CODEWITHSUNDEEP
c++vector
justTom
justTom

Reputation: 11

How to add a column to a multi-dimesional vector (C++)?

This might be easy but I just cant find a solution. I created a 2D vector like this:

vector<vector<int> > matrix;
vector<int> row;

for (int i = 0; i < x;i++) {
    row.push_back(i);
}
for (int i = 0; i < x; i++) {
    matrix.push_back(row);
}

The problem I have is that I now wanna add rows and columns as I need them because I might run out of matrix space over time but I don't know how to do it. The rows are easy I can just .push_back another row to the bottom end of the matrix...but I have no Idea how to add another column.

I'm sorry if this question is super stupid but I'm not that experienced in programming yet and I couldn't find anything on the particular problem of adding columns to a 2D vector.

Upvotes: 0

Views: 2496

Answers (3)

lewis
lewis

Reputation: 1263

vector<vector<T>> is a BAD way to implement a matrix. Well - it may not be a bad backend implementation, but its a bad public API.

I'd suggest using some library designed to support a Matrix. A few come to mind:

https://github.com/SophistSolutions/Stroika/blob/V2.1-Release/Library/Sources/Stroika/Foundation/Math/LinearAlgebra/Matrix.h
https://docs.opencv.org/3.4/d3/d63/classcv_1_1Mat.html
http://arma.sourceforge.net/docs.html#part_classes

or write your own and implement the basic functions directly.

Upvotes: 1

Fureeish
Fureeish

Reputation: 13424

Here is a simple function that will add a column to your matrix at a specified position:

void add_column(std::vector<std::vector<int>>& matrix,
                const std::vector<int>& column, std::size_t position)
{
    if(matrix.size() < position || position < 0) return; // check for position correctness

    std::size_t index = 0;
    for(auto& row : matrix){
        row.insert(row.begin() + position, column[index++]);
    }
}

For example, given a matrix:

std::vector<std::vector<int>> matrix;

and initializing it like so:

std::vector<int> column;
for (int i = 0; i < 3; i++) {
    column.push_back(3);
}
for (int i = 0; i < 3; i++) {
    matrix.push_back(column);
}

you end with with 3x3 matrix of 3s. Now, adding a column in between the first and the second one:

add_column(matrix, std::vector<int>({9, 9, 9}), 1);

and printing it:

for (auto& row : matrix) {
    for (const auto x : row) {
        std::cout << x << ' ';
    }
    std::cout << std::endl;
}

You end up with:

3 9 3 3
3 9 3 3
3 9 3 3

Upvotes: 0

ggorlen
ggorlen

Reputation: 56945

Adding to your code, the idea is to loop over each row in the matrix and push a new element onto each vector. Together, all of these elements are a new column.

#include <iostream>
#include <vector>
using namespace std;

int main() {

    // Initialize a 3x3 matrix
    vector<vector<int>> matrix;
    vector<int> row;
    int x = 3;

    for (int i = 0; i < x; i++) {
        row.push_back(i);
    }

    for (int i = 0; i < x; i++) {
        matrix.push_back(row);
    }

    // Add a column; the matrix is now 3x4
    int newElement = 3;

    for (auto &row : matrix) {
        row.push_back(newElement);
    }

    // Print the matrix
    for (auto &row : matrix) {
        for (auto &cell : row) {
            cout << cell << ' ';
        }

        cout << endl;
    }

    return 0;
}

You'll likely want to separate this code into functions.

Upvotes: 2

Related Questions