Is (4 > y > 1) a valid statement in C++? How do you evaluate it if so?

Question

Is that a valid expression? If so, can you rewrite it so that it makes more sense? For example, is it the same as (4 > y && y > 1)? How do you evaluate chained logical operators?

Answer 1

The statement (4 > y > 1) is parsed as this:

((4 > y) > 1)

The comparison operators < and > evaluate left-to-right.

The 4 > y returns either 0 or 1 depending on if it's true or not.

Then the result is compared to 1.

In this case, since 0 or 1 is never more than 1, the whole statement will always return false.

---

There is one exception though:

If y is a class and the > operator has been overloaded to do something unusual. Then anything goes.

For example, this will fail to compile:

class mytype{
};

mytype operator>(int x,const mytype &y){
    return mytype();
}

int main(){

    mytype y;

    cout << (4 > y > 1) << endl;

    return 0;
}

Answer 2

Expression Validity

Yes, it is a valid expression, assuming y is, or can be implicitly converted into, an integer. If it is not and the > operator is overloaded, it's a different story outside the scope of this question.

It will be evaluated left to right as ((4 > y) > 1).

Assuming y is an integer, let's consider the two possibilities. 4 > y can return true or false. The next part effectively becomes true > 1 or false > 1.

Given the implicit bool to int conversion, there are two possibilities: A) 4 > y returns true. true evaluates to 1. 1 > 1 evaluates to false. B) 4 > y returns false. false evaluates to 0. 0 > 1 evaluates to false.

No matter what, the expression will evaluate to false.

Rewritten Interpretation

I assume what you intend is ((4 > y) && (y > 1)).

Example

(4 > y > 1) is not the same as (4 > y && y > 1).

Logical Operators

The logical operators (!, &&, ||) use short-circuiting logic.

Given a && b, a will be evaluated. If a evaluates to true, then b will be evaluated. Else, b will not be evaluated. As for a || b, short-circuiting logic works in reverse. a will be evaluated. Since expression a is evaluated first, if it is false, there is no possibility that the entire expression will evaluate true.

Given a || b, a will be evaluated. If a evaluates to false, then b will be evaluated. Else, b will not be evaluated. Since expression a is evaluated first, if it is true, there is no possibility that the entire expression will evaluate false.

Chaining the operators is a matter of operator precedence. Better to use parentheses and be clear rather than risk the wrong behavior.

Answer 3

I think it's a valid expression (not a statement), but probably doesn't do what you want. It evaluates left-to-right, as (4 > y) > 1. The test 4 > y will evaluate to either 0 (false) or 1 (true), and the entire expression will always evaluate to 0 (false).

Answer 4

4 > y will evaluate to a boolean value of true or false. The remainder of the expression is then essentially [true|false] > 1, which does not make sense.

Answer 5

4 > y > 1 --> MAY BE ANYWAY if y - is class!!.

 #include <iostream>
 #include <string>

 struct num{ int n; };

 struct op{ bool v; struct num n; };

 struct op  operator > (int x, num n){
  struct op o = { x > n.n, n };
  return o;
 }
 bool operator > (struct op o, int x)
{
    return o.v && o.n.n > x;
}

int main()
{


    struct num y = { 2 } ;
    if (  4 > y > 1 ) {  std::cout << "4 > y > 1 TRUE" << std::endl; }
    else { std::cout << "4 > y > 1  ALWAYS FALSE" << std::endl; }
}