虽然语句不能进入 [英] Cannot enter in while statement

问题描述

可能重复：结果
  无符号和符号比较结果
   unsigned int类型和符号字符比较

我有一个奇怪的现象，当我尝试在这个while语句进入：

I have a strange behavior when i try to enter in this while statement:

unsigned u = 0;
int i = -2;

while(i < u)
{
    // Do something
    i++;
}

但它从来没有进入，即使当我设置一个断点 I = -2 和 U = 0 。
我究竟做错了什么？我怎么能解决这个问题？

But it never enters, even if when i set a break point i = -2 and u = 0. What am I doing wrong? How could i fix this?

推荐答案

这是因为ANSI C标准定义，只要有一个合格的（你的之间的比较无符号整数U ）和非限定的类型（你的 INT I ），非限定类型提升为一种同类型的（因此总是 INT ），但也继承了其他数量的预选赛（即变成无符号）。

It's because the ANSI C standard defines that whenever there is a comparison between a qualified (your unsigned int u) and a non qualified type (your int i), the non qualified type gets promoted to a type of the same type (thus always int), but also inherits the qualifiers of the other quantity (i.e. it becomes unsigned).

当你的 INT ，其值等于 -2 ，becames 无符号的第一个字节经历这种转变： 0000 0010 - ＆GT; 1111 1110 。你的 INT 现在是一个非常大的正数，你的 unsigned int型的肯定更大。

When your int, whose value is equal to -2, becames unsigned the first byte undergoes this transformation: 0000 0010 -> 1111 1110. Your int is now a very large positive number, certainly larger of your unsigned int.

有一个解决方案：强制转换为签署

There is a solution: cast to signed

while(i < (signed) u)
{
    // Do something
    i++;
}

顺便说一句，也许你的编译器应该给你一个警告。

By the way, probably your compiler should give you a warning.

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