C ++反向"for"循环 [英] C++ reverse 'for' loop

问题描述

我有一个向量:

std::vector<int> vec = {1, 2, 3};

我想做一个反向的for循环.当我写的时候它有效:

And I want to make a reverse for loop. It works, when I write:

for(int i = vec.size() - 1; i >= 0; --i) {
    std::cout << i << std::endl; // 2, 1, 0
}

但是如果我写的话，我得到的数字非常大(如18446744073709223794)

But I get a very large number (like 18446744073709223794) if I write:

for(size_t i = vec.size() - 1; i >= 0; --i) {
    std::cout << i << std::endl;
}

但是当我写这篇文章时，它们都可以工作:

But they both work when I write:

for(int i = 0; i < vec.size() - 1; ++i) {
    std::cout << i << std::endl; // 1, 2, 3
}

// Or
for(size_t i = 0; i < vec.size() - 1; ++i) {
    std::cout << i << std::endl; // 1, 2, 3
}

为什么使用size_t时矢量的尺寸错误?

Why do I get the wrong size of the vector when I use size_t?

我认为转换存在问题.

推荐答案

让编译器告诉您出什么问题了！

如果您在启用警告的情况下编译程序，则编译器会告诉您这样的话:

<source>: In function 'int main()':

7:43: warning: comparison of unsigned expression in '>= 0' is always true [-Wtype-limits]
    7 |     for(std::size_t i = vec.size() - 1; i >= 0; --i) {
      |                                         ~~^~~~

那是为什么?这是因为 std::size_t 是 unsigned 类型在C ++中；它仅代表非负数.阅读有关打开警告及其重要性的更多信息:为什么我应该始终启用编译器警告?

Why is that? It's because std::size_t is an unsigned type in C++; it only represents non-negative numbers. Read more about turning on warnings and why it's important: Why should I always enable compiler warnings?

我已决定将此处的答案拆分为一个独立的问题，与OP的错误无关.请继续阅读.

I've decided to split my answer here off to a separate question, independent of OP's bug. Please go read it.

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