A comma operator question

2.6

Consider the following C programs.

// PROGRAM 1
#include<stdio.h>

int main(void)
{
    int a = 1, 2, 3;
    printf("%d", a);
    return 0;
}

The above program fails in compilation, but the following program compiles fine and prints 1.

// PROGRAM 2
#include<stdio.h>

int main(void)
{
    int a;
    a = 1, 2, 3;
    printf("%d", a);
    return 0;
}

And the following program prints 3, why?

// PROGRAM 3
#include<stdio.h>

int main(void)
{
    int a;
    a = (1, 2, 3);
    printf("%d", a);
    return 0;
}

In a C/C++ program, comma is used in two contexts: (1) A separator (2) An Operator. (See this for more details).

Comma works just as a separator in PROGRAM 1 and we get compilation error in this program.

Comma works as an operator in PROGRAM 2. Precedence of comma operator is least in operator precedence table. So the assignment operator takes precedence over comma and the expression “a = 1, 2, 3” becomes equivalent to “(a = 1), 2, 3”. That is why we get output as 1 in the second program.

In PROGRAM 3, brackets are used so comma operator is executed first and we get the output as 3 (See the Wiki page for more details).

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