Pointers and Strings

POINTERS AND STRINGS

i) Character and Pointer

A character pointer is a pointer to the character. It can be declared

char *pointer_character;

A character pointer can also be used to access character arrays and string constants, in the same way as accessing numeric arrays using their respective pointer variable. A character type pointer variable can be assigned an entire string as part of a variable declarations whereas a numerical pointer variable cannot be intialized in the same manner as a numerical array.

ii) String and pointers

A string can conveniently be represented by either using a one‒dimensional character array as a character pointer. A string constant is accessed by a pointer to its first element. If pointer message is declared as

char *pointers message;

The statement pointer message = "I am a student". Allocates memory and stores the character string and then assigns to pointer message a pointer containing the starting address of the character array. It is important to note that there is difference between the following two definitions.

char array message[] =  "I am a student” → array

char *pointer message[ ] ="I am a student"  → Pointer

Array message is big enough to hold the sequence of character and '\0' that initializes it. Individual characters within the array can be changed but array message always refers to the same storage. On the other hand, pointer message initialized to print to a string constant but the pointer may sub‒sequently be modified to point elsewhere.The figure shows the how the weekdays are defined.

Thus, though both array message and pointer message are pointers (store the address), array message is a constant where as pointer message is a variable pointer.

Example 1: Write a program to count number of words.

/* Program to count number of words */

#include <stdio.h>

main()

{

       char str[100], *ps;

       int nword = 0;

       printf ("\n Enter a string");

       ps=str;

       gets(ps);

       while (*ps==32 && *ps!=Null)

             ps++;

       if (*ps!=null)

       nword++;

       for (ps=str; *ps!=NULL; ps++)

       {

              if (*ps!=32 && *(ps‒1)==32)

              nword++;

       }

        printf("%d word(s) in the given string", nword);

}

OUTPUT

Enter a string

I am a student.

4 word(s) in the given string.

EXPLANATION: In the above program str[] declared as a character type array. Variable ps declared as pointer variable. Read the string ps from the terminal. The while loop checks the condition *ps=32 && *ps!=Null if condition satisfies then increment the variable ps by 1. if statement checks the conditions *ps!=Null and executes the block of statement then prints the string nword.

Example 2: Program to demonstrate on strings and pointers.

/* Program to demonstrate strings and pointers */

#include <stdio.h>

#include <conio.h>

void main()

{

/* Local definitions */

char * sl = "abcd";

char s2[] = "efgh";

*Statements */

clrscr();

printf("%s %16lu \n", s1, s1);

printf("%s %16lu \n", s2, s2);

sl = s2;

printf("%s %16lu \n", s1, s1);

printf("%s %16lu \n", s2, s2);

getch();

} /* main */

OUTPUT

abcd 58458287

efgh 58523632

efgh 58523632

efgh 58523632

EXPLANATION: The above program refers the strings and pointers concept. String s1 and s2 are declared as character type. The program prints the string as given as output.