28.1.2001Sudeshna Sarkar, IIT Kharagpur 1 Functions Lecture 9 28.1.2001.

28.1.2001 Sudeshna Sarkar, IIT Kharagpur1

Functions

Lecture 928.1.2001


Class Test

on 7th February Thursday

5:30 to 6:30 pm


A Problem

Suppose we are writing a program that displays messages on the screen.

We want to display rows of =============================== to separate sections of output.


Solution#include <stdio.h>

int main (void)

{

/* produce some output */

/* print banner line */

printf(“==============\n”) ;

/* produce more output */

printf(“==============\n”);

/* produce even more output */

printf(“==============\n”);

/* produce final output */

}


Critique

Redundant code What if we want to change the display

e.g. to print a blank line before and after each line with =’s ?

What if we want to print banner lines in some other program?


The Solution: Functions

Definition: A function is a named code sequence

A function can be executed by using its name as a statement or expression.

The function may have parameters - information that can be different each time the function is executed.

The function may compute and return a value.


Why use functions ?

Functions provide an abstraction when writing a program - allow low-level details to be packaged.

Able to package a computation we need to perform at multiple spots within a program. Write once, use many times.

If changes are needed, they only have to be done once, in one place.


Why use functions ?

Allows the programmer to create a program from smaller, simpler sub-components.

Many programs are far too large to understand all at once.

Functions give us a way to break a large program into smaller pieces

A function should perform a well-defined task.


Common functions

int main (void)

{

. . .

}

Function definitionfor main()

printf (“%d + %d is %d\n”,x,y,z);

scanf (“%d%d”, &x, &y) ;

limit = sqrt ((double) num);

Function calls


More common functions

Every standard C compiler comes with a set of standard libraries.

#include <stdio.h> Tells the compiler you will use the standard IO library

#include <math.h> C’s standard math library functions:

sqrt, pow, sin, cos, exp, ...

isspace, ...


Defining your own functions

You may define your own functions in your programs

You define a function by giving its name and writing the code that is executed when the function is called.


High level programming structuremain ()

{

. . .

SetUpBoard ();

do {

redsTurn ();

blacksTurn ();

} while (gamenotover ());

}


Function definition


void print_banner (void)

{

printf (“=========”);

printf (“=========\n”);

}

heading comment

function name

function body(statements to be executed)A function can haveANY number of ANYkind of statements



void print_banner (void)

{

printf (“=========”);

printf (“=========\n”);

}

voidvoid has two different roles in this function definition.

indicates that the functiondoes not return (have) anoutput value.

indicates that the functionhas no parameters.


Calling a functionint main (void){ /* produce some output */ . . . print_banner ();

/* produce more output */ . . . print_banner (); /* produce final output */ . . . print_banner ();}

To execute the function,it is called or invokedfrom within a program or another function.

Note: A function that does not return a value can be called wherever a statement is allowed.


Terminology

main () is the caller print_banner() is the callee. main() invokes / calls print_banner() 3

times. print_banner() is called from main()


Function Control Flow/* print banner line */void print_banner (void){

printf(“**************\n”);}

int main (void){ . . . print_banner (); . . . print_banner (); return (0);}

main (void){

print_banner ();

print_banner ();

}

print_banner {

}

print_banner {

}


Control Flow

All C programs Start at main () /*no matter where main()is */

Continue in top-to-bottom order, statement by statement, unless the order is changed by: function call function return if loops


Function Type and Value A function can return a value. Like all values in C, a function return value has a type. The function has the type of its returned value.

/* Get number from user */int get_input (void){ int num; printf (“Enter a number:”); scanf (“%d”, &num); return (num);}

function type

return statement

returned value


Calling a function

A value-returning function is called by including it in an expression.

int main (void){ int x, y; x = get_input() ; y = get_input() ; printf (“ %d + %d = %d\n”, x, y, x+y); return (0);}

Note : A value returning function can be used anywhere an expression of the same type can be used.


return

In a value-returning function (result type is not void) return does two distinct things : 1. specify the value returned by the execution of

the function 2. terminate that execution of the function.

In a void function: return is optional at the end of the function body. return may also be used to terminate execution of

the function explicitly. No return value should appear following return.


void compute_and_print_itax (){

double income;scanf (“%f”, &income);if (income < 50000) {

printf (“Income tax = Nil\n”); return;}if (income < 60000) {

printf (“Income tax = %f\n”, 0.1*(income-50000);return;

}if (income < 150000) {

printf (“Income tax = %f\n”, 0.2*(income-60000)+1000);return ;

}printf (“Income tax = %f\n”, 0.3*(income-150000)+19000);

}

Terminate function execution before reaching the end


Function parameters

It is very often useful if a function can operate on different data values each time it is called. Such values are function (input) parameters.

The function specifies its inputs as parameters in the function declaration.

/* Find area of a circle with radius r */double area (double r){

return (3.14159*r*r) ;}

parameter


Arguments

The function call must include a matching argument for each parameter.

When the function is executed, the value of the argument is substituted for the parameter.

int main (void){ . . .

double circum;. . .area1 = area(circum/2.0);. . .

}

double area (double r){

return (3.14*r*r);}

parameter passing

28.1.2001Sudeshna Sarkar, IIT Kharagpur 1 Functions Lecture 9 28.1.2001.

Documents

Transcript of 28.1.2001Sudeshna Sarkar, IIT Kharagpur 1 Functions Lecture 9 28.1.2001.