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Functions

A function is a self-contained block of statements that perform a coherent task of some kind.
Functions break large computation into smaller ones.
Some common tasks written in the form of functions can be reused.

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A simple function

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#include<stdio.h>

float max(float x, float y); main( )

{

float a,b,m;

scanf(“%f %f”, &a, &b); m = max(a,b); printf(“Max : %f\n”, m);

}

float max(float x, float y)

{

if(x > y) return x; else return y;

}

Function declaration

Function call

Function definition

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One more example

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/* function prototype */

#include <stdio.h> int square( int y); int main( )

{

int x;

for ( x = 1; x <= 10; x++ ) printf( "%d ", square( x ) );

printf( "\n" ); return 0;

}

int square( int y )

{

return y * y;

}

Main is also a function.

It returns an int

If unspecified, then by default a function returns int

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What we should learn

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What is the control flow when a function is called.
Function declaration or prototype.
Function call.
Function definition.

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Control Flow

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ii.

iii.

iv.

vi.

vii.

Execution starts with main( ), when a function call like z = func(exp1,exp2, …); is encountered then

exp1, exp2, … are evaluated

The values of exp1, exp2, … are passed to func( ) main is suspended temporarily

func( ) is started with the supplied values func( ) does the processing

func( ) returns a value which is assigned to z main( ) is resumed.

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Function prototype

Every function should have a declaration which specifies the function name, its arguments types and its return type.

This feature was borrowed by ANSI C committee from the developers of C++.
A function declaration (prototype) tells the compiler regarding the arguments type, their order, return type.
This can allow the compiler to detect errors in function call and function definition.
Remember this feature was not there before ANSI C and for compatibility purposes, still the compiler may not give any errors when prototype is missing.

Eg: float func(int j, float k);
Optionally j, k could be omitted, that is, float func(int, float);

is also valid.

This says that func is a function which takes two arguments (first

one is int and second one is float) and return a float.

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void

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What if a function does not return any value at all.
In this case the function returns a void and hence the return type should be void.
Eg: void f1(void);
The function f1 does not take any arguments and does not return any value too!!
Why you need such functions?

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void

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void nothing(void); main( )

{

nothing( );

}

void nothing(void)

{

printf(“when you need to do some fixed thing\n” “then you can use a func. like this \n”);

return;

}

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Function definition

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Return-type function-name( parameter-list)

{

declarations statements

}

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To find maximum of three ints

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#include <stdio.h>

int maximum( int, int, int ); int main()

{

int a, b, c;

/* function prototype */

printf( "Enter three integers: " ); scanf( "%d%d%d", &a, &b, &c );

printf( "Maximum is: %d\n", maximum( a, b, c ) ); return 0;

}

int maximum( int x, int y, int z )

{

int max = x;

if ( y > max ) max = y; if ( z > max ) max = z;

return max;

}

You cannot use max in main( )

Declaration. When this function returns max disappears !!

max is called a local variable for this function.

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Let us swap

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void swap(int, int); main( )

{

int a, b;

a = 10, b = 20;

swap(a, b);

printf(“a = %d, b = %d\n”, a, b);

}

void swap(int x, int y)

{

int t;

t = x; x = y; y = t; return;

}

$./a.out

a = 10, b = 20

This does not work !

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Why swap failed !?

when the function is called the values 10 and 20 are passed.
In swap( ), x = 10 and y = 20.
swap( ) actually swapped x and y
But this doesn’t mean that a and b are swapped.

Is there a way for the swap to correctly work?

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How swap can correctly work

There are three ways how information is passed to a function.

Passing values : this is what our swap is doing
Passing references : In some languages it is possible to pass the actual variables, but this is not present in C
Passing addresses: Explicitly use addresses in the function, so that the function can modify the actual variable using these addresses.

You need to pass addresses of a and b
Based on these addresses, respective values should be swapped
This we will see when we discuss about pointers.

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Tips and traps

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Omitting the return-type in a function definition causes a syntax error if the function prototype specifies a return type other than int.
Forgetting to return a value from a function that is supposed to return a value can lead to unexpected errors.
Returning a value from a function whose return type is void causes a syntax error.
Even though an omitted return type defaults to int, always state the return type explicitly. The return type for main is however normally omitted.

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Recursion

A function can call itself !!
This is a very good strength of the language.
Many big problems can be solved easily with recursions
Eg: n! = n * (n-1)!
Can we do like

int factorial(int n)

{

int t;

t =factorial(n-1); return ( n * t );

}

Is there any problem with this?

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Recursion

If you are not careful then it is easy to fall in infinite recursion.

int factorial(int n)

{

int t;

if ( n < 0) return -1; /* an error code */ else if ( n == 0) return 1;

else {

t = factorial(n-1); return (n * t);

}

what happens when we call

x = factorial(4);

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Recursion

Fibonacci series

0, 1, 1, 2, 3, 5, 8, 13, 21, …
fib(0) = 0
fib(1) = 1
fib(2) = 1
……
fib(n) = fib(n-1) + fib(n-2)

int fib(int n)

{

if(n == 0 || n ==1) return n; else return fib(n-1) + fib(n-2);

}

what happens when we call z = fib(3);

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Recursion

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Recursion is useful when the problem can be defined recursively.

To find sum of first n natural numbers.

Sum(0) = 0
Sum(n) = n + Sum(n-1)

int sum(int n)

{

if (n == 0) return 0; else return n + sum(n-1);

}

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Recursion

One of the common pit-fall is: forgetting or wrongly stating the termination condition.
Whatever you do using recursion can be done without recursion also, but using some loop structures.
A copy of the function is created whenever a function call occurs, and hence recursive functions can be heavy on the memory.
Nevertheless, solution becomes conceptually simpler.

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Replacing recursion

To find sum of first n natural numbers

int sum( int n)

{

int t = 0, j; for(j=1; j<=n; j++)

t = t + j; /* alternatively, t += j; */ return ( t );

}

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Fibonacci without recursion

Algorithm to find fib(n)

n = read from key board.
Let a = fib(0), b = fib(1)
If n == 0 or n ==1 then output a or b and exit;
Let j = 1;
while (j < n) Do

c = a + b;
a = b;
b = c;

4. j = j+1;

Output( fin(n) is c );

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Fibonacci without and with recursion

int fib( int n)

{

int a = 0, b = 1, c; int j = 1;

if (n == 0 || n == 1) return n;

while(j < n) {

c = a+b; a = b;

b = c; j ++;

}

return c;

}

int fib( int n)

{

if (n == 0 || n == 1) return n;

else

return fib(n-1) + fib(n-2);

}

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Recursion

Problems that can be defined using recursion will mostly have good recursive solutions.
Using recursion often simplifies the solution conceptually.
Some of the data structures (like trees) can be easily dealt with recursions.

Mostly these aspects you will be studying in Data Structures Course.

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Functions

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Appropriate use of functions makes your program a very structured one.
It might be easy to debug a well structured program.
A program is normally a set of functions which communicate among themselves using arguments and return values.

Some times functions communicate with each other using global variables and addresses.