for(unsigned char i = 0; i <= 0xff; i ++)产生无限循环 [英] for (unsigned char i = 0; i&lt;=0xff; i++) produces infinite loop

问题描述

为什么下面的c代码以无限循环结尾?

Why does the following c code end up in an infinite loop?

for(unsigned char i = 0; i <= 0xff; i++){}

与以下结果相同:

for(unsigned char i = 0; i <= 0xff; ++i){}

我必须以哪种方式修改代码，以使其按预期工作(不使用int或unsigned int数据类型)?

In which way do I have to modify the code, to get it working as expected (without using int or unsigned int datatype)?

推荐答案

典型的for循环依赖于能够在循环的最后迭代之后 检测终止条件.在您的情况下，正如其他答案所指出的那样，i <= 0xff始终为真(假定i的类型为unsigned char且通常为UCHAR_MAX==0xff).

A typical for loop relies on being able to detect the termination condition after the last iteration of the loop. In your case, as other answers have pointed out, i <= 0xff is always true (given that i is of type unsigned char and that UCHAR_MAX==0xff, which is typical).

您可能会在任何整数类型的边界附近遇到相同类型的问题.例如，这:

You can run into the same kind of problem near the bounds of any integer type. For example, this:

for (int i = INT_MAX - 9; i <= INT_MAX; i ++) {
    /* ... */
}

(可能)也是一个无限循环(除了有符号整数类型的溢出具有未定义的行为，而与无符号整数的定义明确的环绕语义相反，优化的编译器可以利用这一点，但我离题了) ).只是不要这样做.

is (probably) also an infinite loop (except that overflow for signed integer types has undefined behavior as opposed to the well-defined wraparound semantics for unsigned integers, and an optimizing compiler can take advantage of that and -- but I digress). Just don't do that.

许多解决方案之一是将测试移到循环的底部，然后递增:

One of many solutions is to move the test to the bottom of the loop, before the increment:

for (unsigned char i = 0; ; i ++) {
    printf("%d\n", i);
    if (i == 0xff) break;
}

for循环的第二个子句是终止条件，在每次迭代之前进行评估.如果将其保留为空，它将一如既往地为您提供无限循环(for (;;)是简单无限循环的常见用法).我们在循环的底部测试i是否与0xff 相等.如果是，那么我们就执行了最后一次迭代，就可以跳出循环了.

The second clause of a for loop is the termination condition, evaluated before each iteration. If you leave it empty, it's treated as always true, giving you an infinite loop (for (;;) is a common idiom for a simple infinite loop). We test whether i is equal to 0xff at the bottom of the loop. If it is then we've just executed the last iteration, and we can break out of the loop.

(有些人可能更喜欢在这里使用while循环，但我喜欢for，因为它使我们可以将循环控制变量的声明和增量结合在一个结构中.)

(Some might prefer to use a while loop here, but I like the for because it lets us combine the declaration of the loop control variable and the increment in a single construct.)

(严格来说，unsigned char的最大值不一定是0xff或255，但是它将出现在您可能会遇到的任何系统上.某些DSP的实现具有CHAR_BIT > 8，因此UCHAR_MAX > 255.)

(Strictly speaking the maximum value of unsigned char isn't necessarily 0xff or 255, but it will be on any system you're likely to encounter. Implementations for some DSPs have CHAR_BIT > 8, and thus UCHAR_MAX > 255.)

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