FUNCTIONS

FUNCTION??All languages have a construct to separate

and package blocks of code. C uses the "function" to package blocks of code.

A function is self-contained block of statements that perform a coherent task of some kind.

Why Use Functions?Writing functions avoids rewriting the

same code over and over.By using functions it becomes easier to

write programs and keep track of what they are doing(separate the code into modular units).

LOOK LIKE??A function has a name, a list of arguments

which it takes when called, and the block of code it executes when called.

#include<stdio.h> void message(); /*function prototype declaration*/ void main() { message(); /*function call*/ printf(“\n Friends……”); }

void message() /* function definition */

{ printf(“\n Hi……….”); }

It is necessary to mention the prototype of every function that we intend to define in the program.

A function can be called from another function, ‘but a function cannot be defined in another function’.

Communication??Between calling and called functionsMechanism used to convey information to the

function is the ‘argument’(parameter).

Parameter:External value used for processing is called a

parameter.Parameters are of two types:i. Actual parametersii. Formal parametersParameters can passed in two ways:

i. Pass by value ii. Pass by reference

Types of parameters… Actual parameters: The expression passed to a function

by its caller is called the "actual parameters” Formal parameters: The parameter storage local to the function is called the "formal parameter"

Calling Convention…Indicates two things:i. The order in which the arguments are passed to the

functionii. Which function(calling or called)performs the cleanup

of variables when the control returns from the function

When a function call is encountered and the arguments are to be passed to a function two possibilities exist:

i. Arguments can be passed from left to right ii. Arguments can be passed from right to left

• Most common calling convention is “standard calling convention”

Additional features…a) Return type of functionb) Calling functions by value or by referencec) Recursion

Note:Always ‘C’ uses positional correspondence in

functions while passing parametersExample:

void f(int a, int b, int c) {

printf(%d %d %d”,a,b,c); } z=x+y 7main() y=z+5 12{ x+y 15

int x=3,y=4,z=5;f(x+y,y=z+5,z=x+y);

}

Pass by Value…We pass ‘values’ of variables to the ‘called’ functionC passes parameters "by value" which means that the actual

parameter values are copied into local storage. The caller and callee functions do not share any memory(--

they each have their own copy)Examples:

sum=area(a,b); f=fact(a);

Practice: Consider the following C function int a=3;

void f(int a) {

void g();a=a+2;printf(“%d”,a);g(); }

void g(){ a=a+2; printf(“%d”,a);}main(){ f(a); printf(“%d”,a);}

Ans:5 5 5

NOTE… “Pass by Value” is more secure than “Pass by Reference” because

actual data is not affected and a copy of it is maintained. Ex: void f(int i)

{i=i+1;

} main() {

int i=5;f(i);printf(“%d”,i);

}Output: 5Explanation: Here value of i passed as pass by value. So value of i

local to main is not changed.

Disadvantage!!!This scheme is fine for many purposes, but it

has two disadvantages: Because the callee has its own copy,

modifications to that memory are not communicated back to the caller. Therefore, value parameters do not allow the callee to communicate back to the caller.

Sometimes it is undesirable to copy the value from the caller to the callee because the value is large and so copying it is expensive

Solution…….The alternative is to pass the arguments "by

reference". Instead of passing a copy of a value from the caller to the callee, pass a pointer to the value.

In this way there is only one copy of the value at any time, and the caller and callee both access that one value through pointers.

POINTERS

POINTERS??? Def.: A variable which stores the address of

other variable.

Represented by *Also called Indirection operator

There are two pointers based on memory:Near pointer(occupies 2bytes)Far pointer(occupies 4bytes)

Pointer declaration is as ,i location name

6 value at location 100 location number(address)

Types of pointers…Single pointer(*)

Ex: int x=5; int *ptr; ptr=&x;Double pointer(**)

Ex: int **pptr; pptr=&ptr;Triple pointer(***)………N pointer(*………*)NULL pointer: A pointer which holds null value

Ex: int *ptr=NULL;

• WILD pointer: An uninitialized pointerEx: int *ptr;

char *p; float *f;

DANGLING pointer: A pointer that points to an address that doesn’t

exist.Ex: n1 n2 n3 nodes

200 300 NULL 100 200 300

If we delete 200 ptr, then n1 n3 200 NULL 100 300

CONSTANT pointer: int *const ptr=&variable;

FUNCTIONAL pointer:int f; variableint f(); functionint *f; pointerint *f(); function

int (*f)(); functional pointer

SPECIALIZED pointers:int *p;float *fp;char *cp;long *lp;

GENERIC pointer: (void *)void *vp;

