链接多个大于/小于运算符

Question

In an if statement I want to include a range, eg:

if(10 < a < 0)

but by doing so, I get a warning "Pointless comparison". However, this works fine without any warning:

if(a<10 && a>0)

Is the first case possible to implement in C?

Answer 1

Note that the original version if(10 < a < 0) is perfectly legal. It just doesn't do what you might (reasonably) think it does. You're fortunate that the compiler recognized it as a probable mistake and warned you about it.

The < operator associates left-to-right, just like the + operator. So just as a + b + c really means (a + b) + c , a < b < c really means (a < b) < c . The < operator yields an int value of 0 if the condition is false, 1 if it's true. So you're either testing whether 0 is less than c, or whether 1 is less than c.

In the unlikely case that that's really what you want to do, adding parentheses will probably silence the warning. It will also reassure anyone reading your code later that you know what you're doing, so they don't "fix" it. (Again, this applies only in the unlikely event that you really want (a < b) < c) .)

The way to check whether a is less than b and b is less than c is:

a < b && b < c

(There are languages, including Python, where a < b < c means a<b && b<c , as it commonly does in mathematics. C just doesn't happen to be one of those languages.)

Answer 2

这是不可能的，您必须像在案例 2 中那样拆分支票。

Answer 3

No it is not possible.
You have to use the second way by splitting the two conditional checks.

Answer 4

The first does one comparison, then compares the result of the first to the second value. In this case, the operators group left to right, so it's equivalent to (10<a) < 0 . The warning it's giving you is really because < will always yield 0 or 1. The warning is telling you that the result of the first comparison can never be less than 0, so the second comparison will always yield false.

Even though the compiler won't complain about it, the second isn't really much improvement. How can a number be simultaneously less than 0, but greater than 10? Ideally, the compiler would give you a warning that the condition is always false. Presumably you want 0<a<10 and a>0 && a<10 .

You can get the effect of the second using only a single comparison: if ((unsigned)a < 10) will be true only if the number is in the range 0..10. A range comparison can normally be reduced to a single comparison with code like:

if ((unsigned)(x-range_start)<(range_end-range_start))
    // in range
else
    // out of range.

At one time this was a staple of decent assembly language programming. I doubt many people do it any more though (I certainly don't as a rule).

Answer 5

As stated above, you have to split the check. Think about it from the compiler's point of view, which looks at one operator at a time. 10 < a = True or False. And then it goes to do True/False < 0, which doesn't make sense.

Answer 6

不，这不是 if 语句的有效语法，它应该有一个有效的常量表达式，或者其中可能有逻辑运算符，并且仅在括号中的表达式计算结果为真或非零值时执行