Strings and Pointers

6. Strings and Pointers

We can access the string by using the pointer variable. Here is a simple C program that illustrates the use of pointer for string.

Program for accessing the string using a pointer variable.

#include<stdio.h>

#include<conio.h>

#include<stdlib.h>

#define size 10

void main(void)

{

char name[size];

char *i;

clrscr();

printf("\n What is Your name?");

gets(name);

i=name;

printf("\nNow printing your name as ");

while(*i!= '\0')

{

printf("%c", *i);

i++;

}

getch();

}

Output

What is Your name? Parth

Now printing your name as Parth

Now we will use the pointer arithmetic and copy the contents of one string to another using pointer variable.

#include<stdio.h>

#include<stdlib.h>

#include<conio.h>

void main(void)

{

char *ptr1; /* a pointer to type character */

char *ptr2; /* another pointer to type character */

char str1[] = "Hello India";

char str2[20];

clrscr();

printf("\n The first string i.e. str1 is: ");

puts(str1); /* displays first string */

 ptr1 = str1; /* pointer to str1 is set */

 printf("\n The pointer ptr1 contains: %s",ptr1);

 ptr2 = str2; /* pointer to str2 is set */

 while(*ptr1 != '\0')

{ /*copying the contents of ptr1 to ptr2*/

*ptr2++= *ptr1++;

}

*ptr2 = '\0';/*placing the '\0' at the end of the string*/

printf("\n\n The second string i.e. str2 is: ");

puts(str2); /* displays the second string */

getch();/*to avoid pressing alt+F5 to see the console*/

}

Output

The first string i.e. str1 is: Hello India

The pointer ptr1 contains: Hello India

The second string i.e. str2 is: Hello India

In above program, there are two strings str1 and str2. The string str1 contains a string "Hello India"

We have taken two pointer variables: ptr1 and ptr2. The starting address (base address) is assigned to ptr1.

ptr1=str1;

Now we can get the same string "Hello India" in ptr1. Now we have set the ptr2 as a pointer to second string str2.

ptr2 = str2;

There is nothing in str2, so we will copy the contents of first string to second string using pointers

while(*ptr1 != '\0')

{/*copying the contents of ptr1 to ptr2*/

*ptr2++= *ptr1++;

}

To terminate str2 we will put '\0' at the end of str2 by assigning

*ptr2 = '\0';

Finally we have printed the contents of str2.

Key Point: We can store one string in a pointer variable of character type. i.e

char *ptr="Hello";

is possible. We can apply pointer arithmetic to access the characters within the string.

 

Ex. 2.5.8 Write a C program to concatenate two strings using pointer.

Sol. :

#include<stdio.h>

#include <conio.h>

void main()

{

char str1[20],str2[10];

char *ptr1,*ptr2;

printf("\n Enter first string ");}

gets(str1);

printf("\n Enter second string ");

gets(str2);

ptr1=str1;

ptr2=str2;

while(*ptr1!= '\0')//reaching at the end of first string

*ptr1++;

while(*ptr2!= '\0')

{

*ptr1= *ptr2;

*ptr2++;

*ptr1++;

}

*ptr1='\0';

printf("\n The concatenated string is %s", str1);

getch();

}

Output

Enter first string Hello

Enter second string India

The concatenated string is HelloIndia

 

Ex. 2.5.9 Write a "C" function using pointer for string reversal.

Sol. Program for string reversal -

void main()

{

char s1[10],s2[10];

clrscr();

printf("\n Enter The String");

scanf("%s",s1);

char *i=s1;

char *j=s2;

while(*i!= '\0')

i++;

i--; /*reaching at the last character*/

while(i>=&s1[0])/* copying the characters*/

*j++=*i--;

*j='\0';

printf("\n The Reversed string is %s",s2);

getch();

}

 

Ex. 2.5.10 Write pseudo C routine using pointers for checking whether a given string is palindrome.

Sol. Palindrome (char* a) // a is a string

{

int *p, *q; int flag = 0;

p = q = a;

while (* q!= '10')

q++;

q --;

while (p < q)

{

if (* p! = *q)

flag = 1;

p++; q--;

}

if (flag ==0)

printf ("string is palindrome");

else

printf ("\n string is not palindrome");

}

 

Ex. 2.5.11 Write a C program to implement any four string handling functions using functions and pointers.AU May-17, Marks 8

Sol. :

#include<stdio.h>

#include<stdlib.h>

#include<conio.h>

int main(void)

{

int choice;

void str_len(),str_concat(), str_copy(), str_reverse();

char ans='y';

do

{

printf("\n Main Menu");

printf("\n1. Length \n2. Concatenation \n3. Copy \n4. Reverse");

printf("\n Enter your choice: ");

scanf("%d", &choice);

switch(choice)

{

case 1:str_len();

break;

case 2:str_concat();

break;

case 3:str_copy();

break;

case 4:str_reverse();

break;

}

printf("\n Do you want to go to main menu?");

} while(ans=='y' || ans == 'Y');

}

void str_len()

{

char *ptr,str[10];

int len=0;

printf("Enter some string: ");

scanf("%s", str);

ptr=str;

while(*ptr!='\0')

{

*ptr++;

len++;