NOTE::The arithmetic operations that are allowed on pointers are +,-

Example:void main(){

int i=5,*j,**k;j=&I;k=&j;printf(“i=%u”,&i);printf(“i=%u”,j);printf(“i=%u”,*k);printf(“j=%u”,&j);printf(“j=“%u”,k);printf(“k=%u”,&k);printf(“i=%d”,i);printf(“i=%d”,*(&i));printf(“i=%d”,**k);

}

Output: i=1000i=1000i=1000j=2000j=2000k=3000i=5i=5i=5

Pass by Reference…Here we pass the location number(also called

address) of the variable to a function.Example:

void swap(int*,int*); //prototype Swap(&a,&b); //calling function Void swap(int *x,int *y){} //definition

Practice:int x;void f(int *x);{void f2(int);*x=*x*2;f2(*x);}void f2(int x){x=x*2;printf(“%d”,x);}main(){x=3;f(&x);printf(“%d”,x);}

Output::6 12 6

Recursion?...A function is said to be ‘recursive’ if a

statement within the body of a function calls the same function

(OR)Recursion is the process of defining

something in terms of itself Recursion internally uses stack

Example:Q::What is the value of f(5)?int f(int n){

static int r=0;if(n<=0)

return 1;if(n>3){

r=n;return(f(n-1)+2);

}return f(n-1)+r;

}Output:: 18

f(int n){ 0

return 1;} 1f(n){

r=5;return f(n-1)+r; //1+5=6

} 2f(n){

r=5; return f(n-1)+r; //6+5=11

} 3f(n){

r=5;return f(n-1)+r; //11+5=16

} 5 STACK

f(n){

r=5; return(f(n-2)+2); //16+2=18

}

f(n)5

f(n)3

f(n)2

f(n)1

f(n)0

Test ur skills!!! void f(int i){

if(i){

printf(“%d”,i); f(i+1);

}}main(){

f(10);}

Output:: 10,11,12,13,…………,32767,-32767,……….-1

Explanation:: when the value of i becomes ‘0’ that is condition is false and execution stops.

#define<stdio.h>#define print(x) printf(“%d”,x);int x;void Q(int z){

z+=x;print(z);

}void P(int *y){

int x=*y+2;Q(x);*y=x-1;print(x);

}main(){

int x=5;P(&x);print(x);

}

Output:: 12 7 6

#include<stdio.h> int fun(int n, int *f_p) {

int t, f;if(n<=1){

*f_p=1;return 1;

}t=fun(n-1,f_p);f=t+*f_p;*f_p=t;return f;

}int main(){

int x=15;printf(“%d\n”,fun(5,&x));return 0;

}

The value printed is???Output:: 8

main(){

int *ptr, i;int A[]={10,20,30,40,50};ptr=A+4;for(i=1;i<5;i++){

*ptr=*(ptr-1); //*ptr=*ptr--;*ptr=*ptr-1; //*ptr=*ptr-1;

}for(i=0;i<5;i++){

printf(“%d”,A[i]);}

}Output is?????10 20 30 40 39

Output of the following C program is………………? main() {

char s[]={'a','b','c','\n','c','\0'};char *p,*str,*str1; p=&s[3];str=p;str1=s; printf("%d",++*p + ++*str1-32);

}

NOTE:ASCII value of \n(10),\0(0)Output:: 77(‘M’)

main() {

int a[2][2][2] = { {10,2,3,4}, {5,6,7,8} }; int *p,*q; p=&a[2][2][2]; *q=***a; printf("%d----%d",*p,*q);

}

Output: some garbage value……

Explanation:p=&a[2][2][2] you declared only two 2D arrays, but you are trying to access the third 2D(which you are not declared) it will print garbage values. *q=***a …starting address of ‘a’ is assigned to integer pointer. Now q is pointing to starting address of a. If you print *q, it will print first element of 3D array..

Output is……???

main() {

printf("\nab"); printf("\bsi"); printf("\rha");

}

Note::\r-linefeed…\b-backspace

Output is……….?

main() {

printf("%p",main); } Ans :Some address will be printed. Explanation:Function names are just addresses

(just like array names are addresses). main() is also a function. So the address of function main will be printed. %p in printf specifies that the argument is an address. They are printed as hexadecimal numbers..

main(){

static char names[5][20]={"pascal","ada","cobol","fortran","perl"};

int i;char *t;t=names[3]; names[3]=names[4]; names[4]=t; for (i=0;i<=4;i++)

printf("%s",names[i]); } Ans ::Compiler error: Lvalue required in function mainExplanation::Array names are pointer constants. So it cannot

be modified..

