使用 size_t 值反向遍历向量 [英] Traversing a vector in reverse direction with size_t values

问题描述

我想以相反的方向遍历向量的值.如您所知，向量的大小为 size_t.当我使用以下代码时:

I want to traverse through the values of a vector in opposite direction. As you know the size of a vector is of size_t. When I use the following code:

for(size_t r=m.size()-1; r >= 0; r--)
{
    x[r] = f[r];
    for(size_t c = r+1; c < m.size(); c++)
    {
        x[r] -= m[r][c] * x[c];
    }
}

我会超出向量的范围，因为r = 0递减后r会变成4294967295.

I will go out of the range of the vector because the r will become 4294967295 after decrementing r = 0.

我没有改变 r 的类型，因为在我的项目中，我将警告视为错误，所以它应该是 size_t 或者我应该转换它，这并不有趣.

I am not changing the r's type because in my project, I am treating warnings as errors, so it should be size_t or I should cast it which is not interesting.

推荐答案

如果您确实想使用 size_t 进行索引，则可以按如下方式制定循环.

If you actually want to use size_t for indexing, the loop could be formulated as follows.

for(size_t r = m.size(); r > 0; r--)
{
    x[r-1] = f[r-1];
    for(size_t c = r; c < m.size(); c++)
    {
        x[r-1] -= m[r-1][c] * x[c];
    }
}

基本上你会从 m.size() 迭代到 1 并通过在循环内移动来补偿；但是这个解决方案可能有点难以遵循.在这个问题中，建议的解决方案是使用reverse_iterator，可以看作是索引的一个合适的抽象.这个问题更深入地介绍了整个主题.

Basically you would iterate from m.size() to 1 and compensate by shifting inside the loop; but this solution might be a bit hard to follow. In this question, a proposed solution is to use a reverse_iterator, which can be seen as a suitable abstraction of the index. The entire topic is coverd in more depth in this question.

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