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 a , b ; c , d it can be grouped as
(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 ");
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