Two variables in a 'for' loop in C

Question

I am writing some code where I need to use two variables in a for loop. Does the below code seem alright?

It does give me the expected result.

for (loop_1 = offset,loop_2 = (offset + 2); loop_1 >= (offset - 190),loop_2 <= (190 + offset + 2); loop_1--,loop_2++)
{
    if (  (*(uint8_t*)(in_payload + loop_1) == get_a1_byte(bitslip)) &&
         ((*(uint8_t*)(in_payload + loop_2) == get_a2_byte(bitslip)))
       )
    {
          a1_count++;
    }
}

But I am getting a compiler warning which says:

file.c:499:73: warning: left-hand operand of comma expression has no effect

What does this mean?

Answer 1

The problem is the test condition:

loop_1 >= (offset - 190),loop_2 <= (190 + offset + 2)

This does not check both parts. (Well, it does, but only the result of the second part is used.)

Change it to

(loop_1 >= (offset - 190)) && (loop_2 <= (190 + offset + 2))

if you want both conditions to be checked.

Answer 2

Mat is correct, but you should probably consider simplifying your code to:

for (i = 0; i <= 190; i++)
{
    uint8_t *pl1 = (uint8_t *)(in_payload + offset - i);
    uint8_t *pl2 = (uint8_t *)(in_payload + offset + i + 2);

    if (*pl1 == get_a1_byte(bitslip) && *pl2 == get_a2_byte(bitslip))
    {
        a1_count++;
    }
}

(You can obviously hoist the calculation of in_payload + offset out of the loop too, but the optimiser will almost certainly do that for you).