如何从std :: map值的std :: vector找到最小键值？

Question

I have an std::vector of std::map values:

std::vector<std::map<std::string, double>> dataPoints;

I would like to find the lowest low value, 74.0. Here is my application so far:

#include <vector>
#include <map>
#include <iostream>

int main() {
    std::vector<std::map<std::string, double>> dataPoints;

    dataPoints.push_back({{{"high", 77.0}, {"low", 74.0}}});
    dataPoints.push_back({{{"high", 78.0}, {"low", 75.0}}});
    dataPoints.push_back({{{"high", 79.0}, {"low", 76.0}}});
    dataPoints.push_back({{{"high", 80.0}, {"low", 77.0}}});
    dataPoints.push_back({{{"high", 81.0}, {"low", 78.0}}});

    return 0;
}

The closest I have found so far is:

double min = std::min_element(dataPoints.begin(), dataPoints.end(), [](std::map<std::string, double> &a, std::map<std::string, double> &b) { return (a["low"] < b["low"]); })["low"];

But this does not quite work.

In JavaScript, I could achieve this as follows:

low = _.min(_.map(dataSegment, function(dataPoints) {
    return dataPoint.low;
}));

Answer 1

You want min_element , not max_element . And it returns an iterator so you'll want to dereference it.

And I suppose you probably don't want to insert a zero if "low" is not in the map. So at instead of [] ; this also allows us to constify the whole thing across the board.

double min = std::min_element(dataPoints.cbegin(), dataPoints.cend(),
              [](const std::map<std::string, double> &a,
                 const std::map<std::string, double> &b) {
                     return a.at("low") < b.at("low"); 
              })->at("low");

Answer 2

Map is used to store key/value pairs. There is no point using a map if you have 2 keys that are same all the time. You don't need these keys sorted either.

If given first value is high and second low then you can use:

std::pair <double, double>

This way, every time you add a new value you save two std::string sized memory space.

A vector is useful in storing unordered sequence. Depending on whether you want your data to be sorted you can use a set here with a compare based on high or low. This will speed up finding min value to constant time.

Set:

std::set <std::pair <double, double>>.

Set is a sorted container. With set you can use begin() if your lowest is at the top or end() if it at the bottom.

If you choose unsorted then use std::vector . If your container changes frequently and can be large , then you may need std::deque . If your container has a fixed size and you are not going to modify it then use std::array . You may use min_element here in this case. This type of search is linear time.

EDIT

If you want to stick with std::vector < std::map< std::string, double>> the following should work:

//min_element returns std::vector<std::map<std::string, double>>::iterator
auto minDataPoint = std::min_element(dataPoints.begin(), dataPoints.end(), [](std::map<std::string, double> &a, std::map<std::string, double> &b) { 
    return (a["low"] < b["low"]); 
});

std::cout<<"low "<<(*minDataPoint)["low"];

tested here: http://ideone.com/sdG6fk