Output for the following program is??????

main() {

int i=-1; +i; printf("i = %d, +i = %d \n",i,+i);

}

Ans : i = -1, +i = -1 Explanation:Unary + is the only dummy

operator in C. Where-ever it comes you can just ignore it just because it has no effect in the expressions (hence the name dummy operator)..

main() {

char name[10],s[12]; scanf(“ \"%[^\"]\"",s); } How scanf will execute? Ans :First it checks for the leading white space and discards it. Then it matches with a quotation mark and then it reads all character upto another quotation mark..

Output of the following program is………?

void main(){    char *ptr="*******";    int i;    clrscr();    for(i=0;i<8;i++)

{       printf("%*.*s\n",8,i,ptr);    }    getch();}

#include<stdio.h> #include<conio.h>void main(){ char *ptr="*********"; int i,j; clrscr(); for(i=0;i<11;i++){ if(i<5) printf("%*.*s\n",5+i,2*i+1,ptr); else { if(i==7) {*(ptr+3)=' ';*(ptr+4)=' ';*(ptr+5)=' '; } printf("%*.*s\n",9,9,ptr); } }

getch();}

Explanation: Meaning of %*.*s in the printf function:-First * indicates the width i.e. how many spaces will take to print the string.-Second * indicates how many characters will print of any string.

#include<stdio.h> #include<conio.h> int fun(int); int i;void main( ){

int j;for(;;){

if(j=fun(i)) printf("%d",j);else break;

} } int fun(x){

static int v=2; v--; return (v-x);

}

Output:: 1

void main() {

static char *s[3]={"math","phy","che"};typedef char *( *ppp)[3];static ppp p1=&s,p2=&s,p3=&s;char * (*(*array[3]))[3]={&p1,&p2,&p3};char * (*(*(*ptr)[3]))[3]=&array;p2+=1;p3+=2;printf("%s",(***ptr[0])[2]);

}

Output is……….???

Ans: che Explanation: Here

ptr: is pointer to array of pointer to string.

P1, p2, p3: are pointers to array of string.

array[3]: is array which contain pointer to array of string.

As we know p[i]=*(p+i)

(***ptr[0])[2]=(*(***ptr+0))[2]=(***ptr)[2]

=(***(&array))[2] //ptr=&array

=(**array)[2] //From rule *&p=p

=(**(&p1))[2] //array=&p1

=(*p1)[2]

=(*&s)[2] //p1=&s

=s[2]=”che”

What will be output if you will compile and execute the following c code?

#include"conio.h“int display();int(*array[3])();int(*(*ptr)[3])();void main(){

array[0]=display;array[1]=getch;ptr=&array;printf("%d",(**ptr)());(*(*ptr+1))();

}

int display(){

int x=5;return x++;

}

Ans: 5 Explanation:

In this example: array []: It is array of pointer to such function which parameter is void and return type is int data type.

ptr: It is pointer to array which contents are pointer to such function which parameter is void and return type is int type data.

(**ptr)() = (** (&array)) () //ptr=&array

= (*array) () // from rule *&p=p =array [0] () //from rule *(p+i)=p[i]=display () //array[0]=display

(*(*ptr+1))() =(*(*&array+1))() //ptr=&array

=*(array+1) () // from rule *&p=p=array [1] () //from rule *(p+i)=p[i]=getch () //array[1]=getch

Output is……..?

void main(){

static main;int x;x=call(main);clrscr();printf("%d ",x);getch();

}int call(int address){

address++;return address;

}

int dynamic(int,...);void main(){

int x,y;x=dynamic(2,4,6,8,10,12,14);y=dynamic(3,6,9,12);clrscr();printf("%d %d ",x,y);getch();

}

int dynamic(int s,...){

void *ptr;ptr=...;(int *)ptr+=2;s=*(int *)ptr;return s;

}

Output is……?8 12

In C three continuous dots is known as ellipsis which is variable number of arguments of function. In this example ptr is generic pointer which is pointing to first element of variable number of argument. After incrementing it will point third element.

What will be output if you will compile and execute the following c code?

void main(){

int i;float a=5.2;char *ptr;ptr=(char *)&a;for(i=0;i<=3;i++)printf("%d ",*ptr++);

}

Ans: 102 102 -90 64Explanation:

In c float data type is four byte data type while char pointer ptr can point one byte of memory at a time. ptr pointer will point first fourth byte then third byte then second byte then first byte.

Content of fourth byte:Binary value=01100110Decimal value= 64+32+4+2=102

Content of third byte: Binary value=01100110Decimal value=64+32+4+2=102

Content of second byte: Binary value=10100110Decimal value=-128+32+4+2=-90

Content of first byte: Binary value=01000000Decimal value=64

Note: Character pointer treats MSB bit of each byte i.e. left most bit of above figure as sign bit.

THAN

K U