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

Question

I need to extract specific values from the unordered_map. However, unordered_map is unable to lookup with a variable inside its box brackets.

In the below code cout << m[code[i]] << " "; is throwing an error.

#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;
}

Error msg:

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

Do not tell anybody what error you are getting, that would be more interesting — Slava
– Slava, Commented Jun 19, 2017 at 3:36

songyuanyao · Accepted Answer · 2017-06-19 03:48:16Z

1

Given code with type string, code[i] returns a char (but not a string); which doesn't match the key type of the map.

You can change the type of map to unordered_map<char, string>, e.g.

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"}
};

If you want to work with unordered_map<string, string>, you have to pass a string to unordered_map::operator[]. e.g.

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

edited Jun 19, 2017 at 3:48

answered Jun 19, 2017 at 3:38

songyuanyao

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1 Comment

Slava Over a year ago

Somebody need to make a VS plugin that would redirect compiler error directly to SO, these people do not bother to read and comprehend them anyway. They do not even bother to paste error messages to their questions. Why bother, if somebody like @songyuanyao whould answer anyway?

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