Part
– 6: More on Functions
IN lesson 1 we that each C program consists
of one or more functions. main() is a special function because program
execution starts from it.
A function is combined of a block of code
that can be called or used anywhere in the program by calling the name. Body of
a function starts with ‘{‘ and ends with ‘}’ . This is similar to the main
function in our previous programs. Example below shows how we can write a
simple function.
Example
6-1: example6-1.c
#include
/*Function prototypes*/
myfunc();
main()
{
myfunc();
system("pause");
}
myfunc()
{
printf("Hello, this is a test\n");
}
In above example we have put a section of
program in a separate function. Function body can be very complex though. After
creating a function we can call it using its name. Functions can also call each
other. A function can even call itself. This is used in recursive algorithms
using a termination condition (to avoid the program to enter in an infinite
loop). For the time being do not call a function from inside itself.
By the way pay attention to “function
prototypes” section. In some C compilers we are needed to introduce the
functions we are creating in a program above the program. Introducing a
function is being called “function prototype”.
Most of C programming language commands are
functions. For example “printf” is a function that
accepts one or more arguments and does
printing. Bodies of these functions are defined in C library files which are
included in your C compiler. When you use a C function, compiler adds its body
(where the actual code of the function exists) to your program and then creates
the final executable program. Function prototypes (introduction before use) of
internal C functions is in header files (i.e stdio.h, …) which are included at
the top of a program.
Reasons
for using functions
There are many reasons for using functions.
• You may need to reuse a part of code many
times in different parts of a program.
• Using functions, program will be divided to
separate blocks. Each block will do a specific job.
Designing, understanding and managing smaller
blocks of code is easier.
• A block of code can be executed with
different numbers of initial parameters. These parameters
are passed to function with function
arguments.
Assume we want to read scores of students
from a file, calculate their average and print results. If we write the program
just inside a single main() function block we will have a large function. But
if we spread the program functionality to different functions, we will have a
small main() function and several smaller functions which perform their
specific task.
main()
{
Read_scores();
calculate()
write_to_file();
print_results()
}
Now let’s look at this example:
Example
6-2: example6-2.c
#include
#include
add();
subtract();
multiply();
main()
{
char choice;
while(1)
{
printf("\nMenu:\n");
printf("1- Add\n2- Subtract\n");
printf("3- Multiply\n4- Exit");
printf("\n\nYour choice -> ");
choice=getch();
switch(choice)
{
case '1' : add();
break;
case '2' : subtract();
break;
case '3' : multiply();
break;
case '4' : printf("\nProgram Ends.
!");
exit(0);
default:
printf("\nInvalid choice");
}
}
}
add()
{
float a,b;
printf("\nEnter a:");
scanf("%f",&a);
printf("\nEnter b:");
scanf("%f",&b);
printf("a+b=%f",a+b);
}
subtract()
{
float a,b;
printf("\nEnter a:");
scanf("%f",&a);
printf("\nEnter b:");
scanf("%f",&b);
printf("a-b=%f",a-b);
}
multiply()
{
float a,b;
printf("\nEnter a:");
scanf("%f",&a);
printf("\nEnter b:");
scanf("%f",&b);
printf("a*b=%f",a*b);
}
Function
arguments
Functions are able to accept input parameters
in the form of variables. These input parameter variables can then be used in
function body.
Example
6-3: example6-3.c
#include
/* use function prototypes */
sayhello(int count);
main()
{
sayhello(4);
system("pause");
}
sayhello(int count)
{
int c;
for(c=0;c<=""
font="">
printf("Hello\n");
}
In above example we have called sayhello()
function with the parameter “4”. This function receives an input value and
assigns it to “count” variable before starting execution of function body.
sayhello() function will then print a hello message ‘count’ times on the
screen.
Again we use a function prototype before we
can call a function inside main() or another function in our program. As you
see these prototypes are put after pre-compiler commands (those starting with #
sign), before main() function at the top of our program. You can copy function
header from the function itself for the prototype section. However do not
forget that function prototype needs a semicolon at its end while in function
itself it does not.
If you do not put prototypes of your
functions in your programs you might get an error in most of new C compilers.
This error mentions that you need a prototype for each of your functions.
Function
return values
In mathematics we generally expect a function
to return a value. It may or may not accept arguments
but it always returns a value.
y=f(x)
y=f(x)=x+1
y=f(x)=1 (arguments are not received or not
important)
In C programming language we expect a
function to return a value too. This return value has a type as other values in
C. It can be integer, float, char or anything else. Type of this return value
determines type of your function.
Default type of function is int or integer.
If you do not indicate type of function that you use, it will be of type int.
As we told earlier every function must return
a value. We do this with return command.
Sum()
{
int a,b,c;
a=1;
b=4;
c=a+b;
reurn c;
}
Above function returns the value of variable
“c” as the return value of function. We can also use expressions in return
command. For example we can replace two last lines of function with ‘return
a+b;’
If you forget to return a value in a function
you will get a warning message in most of C compilers. This message will warn
you that your function must return a value. Warnings do not stop program
execution but errors stop it.
In our previous examples we did not return
any value in our functions. For example you must return a value in main()
function.
main()
{
.
.
.
return 0;
}
Default return value for an int type function
is 0. If you do not insert ‘return 0’ or any other value in your main()
function a 0 value will be returned automatically. If you are planning to
return an int value in your function, it is seriously preferred to mention the
return value in your function header and make.
“void”
return value
There is another “void” type of function in C
language. Void type function is a function that does not return a value. You
can define a function that does not need a return value as “void”.
void test ()
{
/* fuction code comes here but no return
value */
}
void functions cannot be assigned to a
variable because it does not return value. So you cannot write:
a=test();
