Please explain the comma operator in this program

Question

Please explain the output of this program:

int main()
{    
    int a,b,c,d;  
    a=10;  
    b=20;  
    c=a,b;  
    d=(a,b);  
    printf("\nC= %d",c);  
    printf("\nD= %d",d);  
}

The output which I am getting is:

C= 10  
D= 20

My doubt is what does the "," operator do here?
I compiled and ran the program using Code Blocks.

possible duplicate of [What does the comma operator \`,\` do in C?](http://stackoverflow.com/questions/52550/what-does-the-comma-operator-do-in-c) — Mohit Jain, Jul 09 '15 at 13:04

Eduardo · Accepted Answer · 2013-02-16T14:58:31.513

24

The , operator evaluates a series of expressions and returns the value of the last.

c=a,b is the same as (c=a),b. That is why c is 10

c=(a,b) will assign the result of a,b, which is 20, to c.

As Mike points out in the comments, assignment (=) has higher precedence than comma

edited Feb 16 '13 at 14:58

answered Feb 16 '13 at 14:49

Eduardo

2

+ and `=` has higher precedence than `,`. – Mike Dunlavey Feb 16 '13 at 14:50
@Eduardo +1 but, although answer for `d=(a,b)` is sort of clear but adding that in your explanation will make your answer complete – exexzian Feb 16 '13 at 14:57
@Eduardo Sir, But what happens if there are 3 values. x=(a,b,c,d); So here the last value (d) is assigned to x? – Swamy Feb 16 '13 at 16:08
@Swamy: That's correct, `d` will be the value assigned to `x`. – Eduardo Feb 16 '13 at 17:21

score 12 · Answer 2 · answered Feb 16 '13 at 14:51

12

Well, this is about operator precedence:

c=a,b

is

equivalent to

(c=a),b

The point is, the "," operator will return the second value.

Thus

c=a,b

assigns a to c and returns b

d=(a,b)

returns b and assigns it to d

answered Feb 16 '13 at 14:51

Udo Klein

score 5 · Answer 3 · answered Feb 16 '13 at 14:49

5

The comma operator evaluates all its operands, then yields the value of the last expression.

answered Feb 16 '13 at 14:49

3 Answers3