Compile Error with using lamda function with max_element

Question

I am trying to write some code to find the contour with the maximum size in a std::vector of contour.

I have the following error

error: conversion from ‘__gnu_cxx::__normal_iterator<std::vector<cv::Point_<int> >*, std::vector<std::vector<cv::Point_<int> > > >’ to non-scalar type
‘std::vector<cv::Point_<int> >::iterator {aka __gnu_cxx::__normal_iterator<cv::Point_<int>*, std::vector<cv::Point_<int> > >}’ requested
    std::vector<cv::Point2i>::iterator it = std::max_element(contours.begin(), contours.end()

Below is my code

std::vector<std::vector <cv::Point2i>> contours;
std::vector<cv::Vec4i> hierarchy;

cv::findContours(rImg, contours, hierarchy,CV_RETR_EXTERNAL, CV_CHAIN_APPROX_NONE, cv::Point(0, 0));
cv::Mat blank = cv::Mat::zeros(frame.size(), CV_8UC3);
cv::RNG rng;

std::vector<cv::Point2i>::iterator it = std::max_element(contours.begin(), contours.end(),
                                                        [](const std::vector<cv::Point2i>& p1, 
                                                        const std::vector<cv::Point2i>& p2)
                                                        { return p1.size()< p2.size(); });


std::vector<std::vector<cv::Point2i> > contourV;
contourV.push_back(it);

Would like to know what is wrong and how to correct them

that's why the keyword `auto` was invented. – Walter May 08 '19 at 16:52 — Walter, May 08 '19 at 16:52

Vlad from Moscow · Accepted Answer · 2019-05-08T16:29:03.127

2

You are using an object of the type

std::vector<std::vector <cv::Point2i>> contours;

in the std::max_element algorithm

So the iterator shall correspond to that container. That is

std::vector<std::vector <cv::Point2i>>::iterator it = std::max_element( /*...*/ );

Or it would be even simpler to write

auto it = std::max_element(contours.begin(), contours.end(),
                           [](const std::vector<cv::Point2i>& p1, 
                              const std::vector<cv::Point2i>& p2)
                              { return p1.size()< p2.size(); });

as @melpomene pointed out in a comment.

edited May 08 '19 at 16:29

answered May 08 '19 at 16:22

Vlad from Moscow

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1

Or just `auto` it. – melpomene May 08 '19 at 16:28
1

Even simpler: `auto it = std::max_element(contours.begin(), contours.end(), [](const auto& p1, const auto& p2) { return p1.size() < p2.size(); });` – Jesper Juhl May 08 '19 at 16:50
thanks for the great help....guys... just started out using lamda function in programming... – user1538798 May 09 '19 at 00:33

Compile Error with using lamda function with max_element

1 Answers1