C中的复合条件:if (0.0 <a <1.0) [英] Compound condition in C: if (0.0 &lt; a &lt; 1.0)

问题描述

我最近注意到以下表达式在我的编译器 (Clang) 中编译:

I recently noticed that the following expression compiles in my compiler (Clang):

float a;
if (0.0 < a < 1.0) { ... }

这是否符合我的预期?我希望它的评估结果与此条件相同:

Does this do what I expect? I would expect that it evaluates identically to this condition:

if (0.0 < a && a < 1.0) { ... }

如果是这样，那么何时以及使用哪些其他编译器可以以这种简短形式编写条件?

If it is so, since when and with which other compilers it is possible to write conditions in this short form?

推荐答案

由于 < 运算符的从左到右结合性，表达式条件 (0.0 表示 ((0.0 == 1 <1.0 或 0 <1.0 取决于 a 的值.

Because of left-to-right associativity of < operator the expression condition (0.0 < a < 1.0) means ((0.0 < a) < 1.0) == 1 < 1.0 or 0 < 1.0 depending on value of a.

所以不，它与 if (0.0 < a && a < 1.0)(也许你可能会从 Python 同情规则中混淆)但在 C 中它会被渗透为我在上面解释过.

So no, its not identical to if (0.0 < a && a < 1.0) (perhaps you might confusing from Python compassion rule) but in C it will be interpenetrated as I explained above.

您可以在示例中观察到的差异 --- 0.0 <＜1.0 == true 当 a == 0.0, where as (0.0 < a && a< 1.0) == false，对于a == 0.0，下面是我的代码(阅读评论):

A difference you can observe in an example --- 0.0 < a < 1.0 == true when a == 0.0, where as (0.0 < a && a < 1.0) == false, for a == 0.0, below is my code (read comments):

#include<stdio.h>
void print_(int c){
    c ? printf("True 
"):
        printf("False 
");
}
int main(void){
    float a = 0.0f;
    print_(0.0f < a < 1.0f); // 0.0 < 0.0 < 1.0f == 0 < 1.0f == True
    print_(0.0f < a && a < 1.0f); // 0.0f < 0.0f && ... ==  False && ... = False
    return 0;
}

输出:

True 
False

检查它的工作@Ideone

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