转置矩阵:交换元素不会改变值 [英] Transpose matrix: swapping elements doesn't alter values

问题描述

我正在尝试转置由向量构建的矩阵.

I am trying to transpose a matrix built with vectors.

这是我写的转置函数:

void transpose(std::vector<std::vector<int>>& fill_mat) {
    for (int i = 0; i < fill_mat.size(); ++i) {
        for (int j = 0; j < fill_mat.size(); ++j) {
            std::swap(fill_mat[i][j], fill_mat[j][i]);
        }
    }
}

似乎什么也没做:我的最终结果与开始的结果相同.

It doesn't seem to be doing anything: my final results are the same as the starting ones.

这是我的完整程序:

#include<iostream>
#include<vector>
#include<utility>

void print_matrix(std::vector<std::vector<int>>& to_print) {
    for (int i = 0; i < to_print.size(); ++i) {
        for (int j = 0; j < to_print.size(); ++j) {
            std::cout << " " << to_print[i][j];
        }
        std::cout << std::endl;
    }
}

void make_matrix(std::vector<std::vector<int>>& fill_mat) {
    for (int i = 0; i < fill_mat.size(); ++i) {
        for (int j = 0; j < fill_mat.size(); ++j) {
            fill_mat[i][j] = rand() % 15;
        }
    }
}

void transpose(std::vector<std::vector<int>>& fill_mat) {
    for (int i = 0; i < fill_mat.size(); ++i) {
        for (int j = 0; j < fill_mat.size(); ++j) {
            std::swap(fill_mat[i][j], fill_mat[j][i]);
        }
    }
}


int main() {
    int size = 3;
    std::vector<std::vector<int>> matrix_sample(size, std::vector<int>(size));

    make_matrix(matrix_sample);
    print_matrix(matrix_sample);

    transpose(matrix_sample);

    std::cout << "----## transpose ##-----" << std::endl;
    print_matrix(matrix_sample);
}

我希望打印出转置矩阵，但是输入最终与输出相同.我在做什么错了?

I expected to print out the transposed matrix but the input ends up being the same as the output. What am I doing wrong?

推荐答案

您将(i，j)与(j，i)交换了两次！这就是为什么它无效的原因.

You swap (i,j) with (j,i) twice! That's why it has no effect.

您应该只处理矩阵的一半.加上未成年人的改善，我们得到:

You should only work on one half of your matrix. Plus minors improvements, we get:

void transpose(std::vector<std::vector<int>>& fill_mat)
{
    using size_type = decltype(fill_mat)::size_type; // better use your matrix' size type
    for (size_type i = 0; i < fill_mat.size(); ++i) {
        for (size_type j = 0; j < i; ++j) {
            using std::swap; // see swap idiom
            swap(fill_mat[i][j], fill_mat[j][i]);
        }
    }
}

这篇关于转置矩阵:交换元素不会改变值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！