乘法和除法:C语言中的奇怪输出 [英] Multiplication and division: weird output in c

问题描述

我读到*(乘法)比/(除法)具有更高的优先级.因此，如果同时存在*和/的方程，则*必须首先发生.

I read that * (multiplication) has has higher presedence than / (division). Thus if there is an equation with both * and /, then * must take place first.

但是我看过一个程序，它输出一些奇怪的东西

But I've seen a program that output something strange

#include<stdio.h>
#define SQUARE(x) x*x

int main()
{
    float s=10, u=30, t=2, a;
    a = 2*(s-u*t)/SQUARE(t);
    printf("Result = %f", a);
    return 0;
}

运行此命令时，我以为输出为-25，但实际上为-100.

When running this, I thought that the output would be -25, but in fact it was -100.

当我寻找解释时

 Step 2: a = 2*(s-u*t)/SQUARE(t); becomes,

// Here SQUARE(t) is replaced by macro to t*t
=> a = 2 * (10 - 30 * 2) / t * t;

=> a = 2 * (10 - 30 * 2) / 2 * 2;

=> a = 2 * (10 - 60) / 2 * 2;

=> a = 2 * (-50) / 2 * 2 ;  

/*till here it's OK*/
/*why it divided -50 by 2 before multiplying 2*2 and -50*2 */
=> a = 2 * (-25) * 2 ;

=> a = (-50) * 2 ;

=> a = -100; 

有人可以解释吗?

推荐答案

肢体偏执狂！您的宏应为:

Parenthesis paranoia! Your macro should be:

#define SQUARE(X) ((x)*(x))

否则，优先级规则和宏扩展会做一些奇怪的事情.您的宏:

Or else precedence rules and macro expansion will do weird things. Your macro:

100 / SQUARE(2)

将扩展为:

100 / 2*2

，它的意思是:

(100/2) * 2

是100，而不是25.

which is 100, not 25.

其他异常，而不是代码中的异常是，如果您尝试对表达式求平方:

Other anomaly, not in your code is if you try to square an expression:

SQUARE(2+2)

将扩展为

2+2*2+2

是8，而不是预期的16.

结论:在宏中写很多括号.到处都是.

Conclusion: in macros write a lot of parentheses. Everywhere.

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