How to get indices of a Mat with specific value? - c++

Question

I want to find indices of an array that equal with specific value. so i've Written this code:
vector<int> _classes = { 2,2,1,1,3,3,3,3,5,5,4,4,5,6,6 };
vector<int> labelVec = {1,2,3,4,5,6};
vector<int> index;
for (int i = 0; i < labelVec.size(); i++)
{
compare(_classes, labelVec[i], index, CMP_EQ);
std::vector<int>::iterator nn = find(index.begin(), index.end(), 255);
}
but i have this error : Unhandled exception at 0x760B5608 in compareFuncTest.exe: Microsoft C++ exception: cv::Exception at memory location 0x004DDC44. if i define index as Mat, this problem will be resolved. but if i define index as Mat, i can't use from find(). also in this documentation states: output array (in my code as index) that has the same size and type as the input arrays. PLZ help me to fix this code.

Answer 1

I still do not get what is the point of this test, I guess this will be in some other algorithm... So, I give you two possible solutions.
1) Without OpenCV
First, you must know that
std::vector<int>::iterator nn = find(index.begin(), index.end(), 255);
Will only give you the first occurrance. Knowing this, here is a way you could check if the label is inside the _classes vector.
#include <iostream>
#include <vector>
#include <algorithm>
int main()
{
std::vector<int> _classes = { 2,2,1,1,3,3,3,3,5,5,4,4,5,6,6 };
std::vector<int> labelVec = {1,2,3,4,5,6,7};
for (const auto& label: labelVec)
{
std::vector<int>::iterator nn = find(_classes.begin(), _classes.end(), label);
if (nn != _classes.end())
{
std::cout << "I got the value from _classes: " << *nn << std::endl;
} else
{
std::cout << "I couldn't find the value with label:" << label << std::endl;
}
}
}
Here I iterate over all the labels (as you did) and then use the find directly in the classes, but with the label variable. Then I check if I found the label or not, if not, it will give you a value equal to _classes.end() which will give error if you try to use it (look at the extra label 7 which is not found).
This example can be tested here online.
2) With OpenCV
no oline test here. But this one is also easy to do. If you have a Mat in index you will only need to change the iterators to be templated. Like this:
auto nn = find(index.begin<int>(), index.end<int>(), 255);
If you a cv::Mat of classes you can also do it as in the method before and skip the comparison part (this would be faster)
Update
Since you want is the indices and all of them, then you have to iterate over it :/ if you wanted the values you could have used copy_if. You can create a lambda function to easily do the job.
like this:
#include <iostream>
#include <vector>
#include <algorithm>
int main()
{
auto getIndices = [](const std::vector<int>& vec, const int value){
std::vector<int> result;
for (size_t t = 0; t < vec.size(); t++)
{
if (vec[t] == value)
{
result.push_back(static_cast<int>(t));
}
}
return result;
};
std::vector<int> _classes = { 2,2,1,1,3,3,3,3,5,5,4,4,5,6,6 };
std::vector<int> labelVec = {1,2,3,4,5,6,7};
for (const auto& label: labelVec)
{
std::vector<int> nn = getIndices(_classes, label);
std::cout << "I got the following indices for value"<< label<< ": [ ";
for (const auto& n : nn)
{
std::cout << n << ",";
}
std::cout << " ]" << std::endl;
}
}