在C中使用char迭代器的FOR循环是否可行? [英] Are FOR loops with char iterators in C possible?

问题描述

使用无符号字符作为迭代器时遇到问题.使用以下代码将导致卡在FOR循环中. 输出看起来像这样.

I'm having a problem using an unsigned char as an iterator. Using the following code results in being stuck inside a FOR loop. The output looks like this.

unsigned char i;
char * arr;
int * freq;
arr = (char *)(malloc(256*sizeof(char)));
freq = (int *)(malloc(256*sizeof(int)));
for (i=0; i<=255;i++){
    arr[i]=i;
    freq[i]=0;
    printf("%c",i);
}

我的问题是为什么会这样?是由于使用无符号字符作为迭代器吗?

My question is why this happens? Is it due to using an unsigned char as an iterator?

推荐答案

i <= 255

如果i的类型为unsigned char，并且在您的平台上该类型为8位(可能是8位)，则表达式将始终为true，并且您将遇到无限循环.但是，这是定义明确的行为. unsigned char会简单地回绕，一旦达到255，其值就会再次从0开始.

If i is of type unsigned char and that type is 8 bit on your platform (which it likely is), then the expression will always be true and you have an infinite loop. It is well-defined behavior, though. The unsigned char will simply wrap around and its value will start at 0 again once it has reached 255.

解决方案当然很简单:将i的类型更改为int.

The solution is simple of course: change the type of i to int.

为避免将来出现此类意外情况，请确保打开编译器的警告.如果我使用GCC编译假循环并将其传递给-Wall和-Wextra标志，它将告诉我.

To avoid such surprises in the future, make sure to turn on your compiler's warnings. If I compile the bogus loop with GCC and pass it the -Wall and -Wextra flags, it will tell me this.

main.c: In function ‘main’:
main.c:5:17: warning: comparison is always true due to limited range of data type [-Wtype-limits]
   for (i = 0; i <= 255; ++i)
                 ^

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