Thread: Conditional operator: second operand is neither a throw-expression nor of type void

hello,
have this piece of code and wonder why void funtion is causing the problem:

Code:

void TurnLed( int flag){
    if(flag){
        digitalWrite(PinLed, HIGH);
        } else {
            digitalWrite(PinLed, LOW);
        }
    }

  void loop() {
  Serial.println(digitalRead(PinBtn));

  (state == digitalRead(PinBtn)) ? : (state=digitalRead(PinBtn) , TurnLed(state) ) ;

  delay(300);               // wait for a second
 }

Code:

/home/az/work/Arduino/Sketches/Examples/Push_Led/Push_Led.ino:35:82: error: third operand to the conditional operator is of type 'void', but the second operand is neither a throw-expression nor of type 'void'
   (state == digitalRead(PinBtn)) ? : (state=digitalRead(PinBtn) , TurnLed(state) ) ;
                                                                                  ^

[Stino - Exit with error code 1.]

Hi,
The second and third operand of a ? : operator need to have the same type. Because there is no second operand the compiler can't deduce the type of the operand.
Try to rewrite the statement as an if/then/else expression.

Cheers

Code:

Serial.println(digitalRead(PinBtn));

In addition, this is a C forum, and the code you posted is C++.

Originally Posted by talahin

Hi,
The second and third operand of a ? : operator need to have the same type. Because there is no second operand the compiler can't deduce the type of the operand.
Try to rewrite the statement as an if/then/else expression.

Cheers

i've been trying to get the conditional operator to work. Always used "if/then".

(state == digitalRead(PinBtn)) ? : (state=digitalRead(PinBtn) , (void)TurnLed(state) ) ;
nor
(state == digitalRead(PinBtn)) ? void : (void)(state=digitalRead(PinBtn) , TurnLed(state) ) ;

make any difference.

Originally Posted by rstanley

Code:

Serial.println(digitalRead(PinBtn));

In addition, this is a C forum, and the code you posted is C++.

yeah, you are right of-course. Just thought that there is no difference between implementations in these two languages.

Hi,
Try

Code:

if (state != digitalRead(PinBtn)) {
    state = digitalRead(PinBtn) , TurnLed(state) ;
}

Also you're reading the button 3 times in your loop. What happens if the button is pressed or released between reading the button states.

Cheers

Hi,

thank you. So , no way to twist the "?" operator?

First it's not the "?" operator, but "? :" operator. It's a ternary operator, meaning it needs 3 operands.
As you noticed, leaving out one of the operands will make the compiler complain.

Code:

state = digitalRead(PinBtn) , TurnLed(state);

The comma character in your statement is an operator not a seperator. It will first evaluate the statement on the left side of the comma, drop the result and then evaluate the right side and use the result of that. This is why it was complaining in your original code, because the result type of TurnLed() is void.

Cheers

Originally Posted by talahin

Hi,

The comma character in your statement is an operator not a seperator. It will first evaluate the statement on the left side of the comma, drop the result and then evaluate the right side and use the result of that. This is why it was complaining in your original code, because the result type of TurnLed() is void.

Cheers

i did not know that. Thank you!

Originally Posted by babo

yeah, you are right of-course. Just thought that there is no difference between implementations in these two languages.

There is no difference in the use of the ternary operator in C and C++, but since you have not provided the entire code, you might have other questions or problems that would be better asked in the C++ forum.