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Chapter-4
User Defined Functions (UDFs)

syntax :

return type name(datatype variable, datatype variable,….); functions are set of instructions combined together and given the name & can be used whenever you want by making a call to that function. There are two types of functions : User defined functions & Library functions. Both printf and scanf are examples of functions, Other functions supplied by the system in what are called libraries.

A function without any arguments
 
main()
{
display();
}

display()
{
printf("Syspro");
}

This program makes a call to ‘display’ function which prints Syspro on screen. Its is necessary that the function to which the call is made should be defined in the same file.
 
 

Passing the arguments to function :

while passing the arguments it is important to define datatype of the argument. By default C considers that the passed value is of integer type.
 
main()
{
sqr(8);
}

sqr(int val)
{
printf("squre of %d is %d", val, val*val);
}


 

The above program makes a call to sqr function which calculates the squre of the number, as well as displays the square of the number.

As per the programming rules any function should be cohesive which means it should solve only one purpose, it should not display the square of the number passed. Just calculate the square and return it.

Returning the value from the function

This program makes a call to sqr function which only calculates the square and returns calculated value which is stored in x variable.
 
main()

{
int x;
x=sqr(8);
printf("\nsqure of 8 is %d", x);
}

sqr(int val)
{
return(val*val);
}


 

Passing & returning different datatypes.

This program makes a calls to function
float area_of_circle( float );

main()
{

float area;
area=area_of_circle(2.4);
printf("Area of circle is %f",area);
}

area_of_circle(float radius)
{
return(3.14*radius*radius);
}

Scope of Variable

Variables can be of two types one local, another global, depending upon place of declaration scope of variable changes. Variable declared within a function is treated as local variable. Variable declared before main function is treated as global variable. Local variable is accessible locally to that function and not outside that function, but global variables can be accessed anywhere in the program.
 
main()
{
int loc=30;
printf("\n loc = %d",loc);
callfunc();
}

callfunc()
{
printf("\n loc = %d",loc); 
/* this statement will give the error message undefined symbol loc */
}

Above program will give an error message in callfunc() function since the variable loc is defined in main function and is not available for callfunc() function. The same variable can be made available by making it of global type.
 
int g;

main()
{
g=34;
printf("Value of g is %d",g);
showg();

}

showg()
{
printf("Value of g is %d",g);
}


 

int g;

This program will show the g variable since it is of global variable. A global variable can be declared by defining it above the main function.

Call by Value

Prameters passed to function are transfered through call by value method. Which means though we transfer the variables to function but it takes value only from them. Actual variables are not getting affected.
 
main()
{
int x=20,y=30;
printf("\n In main Function x = %d, y=%d",x,y);
swap(x,y);
printf("\n In main Function x = %d, y=%d",x,y);
}

swap(int x,int y)
{

int t;
t=x;
x=y; 
y=t;
printf("\n In swap Function x = %d, y=%d",x,y);
}

This program shows that variables defined in main function when passed to swap function swaps the contents but as soon as you return to main function they values you find are intact.
 

Here swap function is having its own local variables with the same name as x and y. These variables are modified by swaping but actual x and y variable remain intact as they were since variables are never passed to function only values of variables are transfered.

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