使用涉及地图的比较器对项目向量进行排序

Question

I have a use case in which I map all my elements in a vector to values in a map (Hash Table). I now want to sort the vector using these values stored in the map.

static map<string, string> canonForm;
static bool myfunction(string a, string b){
    return (canonForm[a] < canonForm[b]);
}

EDIT: For example, here, canonForm will hold strings (value) for each string (key) in my vector. The snippet above contains a function I want to use as a comparator to sort the vector of strings. How would I go about implementing this? The above snippet pops an error during compilation.

Please let me know if I can improve the question further

Answer 1

Here is an example of how to do this. See the notes inline:

#include <unordered_map>
#include <string>
#include <vector>
#include <algorithm>

// Note #1 - consider using an unordered_map here for better performance
static std::unordered_map<std::string, std::string> canonForm;

static bool myfunction(std::string const& a, std::string const& b)
{
    // Note #2 - use the `at` member-function - it works on const maps and does not create a new entry if the key is not found
    return canonForm.at(a) < canonForm.at(b);
}


void sort_by_canonform(std::vector<std::string>& keys)
{
    // Note #3 - simply supply myfunction as the comparison function
    std::sort(std::begin(keys), std::end(keys), myfunction);
}