C if statement comparing two integer variables to the same constant

Question

I am trying to compare two integer variables to a same constant. Apparently in this case, both variables have the same default values. I tried to simplify the comparison statement like this:

if (po->app_tag == po->inst_id == 0)
   return 1;

It didn't work.

I had to code it like:

if ((po->app_tag == 0) && (po->inst_id == 0))
   return 1;

To make it work. Can anyone explain to me the difference between the two? Somehow I feel that C compiler should have a provision for this kind of syntax when it comes to comparison of two integer variables. I can be wrong asking for it.. :), but want to understand a reason.. Why am I wrong here.

Answer 1

Due to associativity of the == operator (it associates from left to right),

po->app_tag == po->inst_id == 0

is interpreted as:

(po->app_tag == po->inst_id) == 0

That's how the language is defined.

Using

(po->app_tag == 0) && (po->inst_id == 0)

fixes that problem by evaluating po->app_tag == 0 and po->inst_id == 0 independently and performing a boolean AND between those two.

See http://en.cppreference.com/w/c/language/operator_precedence for more info on operator precedence and associativity.