Call By Value:
Creates a new memory loaction for use within the 
subroutine.The memory is freed once it leaves the 
subroutine.Changes made to the variable are not affected 
outside the subroutine.
Call By Reference:
Passes a pointer to the memory location.Changes made to the 
variable within the subroutine affects the variable outside 
the subroutine.

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Call by value method:
passing the value of variable to the function.
void main()
{
int x=10,y=20;
printf("%d%d',x,y);
swap(x,y);
}
void swap(int a,int b)
{
int c;
c=a;//changes here do not affect in values
a=b;//of x and y in main function..
b=c;
}

call by reference method:
passing the address of variable to the function.
swap(&a,&b)
&c=&a;
&a=&b;
&b=&c;
changes made in subfunction causes changes in address of 
variables n thus in main () also.. 

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call by value :
call by value means programmer send some value coping from
one function to another.At the time of function calling a
programmer can send a copy of variable of value.

call by referance :
it means sending sending the address of variable to the
called function means a user can send the address of 
variable.