警告：逗号右手操作数无效gcc 4.4.7

Question

I need your help for the following warning in Linux gcc 4.4.7

for (int iLoop1= 0; iLoop1< iLoopN; ++iLoop1, ++iLoop2, !iIsOk)

I have the following warnings coming from the last argument after the last comma

error: right-hand operand of comma has no effect

I have read the wiki page on comma operator but I do not understand the problem https://en.wikipedia.org/wiki/Comma_operator

EDIT : effectively !iIsOk does nothing I have tested the following code

// Example program
#include <iostream>
#include <string>
#include <iostream>

int main()
{

    int iIsOk = 0;

    for (int iLoop1 = 0; iLoop1 < 2; iLoop1++, !iIsOk)
    {
       std::cout << "IsOk=" << iIsOk << std::endl;
    }

      for (int iLoop1 = 0; iLoop1 < 2; iLoop1++, iIsOk = !iIsOk)
    {
       std::cout << "IsOk2=" << iIsOk << std::endl;
    }
}

Output:

IsOk=0
IsOk=0
IsOk2=0
IsOk2=1

Answer 1

!iIsOk although evaluated, does not do anything; ie removing it will have absolutely no effect on the program.

Your helpful compiler is warning you of this.

Did you mean to write !iIsOk as part of the stopping condition in the loop:

iLoop1< iLoopN && iIsOk

or toggle it using

iLoop1++, iIsOk = !iIsOk

or the flashier (which personally I find clearer since there are fewer variable repeats)

iLoop1++, iIsOk ^= true

Answer 2

First, let me tell you the reason behind the warning message. As per the comma operator properties, from C11 , chapter §6.5.17 ( emphasis mine )

The left operand of a comma operator is evaluated as a void expression; there is a sequence point between its evaluation and that of the right operand. Then the right operand is evaluated; the result has its type and value.

So, here, in your case, the final RHS operand is .... , !iIsOk , for which , the produced value is unused. Hence the warning. You need to either make use of the value, or you can get rid of the statement, as in it's current state, it has no effect on the program whatsoever.

That said, as mentioned in the other answer(s) by tilz0R and Bathsheba , per your "specific" requirement, the statement should be iIsOk = !iIsOk but I'd recommend putting this as the last statement in the loop body, for the sake of readability.

Answer 3

!iIsOk should be iIsOk = !iIsOk .

If C has operator like i++ or ++i , thing you did with !iIsOk does nothing. If you want result, you have to assing it somewhere.

To negate variable, use iIsOk = !iIsOk; and you will have desired effect.