The comma operator has two uses - as a separator and as an operator. As a separator, it is used to separate expressions, variable declarations, function calls, and other items. As an operator, it evaluates the first operand and discards the result, then evaluates the second operand and returns its value. It has the lowest precedence of any C operator and associates from left to right. The document provides several examples demonstrating the comma operator's usage and precedence relative to other operators like assignment, brackets, and semicolons.

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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);

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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.

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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” );

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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;
}

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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 = %dn", x), ++x, printf("x = %dn", x), x++);
printf ("y = %dn", y);
printf ("x = %dn", 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");

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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 ");