This looks so right, but its so wrong? multiple operators in a condition

[arizona99]

int a = 0;
if ( 5 < a < 10 )
   std::cout << "Yep";

It would seem like C++ should evaluate the condition as 5 < a AND a < 10. Why doesn't it work that way? And is there a term for this situation?

-thanks

[teknomage1]

The term is improper syntax. What you're really saying is either (true > 10) or (5 < true) depending on how the compiler implements it, neither of which make any sense. The proper syntax would be ( 5 < a && a < 10 ).

EDIT: I hope that doesn't come off as abrasive, so here's a bit more explanation. The < and > operators are not greater than and less than from mathematics, they are comparison operators that compare the value of the expression on the left to the value of the expression on the right and return a boolean true or false.
As such, they cannot be used declaratively like in mathematics because as soon as one is evaluated the comparison no longer has any concept of its input. That's why you need to add a logical operator as glue for the two comparisons, because the logical operator deals with the true or false answers the comparison operators yeild.

[arizona99]

its alright. thank you

[kyoryu]

Originally Posted by arizona99

int a = 0;
if ( 5 < a < 10 )
   std::cout << "Yep";

It would seem like C++ should evaluate the condition as 5 < a AND a < 10. Why doesn't it work that way? And is there a term for this situation?

-thanks

Specifically, you have to remember that the statements are evaluated individually. Any compound boolean statement has to be broken down into individual statements, which are then evaluated and chained together as appropriate. The statement 5 < a < 10 is the same as ( 5 < a ) < 10.

In this case, 5 < a < 10, the first thing to be evaluated is '5 < a'

This is false. False values are evaluated as 0, while true ones (from boolean operators, anyway), are evaluated as 1. So we swap out '5 < a' for '0'.

This leaves us with '0 < 10', which is true, so the code executes. While tekno's right on about how it should be implemented, it's actually not improper syntax, per se. It's just not syntax that's doing what you want.

[teknomage1]

You are correct, kyoryu, but trusting the compiler to implement things from left to right is not always a valid assumption. And though booleans can be interpreted as 1 and 0, it's probably not the way you want to think about them most of the time.

[kyoryu]

Originally Posted by teknomage1

You are correct, kyoryu, but trusting the compiler to implement things from left to right is not always a valid assumption.

If you're using a compiler that's even reasonably compliant, it is.

http://www.cppreference.com/operator_precedence.html

It's not a good idea, though, and separating things with parens never hurts.
Confusing code is bad code.

My point was explaining WHY and WHAT was happening. It's worth noting, especially for new programmers, that there's only one ternary operator in c/c++, and that < isn't it.

Originally Posted by teknomage

And though booleans can be interpreted as 1 and 0, it's probably not the way you want to think about them most of the time.

No, but knowledge of how C++ actually handles bools, and how it resolves complex statements can help when running into similar problems. You handled the "how do I fix it" bit very well, I just wanted to add the "and this is why it didn't work in the first place" bit