comma operator usage with assignment operator in C

Question

#include<stdio.h>
#include<stdlib.h>
int main() {

    int x  = 5;
    int y = 0;
    x++, y = x*x;
    printf("x is %d\n", x);
    printf("y is %d\n", y);

}

Question: Why the output of the above code is:

x is 6
y is 36

instead of

x is 6
y is 25

?

Reasoning: I am thinking it should be the latter because assignment operator has higher precedence than comma and therefore first an assignment to y should happen setting it to 25 and then x should be evaluated and set to 6.

Please see [What does the comma operator `,` do in C?](http://stackoverflow.com/a/18444099/1708801) — Shafik Yaghmour, Jun 20 '14 at 20:41
What makes you think that precedence has anything to do with the order of evaluation? They're totally different things. — Jim Balter, Jul 01 '14 at 09:42

score 3 · Answer 1 · answered Jun 20 '14 at 20:42

The LHS of the comma operator has to be evaluated before the RHS of the comma operator; there is a full sequence point between the two.

Therefore, x++ has to be evaluated and all side-effects (the increment) must take place before the y = x * x part of the expression is considered or any part of it evaluated.

score 3 · Accepted Answer · edited Jun 20 '14 at 21:30

Precedence has to do with the syntax tree of your program, not with how the program executes. What those precedence levels are doing is disambiguating between

x++, (y = x*x) /* this is how your programs gets parsed */

and

(x++ ,  y) = x*x  /* this is NOT how it is parsed */

After your program gets parsed, the execution rules for , state that expressions are evaluated from left to right so the x++ gets run before the y = x*x. In the end, the , is very similar to a ;, except that you can put it inside places that expect expressions instead of statements.

score 1 · Answer 3 · answered Jun 20 '14 at 20:43

1

The precedence doesn't determine the order of evaluation. = binds highly than , so the expression is:

(x++), ( y = x*x;)

Comma is evaluated left to right and includes a sequence point so your expression is similar to:

x++ ; y = x*x ;

answered Jun 20 '14 at 20:43

this

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comma operator usage with assignment operator in C

3 Answers3