unordered_map 无法检索在参数中指定为变量的键的值

Question

我需要从 unordered_map 中提取特定值。但是，unordered_map 无法在其方括号内查找变量。

在下面的代码中 cout << m[code[i]] << " "; 抛出错误。

#include <iostream>
#include <string>
#include <unordered_map>
using namespace std;
int main()
{
    string code;
    cout << "Enter code: ";
    getline(cin, code);

    unordered_map<string, string> m = {
        {"A","Jan"},{"B","Feb"},{"C","Mar"},{"D","Apr"},
        {"E","May"},{"F","Jun"},{"G","Jul"},{"H","Aug"},
        {"I","Sep"},{"J","Oct"},{"K","Nov"},{"L","Dec"}
    };

    for(int i=0 ; i<code.length() ; i++) {
        cout << m[code[i]] << " ";
    }

    return 0;
}

错误信息：

main.cpp:18:18: No viable overloaded operator[] for type 'unordered_map' (aka 'unordered_map, allocator >, basic_string, allocator > >')

Answer 1

给定 code 类型 string，code[i] returns 一个 char（但不是 string）；这与地图的键类型不匹配。

您可以将地图类型更改为unordered_map<char, string>，例如

unordered_map<string, string> m = {
    {'A',"Jan"},{'B',"Feb"},{'C',"Mar"},{'D',"Apr"},
    {'E',"May"},{'F',"Jun"},{'G',"Jul"},{'H',"Aug"},
    {'I',"Sep"},{'J',"Oct"},{'K',"Nov"},{'L',"Dec"}
};

如果您想使用 unordered_map<string, string>，您必须将 string 传递给 unordered_map::operator[]。例如

cout << m[string(1, code[i])] << " ";

unordered_map 无法检索在参数中指定为变量的键的值

unordered_map is unable to retrieve value for a key specified as a variable in argument

c++

unordered-map