如何修复C ++中的“哈希图错误：”错误

Question

I want to use hashmap in C++ in Ubuntu 18.04. Butthis error makes me confusing.

I want to use hashmap in C++ in Ubuntu 18.04. Butthis error makes me confusing.

The complete error code is "Hashmap error: no matching function for call to 'std::map, int>::find(__gnu_cxx::__alloc_traits

::value_type&)"

#include <iostream>
#include <map> using namespace std;

int main() {
    string s = "dfsfsdfsf";
    int i = 0, j = 0, ans = 0, leng;
    map<string,int> window;
    leng = s.length();
    for(;j < leng; j++){
        if(window.find(s.at(j)) != window.end()){
            i = max(i, window[s.at(j)])
        }
    }
    return 0; }

I do not know what happened. Please someone helps me. ^~^

Answer 1

std::string::at returns a reference to a char , a single character. Meanwhile std::map::find expected a reference to the key type, a std::string . There is no implicit conversion from a single character to a string. So you don't give find the argument it expected.

A possible workaround is to use std::basic_string::substr instead of at . This member function returns a string, by value, that holds a substring. You give it a position and how many characters to read. So the immediate fix to your code can be:

window.find(s.substr(j,1)) != window.end()

Other than that, do note that std::map is an ordered map . As such it is usually implement by using a self-balancing binary tree. If you don't need order, and indeed want amortized constant lookup time like a hash would provide, you need std::unordered_map .