}

printf("\n The length of string is: %d ",len);

}

void str_concat()

{

char str1[20], str2[10];

char *ptr1,*ptr2;

printf("\n Enter first string ");

scanf("%s", str1);

printf("\n Enter second string ");

scanf("%s",str2);

ptr1=str1;

ptr2=str2;

while(*ptr1!= '\0')//reaching at the end of first string

*ptr1++;

while(*ptr2!='\0')

{

*ptr1=*ptr2;

*ptr2++;

*ptr1++;

}

*ptr1='\0';

printf("\n The concatenated string is %s", str1);

}

void str_copy()

{

char *ptr1; /* a pointer to type character */

char *ptr2; /* another pointer to type character */

char str1[10],str2[20];

printf("\n Enter the first string: ");

scanf("%s", str1);

printf("\n The first string i.e. str1 is: ");

puts(str1); /* displays first string */

ptr1 = str1; /* pointer to str1 is set */

ptr2 = str2; /* pointer to str2 is set */

while(*ptr1 != '\0')

{ /*copying the contents of ptr1 to ptr2*/

*ptr2++ = *ptr1++;

}

*ptr2 = '\0';/*placing the '\0' at the end of the string*/

printf("\n\n The copied string is: ");

puts(str2); /* displays the second string */

}.

void str_reverse()

{

char str1[10],str2[10];

printf("\n Enter The String");

scanf("%s", str1);

char *ptr1=str1;

char *ptr2=str2;

while(*ptr1!= '\0')

ptr1++;

ptr1--;

while(ptr1>=&str1[0])/*copying the characters*/

*ptr2++= *ptr 1--;-

*ptr2='\0';

printf("\n The Reversed string is %s", str2);

}

 

7. Pointer to Function

The pointer to the function means a pointer variable that stores the address of function. The function has an address in the memory same like variable. As address of function name is a memory location we can have a pointer variable which will hold the address of function. The data type of pointer will be same as the return type of the function. For instance: if the return type of the function is int then the integer pointer variable should store the address of that function.

Syntax

Return_Type *pointer_variable (data_type);

 For example,

float (*fptr)(float);

Here fptr is a pointer to the function which has float parameter and returns the value float. Note that the parenthesis around fptr otherwise the meaning will be different. For example,

float *fptr(float);

This means fptr is a function with a float parameter and returns a pointer to float.

float fun(float);

float (*fptr) (float);

fptr=&fun;

Thus

/*function returning pointer to float */

float *fptr (float a);

/*pointer to function returning float */

float (*fptr) (float a);

The following program illustrates the use of pointer to the function

#include<stdio.h>

#include<conio.h>

void main()

{

void display(float(*)(int),int);

float area(int);

int r;

printf("\n Enter the radius ");

scanf("%d", &r);

display(area,r);/*function is passed as a parameter to another function*/

}

void display(float (*fptr) (int), int r)

{

/*call to pointer to function*/

printf("\n The area of circle is %f", (*fptr) (r));

}

float area(int r)

{

return (3.14*r*r);

}

Output

Enter the radius 10

The area of circle is 314.000000

Program Explanation

The function display calls the function area through pointer variable fptr. Thus fptr is actually a pointer to the function area. As function area returns the float value we have the pointer as of float type.

Now we will discuss "How to pass a pointer variable as a parameter to the function?" Let us understand it with the help of following C program

Passing a pointer variable to the function

#include <stdio.h>

#include <conio.h>

#include <stdlib.h>

void main()

{

int *m;

void fun(int *);

*m=5;

clrscr();

printf("\n Following value is just before function call \n");

printf("%d", *m);

printf("\n Following value is obtained from the function \n");

fun(m);

printf("%d",*m);

}

void fun(int *x)

{

*x=10;

}

Output

Following value is just before function call

5

Following value is obtained from the function

10

 

Ex. 2.5.12 What is the effect of the following statements :

i) int a, *b = Ɛa ii) int p, *p iii) a = (float *)Ɛ x iv) int (*fun) ().

Sol. :

i) The pointer variable b is initialized by an address of a.

ii) It will cause an error "multiple declaration for p"

iii) Variable x may be of int type, hence to store the address of x into a float type variable, the type casting to pointer type float value is done.

iv) This is pointer to a function. This function returns the pointer to integer value.

 

Ex. 2.5.13 Write a "C" function using pointers to multiply two matrices and return the resultant matrix to calling function.

Sol. : Multiplication of two matrices using pointers

int *mul(int m,int n,int (*a) [3], int (*b)[3], int (*c)[3])

{

int i,j,k;

for(i=0;i<m;i++)

{

for(j=0;j<n;j++)

{

*(*(c+i)+j)=0;

for(k=0;k<m;k++)

*(*(c+i)+j)= *(*(c+i)+j)+((*(*(a+i)+k))*(*(*(b+k)+j)));

}

}

return *(c+0);

}

Review Questions

1. What is a pointer variable? Explain declaration, initialization and accessing a pointer variable with an example.

2. What is the data type pointer in C? How do we declare the pointer? Give any four advantages of using pointers.

3. Explain pointer variable with example.

4. Explain about how to declare pointer to a function with an example. AU: May-17, Marks 8