矢量分配崩溃，不知道为什么 [英] Vector assignment crashing, not sure why

问题描述

vector< vector<int> > resizeVector(vector< vector<int> > m)
{
    vector< vector<int> > newMatrix;
    int i,j;

    for (i = 0; i < m[i].size(); i++)
    {
        for(j = 0; j < m[j].size(); j++)
        {
            newMatrix[i][j] = m[i][j];
        }
    }
    return (newMatrix);
}

我正在制作一个可以进行大量矩阵操作的程序，这部分是崩溃，我不知道为什么。我把它缩小到一行：

I am making a program that will do a whole lot of matrix manipulation, and this section is crashing and I don't exactly know why. I have narrowed it down to the line:

newMatrix[i][j] = m[i][j];

它在这里崩溃，我不知道为什么。

It crashes right here, and I am not sure why.

推荐答案

除了@Saurav发布的内容之外， newMatrix 是空的，因此您不能将值分配给 newMatrix [i] [j] 。您可以通过初始化给定大小的向量来解决此问题：

In addition to what @Saurav posted, newMatrix is empty so you cannot assign values to newMatrix[i][j]. You can fix this by initializing the vectors with a given size:

vector< vector<int> > resizeVector(vector< vector<int> > m)
{
    vector< vector<int> > newMatrix(m.size());
    int i,j;

    for (i = 0; i < m.size(); i++)
    {
        newMatrix[i].resize(m[i].size());
        for(j = 0; j < m[i].size(); j++)
        {
            newMatrix[i][j] = m[i][j];
        }
    }
    return (newMatrix);
}

在for循环之前，我们初始化 newMatrix 有 m.size（）其中有许多空向量（由于它们的默认构造函数，向量为空）。在外部for循环的每次迭代期间，我们确保 newMatrix 中的每个向量使用 resize 成员具有正确的大小

Before the for-loops we initialize newMatrix to have m.size() many empty vectors inside of it (the vectors are empty due to their default constructor). During each iteration of the outer for-loop we ensure that each vector within newMatrix has the correct size using the resize member function.

请注意，如果你想要一个矢量的副本，你可以简单地写：

Note that if you want a copy of a vector you can simply just write:

vector< vector<int> > newMatrix(m);

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