multidimensional array - Delete column in 2D std::vector depending on comparison of 2 rows in same column in C++

Question

I have a 2D std::vector < std::vector <double> > array consisting of three rows and an unknown number of columns. I would like to delete any column where the value of row 0 > row 1. Here is my effort:

#include <iostream>
#include <vector>

using namespace std;

template <typename T>
inline void printArray (const std::vector< std::vector< T > >& key_array) {
    for (int i = 0; i < key_array.size(); i++) {
        for (int j = 0; j < key_array[i].size(); j++) {
            std::cout << key_array[i][j] << "";
        }
        std::cout << "
";
    }
    std::cout << "
";
}

int main()
{
    std::vector< std::vector <int> > fog {{1,1,2,1},{2,2,1,2},{3,3,3,3}};
    
    printArray(fog);
    
    for (int i = 0; i < fog[0].size(); ++i) {
                for (int j = 0; j < fog.size(); ++j) {
                    if (fog[0][i] > fog[1][i]) {
                        fog[j].erase(fog[j].begin() + i);
                    }
                }
            }
            
    printArray(fog);

The array is initially:

1       1       2       1                                                                                                      
2       2       1       2                                                                                                      
3       3       3       3 

I would like it to become:

1       1       1                                                                                                      
2       2       2                                                                                                      
3       3       3 

Instead I get:

1       1       1                                                                                                              
2       2       1       2                                                                                                      
3       3       3       3   

I imagine the problem is because the size of the array is changing as elements are deleted, invalidating the loop conditions, and/or iterators are being removed. But with this observation, I have reach my (rather circumscribed) limits.

I would be most grateful for any advice.

question from:https://stackoverflow.com/questions/65898918/delete-column-in-2d-stdvector-depending-on-comparison-of-2-rows-in-same-column

Answer 1

Your code is "correct" but does not work in-place. One solution would be to copy the array and replace it in the copy while iterating over the old one:

#include <iostream>
#include <vector>

using namespace std;

template <typename T>
inline void printArray (const std::vector< std::vector< T > >& key_array) 
{
    for (int i = 0; i < key_array.size(); i++) {
        for (int j = 0; j < key_array[i].size(); j++) {
            std::cout << key_array[i][j] << "";
        }
        std::cout << "
";
    }
    std::cout << "
";
}

int main()
{
    std::vector< std::vector <int> > fog {{1,1,2,1},{2,2,1,2},{3,3,3,3}};
    
    printArray(fog);
    
    auto fog2 = fog;

    for (int i = 0; i < fog[0].size(); ++i) 
    {
        for (int j = 0; j < fog.size(); ++j) 
        {
            if (fog[0][i] > fog[1][i]) 
            {
                fog2[j].erase(fog2[j].begin() + i);
            }
        }
    }
    
    printArray(fog2);      
}

If want to do it in-place you need to change the for-loop logic.