Sorting vectors in a loop

c++, gcc, mingw, sorting, vector

Solution

The two versions that don't work are making copies of the vectors, and sorting these, leaving your outer vector unchanged. You can use references instead:

for (unsigned int i = 0; i < myVectors.size(); i++) {
    vector<int>& vec = myVectors[i];
    sort(vec.begin(), vec.end());
}

and

for (auto& vec : myVectors) {
    sort(vec.begin(), vec.end(), pointCompare);
}

Problem

Sometimes I like c++ very much and sometimes get stuck in things I do not understand. I want to sort values in a vector in vectors. What is working: ``` for (unsigned int i = 0; i < myVectors.size(); i++) { sort(myVectors[i].begin(), myVectors[i].end()); } ``` What does not work: ``` for (unsigned int i = 0; i < myVectors.size(); i++) { vector<int> vec = myVectors[i]; sort(vec.begin(), vec.end()); } ``` and: ``` for (auto vec : myVectors) { sort(vec.begin(), vec.end()); } ``` The last two don't work and I don't know why I am using `mingw64, c++11`; Here is the full example: ``` #include <iostream> #include <vector> #include <algorithm> using namespace std; int main() { cout << "i vec1 vec2" << endl; for (int i = 0; i < 3; i++) { // create vectors vector<int> vec1; vec1.push_back(5); vec1.push_back(1); vector<int> vec2; vec2.push_back(4); vec2.push_back(2); vector<vector<int>> myVectors; myVectors.push_back(vec1); myVectors.push_back(vec2); switch(i) { case 0: for (unsigned int i = 0; i < myVectors.size(); i++) { sort(myVectors[i].begin(), myVectors[i].end()); } break; case 1: for (unsigned int i = 0; i < myVectors.size(); i++) { vector<int> vec = myVectors[i]; sort(vec.begin(), vec.end()); } break; case 2: for (auto vec : myVectors) { sort(vec.begin(), vec.end()); } break; } cout << i << " "; for (auto vec: myVectors) { for (int i : vec) { cout << i << " "; } cout << " "; } cout << endl; } return 0; } ``` Output: ``` i vec1 vec2 0 1 5 2 4 1 5 1 4 2 2 5 1 4 2 ```

Original source