Comma operator

For more details: http://comsciguide.blogspot.in/2015/06/comma-lowest-priority-operator.html

Comma Operator :

It is a binary operator (present in between the two operands),

mainly used in two ways :

1) As a separator

2) As a operator

As a Separator :

The comma is used to separate the expressions, and as a

separator in functional calls and definitions, multiple variable

declarations, enum declarations etc, like semicolon is used to

separate the statements (i.e. terminate the statements).

int a =5 , b=6;

void fun( a , b );

As a operator :

As a operator, It evaluates its first operand and discards the

result, then it evaluates the second operand and returns the value. It

has the lowest precedence (priority) of any c operator and has the

associativity from left to right.

int j ;

j = 1, 2, 3;

j = (1, 2, 3);


In first statement, assignment operator (=) has high priority as

compared to comma. So the expression becomes (j = 1), 2, 3; which

implies j =1;

In second statement, bracket has highest priority than any

other operator. So comma is executed first.

1, 2 will return 2 // left to right associativity

2, 3 will return 3

Therefore j = 3;

Note :

Assigning more than one value or initialization of multiple

variables at a time will results in compilation errors.

Example:

int a = b = c = 1; and int j = 1, 2, 3; results in errors.

int a, b , c; a = b = c = 1;

int j; j = 1 , 2 , 3;

We know that Comma operator has the lowest precedence as

compared to other operators. But in combination of semicolon

and comma, then semicolon has lowest precedence.


In expression it can be grouped as (a , b) ; (c , d). a , b ; c , d

Comma operator act as a Sequence point.

int i = 3 , j = 5;

int k;

k = i++, j--;

l = j--, i++;

In the above expressions, order of evaluation is important. Both

k, l have different values. Like if we take + operator, then

k = (i++) + (j--);

l = (j--) + (i++);

Here order of evaluation is not important. Both k, l have same

values.

Sometimes, the use of comma as separator is confused with as

operator.

void fun ( a() , b() ) // comma acts as a separator

Here we don’t consider which function is called first, as comma

just act as separator and don’t enforce any sequence. ( the standard

calling convention is always from right to left).

Example :

Printf( “ajay” , “ganesh” );


You may thought that, the answer should be “ganesh” ( left to

right associativity). Before evaluating the statement, we have to

check whether comma acts as an operator or separator. Here if we

consider as an operator, “ ganesh “ has to be printed.

But the output is “ajay”, because it acts as separator in function

parameters. (As I mentioned above standard calling convention,

parameters are passed from right to left ). In some books, it is also

mentioned that “In case of printf statement, once comma is read

then it will consider preceding things as variable or values for format

specifier”.

Comma acts as an operator only in expressions.

int i = 1, j = 2; // comma acts as a separator

int i;

i = 1, 2; // comma acts as a operator

Comma can be used to avoid a block and associated braces, as

in :

if (x == 1) { y = 2; z = 3; cout<<y<<z; }


Using comma’s, the above program is also written as

if (x == 1)

y = 2, z = 3, cout<<y<<z;

For practice :

1) int x = 10;

int y = (x++, ++x); printf("%d", y);

2) int x = 10, y;

y = (x++, printf("x = %d\n", x), ++x, printf("x = %d\n", x), x++); printf ("y = %d\n", y); printf ("x = %d\n", x); 3) a) return a=1, b=2, c=3; b) return (1), 2, 3; 4) int choice = 2 ; switch(choice)

{ case 1,2,1: printf("\ncase 1"); break;


case 1,3,2: printf("\ncase 2"); break; case 4,5,3: printf("\ncase 3"); break;

} 5) int x =1 , y =2; if ( y > x , y < x ) printf("\n y < x "); else printf("\n x < y ");

Education

Comma operator