Comma Operator in C - Sanfoundry

This C Tutorial explains Comma Operator in C with examples.

What is a Comma operator in C?
The comma operator is a binary operator that evaluates its first operand and discards the result, then evaluates its second operand and returns its value. Comma operator represented by ‘,’ ensures the evaluation from left to right, one by one, of two or more expressions, separated with commas, and result of entire expression is value of rightmost expression.

For example:

    int a = 10, b = 20, c = 30;
    int x;
 
    printf("Value of exp. "x = a + b, b + c, c + a;" is %d\n", 
        (x = a + b, b + c, c + a));
 
    /* value of x is value of rightmost sub-exp. c + a */

But what happens if we mistakenly write the above example this way:

     int a = 10, b = 20, c = 30;
     int x;
 
     printf("Value of exp. "x = a + b, b + c, c + a;" is %d\n", 
             x = a + b, b + c, c + a);
     /* value of x is value of leftmost sub-exp. a + b */

Actually, in the above printf() statement, this time there are three arguments, separated with commas, following format string for one format specifier. All expressions are evaluated but value of x = a + b is printed. To avoid such problems, enclose the entire expression in pair of parenthesis.

Sanfoundry Certification Contest of the Month is Live. 100+ Subjects. Participate Now!

Example 1:

    int x, y, z, a = 10, b = 20, c = 30;
 
    x = a + b, y = b + c, z = c + a;
 
    printf("In x = a + b, y = b + c, z = c + a; x is %d y is %d"
           "and z is %d", x, y, z);
    /* above statement is same as x = a + b; y = b + c; z = c + a; */

Example 2:

/* comma1.c -- how comma operator works */
#include <stdio.h>
int main(void)
{
    int x, y, z;
    int  a = 10, b = 20, c = 30;
 
    printf("The value of entire expression a + b, b + c, c + a" 
             "is %d\n", (a + b, b + c, c + a));
    printf("Value of the entire exp. a, a++, a++; is %d\n",
             (a, a++, a++));
 
    return 0;
}

Usage of Comma Operator in C:

This comma operator is of great worth in multiple initializations, updating values in for and while loop statements.

Check this: BCA MCQs | Advanced C Programming Videos

Example 1: (comma operator using for loop)

/*comma2.c----usage of comma operator */
#include <stdio.h>
int main(void)
{
    int x, y, z;
 
    for (x = getval(), y = x + 1; y >= 0 ; x++, y = x + 1) {
          ...
        x = getval();
    }         
 
    return 0;
}

Example 2: (comma operator using while loop)

/*comma3.c----use with while loop test exp. */
#include <stdio.h>
int main(void)
{
    int a;
 
    a = getval();
    count(a);
    while (a > 0) {
        ...	
        a = getval();
        count(a);
    }
 
    return 0;
}

In the above program, test exp is preceded by two statements to obtain the value of a and the same pair of statements are needed at the end of while loop. However, with comma operator, we can rewrite this loop as:

/*comma3.c----use with while loop test exp. */
#include <stdio.h>
int main(void)
{
    int a; 
 
    while (a = getval(), count(a), a > 0) { 
        ...
    }
 
    return 0; 
}

Or even in more compact form as:

    while (count(a = getval()), a > 0) {
        ...
    }

With this approach, just one copy of the code is needed. Comma operator makes the source program easier to maintain. In future, if there needs some changes, only one copy of the code is to be fixed.

Sanfoundry Global Education & Learning Series – 1000 C Tutorials.

If you wish to look at all C Tutorials, go to C Tutorials.

« Prev - Conditional Operator in C

» Next - What does sizeof Operator Return in C?

Next Steps: