逗号表达式的左侧操作数无效

Question

for (count = index, packet_no = 0; 
     count < TOTAL_OBJ, packet_no < TOTAL_PKT; 
     count++, packet_no++)

=> left-hand operand of comma expression has no effect.

I find the above code is correct and could not understand why this error comes.

Answer 1

This is how the comma operator works, what you want to do is to use OR or AND (probably AND in your case):

// the condition for resuming the loop is that one of the conditions is true
count < TOTAL_OBJ || packet_no < TOTAL_PKT
// the condition for resuming the loop is that both conditions are true
count < TOTAL_OBJ && packet_no < TOTAL_PKT

Answer 2

You have three comma operators in each of the three terms of the for statement. The warning is for term 2.

Both expressions of terms 1 and 3 are executed as expected.

The left operation of the term 2 is evaluated only as a void, doesn't take part in the for condition, and therefore leads to your warning.

Answer 3

The conditional statement(for, while or if ) having conditional expressions with comma operator(s), the value of last expression is the conditional value(True or False) of conditional statement. For ex.

int i = 1;
int j = 0;
int k = 1;
if(i, j, k) {
   printf("Inside");
}else {
   printf("Outside");
}

prints "Outside" as comma operator is evaluated from left to right and k is the last expression evaluated in if statement which returns false.

int i, j;
if(i = 0 , j = 1) {
   printf("Inside");
}else {
   printf("Outside");
}

Above prints "Inside". j = 1 is the last expression evaluated inside the if statement which has true value.

Answer 4

int i = 1;
int j = 0;
int k = 1;
if(i, j, k) {
   printf("Inside");
}else {
   printf("Outside");
}

Correction to above: This code will print "Inside" as the comma operator is evaluated from left to right and k is the last expression evaluated in the if statement, which returns true since k = 1.