Question

Other than in a for statement, when is the comma operator used?

Answer 1

The comma operator is commonly used to separate variable declarations, function arguments, and expressions, as well as the elements of a for statement. Look closely at the following program, which shows some of the many ways a comma can be used:

#include <stdio.h>
#include <stdlib.h>
void main(void);
void main()
{
     /* Here, the comma operator is used to separate
        three variable declarations. */
     int i, j, k;
     /* Notice how you can use the comma operator to perform
        multiple initializations on the same line. */
     i = 0, j = 1, k = 2;
     printf("i = %d, j = %d, k = %d\n", i, j, k);
     /* Here, the comma operator is used to execute three expressions
        in one line: assign k to i, increment j, and increment k.
        The value that i receives is always the rightmost expression. */
     i = (j++, k++);
     printf("i = %d, j = %d, k = %d\n", i, j, k);
     /* Here, the while statement uses the comma operator to
        assign the value of i as well as test it. */
     while (i = (rand() % 100), i != 50)
          printf("i is %d, trying again...\n", i);
     printf("\nGuess what? i is 50!\n");
}

Notice the line that reads

i = (j++, k++);

This line actually performs three actions at once. These are the three actions, in order:

1. Assigns the value of k to i. This happens because the left value (lvalue) always evaluates to the rightmost argument. In this case, it evaluates to k. Notice that it does not evaluate to k++, because k++ is a postfix incremental expression, and k is not incremented until the assignment of k to i is made. If the expression had read ++k, the value of ++k would be assigned to i because it is a prefix incremental expression, and it is incremented before the assignment is made.

2. Increments j.

3. Increments k.

Also, notice the strange-looking while statement:

while (i = (rand() % 100), i != 50)
     printf("i is %d, trying again...\n");

Here, the comma operator separates two expressions, each of which is evaluated for each iteration of the while statement. The first expression, to the left of the comma, assigns i to a random number from 0 to 99.

The second expression, which is more commonly found in a while statement, is a conditional expression that tests to see whether i is not equal to 50. For each iteration of the while statement, i is assigned a new random number, and the value of i is checked to see that it is not 50. Eventually, i is randomly assigned the value 50, and the while statement terminates.