如何检查整数是偶数还是奇数? [英] How do I check if an integer is even or odd?

问题描述

如何在 C 中检查给定数字是偶数还是奇数?

How can I check if a given number is even or odd in C?

推荐答案

使用模(%)运算符判断除以2时是否有余数:

Use the modulo (%) operator to check if there's a remainder when dividing by 2:

if (x % 2) { /* x is odd */ }

一些人批评了我上面的回答，指出使用 x &1 是更快"或更高效".我不相信是这样的.

A few people have criticized my answer above stating that using x & 1 is "faster" or "more efficient". I do not believe this to be the case.

出于好奇，我创建了两个简单的测试用例程序:

Out of curiosity, I created two trivial test case programs:

/* modulo.c */
#include <stdio.h>

int main(void)
{
    int x;
    for (x = 0; x < 10; x++)
        if (x % 2)
            printf("%d is odd
", x);
    return 0;
}

/* and.c */
#include <stdio.h>

int main(void)
{
    int x;
    for (x = 0; x < 10; x++)
        if (x & 1)
            printf("%d is odd
", x);
    return 0;
}

然后我在我的一台机器上用 gcc 4.1.3 编译了 5 次:

I then compiled these with gcc 4.1.3 on one of my machines 5 different times:

• 没有优化标志.
• 带 -O
• 使用 -Os
• 使用 -O2
• 使用 -O3

我检查了每次编译的汇编输出(使用 gcc -S)，发现在每种情况下，and.c 和 modulo.c 的输出都是相同的(它们都使用了 andl $1, %eax 指令).我怀疑这是一个新"功能，我怀疑它可以追溯到古代版本.我也怀疑任何现代(过去 20 年制造的)非神秘编译器，无论是商业的还是开源的，都缺乏这种优化.我会在其他编译器上进行测试，但目前没有可用的.

I examined the assembly output of each compile (using gcc -S) and found that in each case, the output for and.c and modulo.c were identical (they both used the andl $1, %eax instruction). I doubt this is a "new" feature, and I suspect it dates back to ancient versions. I also doubt any modern (made in the past 20 years) non-arcane compiler, commercial or open source, lacks such optimization. I would test on other compilers, but I don't have any available at the moment.

如果其他人愿意测试其他编译器和/或平台目标并得到不同的结果，我很想知道.

If anyone else would care to test other compilers and/or platform targets, and gets a different result, I'd be very interested to know.

最后，模版本保证无论整数是正数、负数还是零，无论实现对有符号整数的表示形式如何，模数版本都能正常工作.按位与版本不是.是的，我意识到二进制补码无处不在，所以这不是一个真正的问题.

Finally, the modulo version is guaranteed by the standard to work whether the integer is positive, negative or zero, regardless of the implementation's representation of signed integers. The bitwise-and version is not. Yes, I realise two's complement is somewhat ubiquitous, so this is not really an issue.

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