Is it possible to insert an if condition inside a return statement in C?

Question

My purpose is trying to write a function as concise and short as possible.

int func(void)
{
    int a;
    return (
        a = 42,
        a++,
        if (a > 42) a *= -1,
        a);
}

I was expecting to return a -43. Instead I've got a compilation error.

Answer 1

My purpose is trying to write a function as concise and short as possible.

int a;

return (
   a = 42,
   a++,
   if (a > 42) a *= -1,
   a);

Can simply be rewritten as:

return -43;

Or if that's too short for your liking, then you're looking for the conditional operator (colloquially referred to as the ternary operator), which has the form:

/* if a is logically true (does not evaluate to zero) 
*  then evaluate expression b, otherwise 
*  evaluate expression c
*/
a ? b : c;

So the return statement can be rewritten as:

a = 42;
return ++a > 42 ? -a : a;

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As of your objective, then there's no merit to it. You should not write clever code. It harms readability and maintainability. (Although in this case, it doesn't)

Remember:

1. Simplicity is the ultimate sophistication. — Leonardo da Vinci
2. Any fool can write code that a computer can understand. Good programmers write code that humans can understand. ― Martin Fowler
3. Everything should be made as simple as possible — but no simpler. — Albert Einstein (attributed).¹

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[1] — credit: @SteveSummit