
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How to update std::map after using the find method?
提问by jaykumarark
How to update the value of a key in std::mapafter using the findmethod?
std::map使用find方法后如何更新key的值？
I have a map and iterator declaration like this:
我有一个像这样的地图和迭代器声明：
map <char, int> m1;
map <char, int>::iterator m1_it;
typedef pair <char, int> count_pair;
I'm using the map to store the number of occurrences of a character.
我正在使用地图来存储字符出现的次数。
I'm using Visual C++ 2010.
我正在使用 Visual C++ 2010。
回答by James McNellis
std::map::findreturns an iterator to the found element (or to the end()if the element was not found). So long as the mapis not const, you can modify the element pointed to by the iterator:
std::map::find将迭代器返回到找到的元素（end()如果未找到元素，则返回到 ）。只要map不是const，就可以修改迭代器指向的元素：
std::map<char, int> m;
m.insert(std::make_pair('c', 0)); // c is for cookie
std::map<char, int>::iterator it = m.find('c');
if (it != m.end())
it->second = 42;
回答by Martin York
I would use the operator[].
我会使用运算符 []。
map <char, int> m1;
m1['G'] ++; // If the element 'G' does not exist then it is created and
// initialized to zero. A reference to the internal value
// is returned. so that the ++ operator can be applied.
// If 'G' did not exist it now exist and is 1.
// If 'G' had a value of 'n' it now has a value of 'n+1'
So using this technique it becomes really easy to read all the character from a stream and count them:
因此，使用这种技术，从流中读取所有字符并计算它们变得非常容易：
map <char, int> m1;
std::ifstream file("Plop");
std::istreambuf_iterator<char> end;
for(std::istreambuf_iterator<char> loop(file); loop != end; ++loop)
{
++m1[*loop]; // prefer prefix increment out of habbit
}
回答by Manish Sogi
You can use std::map::atmember function, it returns a reference to the mapped value of the element identified with key k.
您可以使用std::map::at成员函数，它返回对以键 k 标识的元素的映射值的引用。
std::map<char,int> mymap = {
{ 'a', 0 },
{ 'b', 0 },
};
mymap.at('a') = 10;
mymap.at('b') = 20;
回答by abhinav1602
If you already know the key, you can directly update the value at that key using m[key] = new_value
如果您已经知道密钥，则可以使用直接更新该密钥处的值 m[key] = new_value
Here is a sample code that might help:
这是一个可能有帮助的示例代码：
map<int, int> m;
for(int i=0; i<5; i++)
m[i] = i;
for(auto it=m.begin(); it!=m.end(); it++)
cout<<it->second<<" ";
//Output: 0 1 2 3 4
m[4] = 7; //updating value at key 4 here
cout<<"\n"; //Change line
for(auto it=m.begin(); it!=m.end(); it++)
cout<<it->second<<" ";
// Output: 0 1 2 3 7
回答by ZAFIR AHMAD
You can update the value like following
您可以像下面这样更新值
auto itr = m.find('ch');
if (itr != m.end()){
(*itr).second = 98;
}
回答by chunky
You can also do like this-
你也可以这样做——
std::map<char, int>::iterator it = m.find('c');
if (it != m.end())
(*it).second = 42;
