Comma Operator in C

Introduction

Comma operator is a special operator which serves two major purposes:

1. Comma as a separator.
2. Comma as an operator.

Let’s understand the difference between the two one by one.

Comma as a Separator

We have used commas as a separator several times in our programs. We use it to declare multiple variables in the same line. The following program demonstrates this fact:

#include <stdio.h>

int main()
{
    int x = 10, y = 20, z = 30;
    printf("%d %d %d", x, y, z);
    return 0;
}

Output:

10 20 30

In the above program, commas are used to separate three variables, x, y, and z, on the same line. Additionally, inside the printf function, commas are used to separate variables and the format string. This demonstrates how a comma can be used as a separator.

Comma as an Operator

Comma can be used as an operator which works a bit differently. 

Consider the following program:

#include <stdio.h>

int main()
{
    int x = 5;
    int y = (x++, ++x);
    printf("%d", y);
    return 0;
}

Can you guess the output?
The output of the above program is 7. But why?
Inside parentheses, the first operand (x++) will be evaluated first. The increment will happen, but it is not assigned to y. It means that the result of x++ is ignored. After that, ++x is evaluated and its result is assigned to y. This means y has received value 7. 

Note that it is important to use parentheses if you want to treat the comma as an operator, because the comma operator has the lowest precedence of all operators, even lower than the assignment operator.

Consider the following program:

#include <stdio.h>

int main()
{
    int x = 5;
    int y = x++, ++x;
    printf("%d", y);
    return 0;
}

Output:

Error: expected identifier or '(' before '++' token

Because the assignment operator has a higher precedence than the comma operator, the statement y = x++ will be evaluated first. However, the compiler then expects the Lvalue of ++x, which is not available, so it throws an error.

Precedence and Associativity of Comma Operator

Comma operator has the least precedence among all the operators as shown in the following table:

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