Question

In C++, the usual way of determining if some value, x, is between two limits is to:

  //This is (A)
double x = 0.0d;
double lower = -1.0d;
double upper = +1.0d;

if(x > lower && x < upper){
  // Do some stuff
}

But today I discovered by accident that I can do this:

  // This is (B)
double x = 0.0d;
double lower = -1.0d;
double upper = +1.0d;

if(lower < x < upper){
  // Do some stuff
}

It seems to work fine, but I’ve never heard of this being done before, with “lower < x < upper”. Does this produce executable code as you would expect it to? IE, is (A) equivalent to (B)?

I think a lot of people won’t know about this, and I suspect that might be because the compiler interprets (A) differently to (B). It this right?

Answer 1

No, A and B are not equivalent, you cannot do this.

Or, obviously you can (as you discovered) but you’re not doing what you think you’re doing.

You’re evaluating (lower < x) < upper, i.e. the value of lower < x (which is false or true, but which convert to int for the comparison) is compared to upper.

See this table of operator precedence for more information.