在评估中使用多个“大于/小于”比较器是否合适？

Question

I was trying to see if a variable was within a range by using multiple greater-than (>) 's and less-than (<) 's in one evaluation. I was unsure whether this works or not though. It seems than using multiple =='s is illigal , but is it okay with < and >'s?

Here's an example of what I'm trying to do:

if (bottomOfRange < variable < topOfRange) {
//Do stuff
}

I know that this:

if (bottomOfRange < varialbe && variable < topOfRange) {
//Do stuff
}

works, I'm just looking for an even more efficient way of comparing.

Will this evaluate how I want, or will it act differently?

Answer 1

No. This code will probably compile, depending on the types of the variables, but won't give the comparison you want. It will:

• evaluate bottomOfRange < variable to give a boolean result;
• promote that result to a numeric type with value 0 or 1;
• compare that value with topOfRange .

If you want to compare a value against two other values, then you need to write out the two comparisons:

if (bottomOfRange < variable && variable < topOfRange)

Answer 2

Assuming that bottomOfRange and topOfRange are built-in types, then no, it won't accomplish anything useful.

If you want to badly enough, you could define a type that overloaded operator< to make it actually work though:

class range {
    int lower;
    int val;
    range(int lower, int val) : lower(lower), val(val) {}
public:
    bool operator < (int upper) {
        return lower < val && val < upper;
    }

    class start {
        int t;
    public:
        start(int t) : t(t) {}
        range operator < (int val) { return range(t, val); }
    };
};

#ifdef TEST

#include <iostream>

int main(){
    static char const *s [] = { "Out of range", "In range" };

    int variable = 20;
    range::start bottomOfRange(10);
    int topOfRange(30);

    if (bottomOfRange < variable < topOfRange)
        std::cout << "In range\n";
    else
        std::cout << "out of range\n";

    bottomOfRange = 30;
    topOfRange = 10;

    if (bottomOfRange < variable < topOfRange)
        std::cout << "In range\n";
    else
        std::cout << "Out of range\n";
}
#endif

Result:

In range
Out of range

Warning: this is intended primarily (purely?) as a curio. Although it obviously supports exactly what you've asked for, I'd be pretty hesitant about using it in my own code, and I'm not really recommending that you use it in yours either.