Strings and Pointers

Lecture 17

In the previous lecture, we have seen some of the string library functions. The best use of pointers to manipulate string can be illustrated with the help of making those library functions into user defined functions- which means you can develop user defined functions that will perform exactly the same way strln ( ), strcpy ( ), and strcmp ( ) works.

strln ( )

Take a look at the following program. It performs the same task the library function strln () does.

#include<stdio.h>

#include<conio.h>

int strlen(char *);

void main(){

      clrscr();

      char str1[]="Dept of CSE";

      int len1,len2;

      len1=strlen(str1);

      len2=strlen("Department of CSE");

      printf("String: %s, Length: %d\n",str1,len1);

      printf("String: %s, Length: %d","Department of CSE",len2);

      getch();

}

int strlen(char *s){

      int length=0;

      while (*s!='\0'){

            length++;

            s++;

      }

      return length;

}

strcpy ( )

Take a look at the following program. It performs the same task the library function strcpy () does.

#include<stdio.h>

#include<conio.h>

void strcpy(char *,char *);

void main(){

      clrscr();

      char source[]="Dept of CSE";

      char target[20];

      strcpy(source,target);

      printf("Source: %s, Target: %s\n",source,target);

      getch();

}

void strcpy(char *s,char *t){

      while (*s!='\0'){

            *t=*s;

            s++;

            t++;

      }

      *t='\0';

}

strcmp ( )

Take a look at the following program. It performs the same task the library function strcmp () does.

#include<stdio.h>

#include<conio.h>

int strcmp(char *,char *);

void main(){

      clrscr();

      int variable;

      char str1[20],str2[20];

      printf("Enter String 1: ");

      gets(str1);

      printf("Enter String 2: ");

      gets(str2);

      variable=strcmp(str1,str2);

      printf("%d\n",variable);

      getch();

}

int strcmp(char *s1,char *s2){

      while (*s1 == *s2){

            if(*s1 == '\0')

                  return 0;

            s1++;

            s2++;

      }

      return *s1-*s2;

}

strcat ( )

Take a look at the following program. It performs the same task the library function strcat () does.

#include<stdio.h>

#include<conio.h>

void strcat(char *,char *);

void main(){

      clrscr();

      int variable;

      char str1[20],str2[20];

      printf("Enter String 1: ");

      gets(str1);

      printf("Enter String 2: ");

      gets(str2);

      strcat(str1,str2);

      printf("%s",str1);

      getch();

}

void strcat(char *s1,char *s2){

      while (*s1!='\0'){

            s1++;

      }

      while (*s2!='\0'){

            *s1=*s2;

            s1++;

            s2++;

      }

      *s1='\0';

}

I hope that you will understand the use of pointers in string manipulations after I finish my lecture and after you conduct these programs by yourself.