Rewrite appropriate programs from earlier chapters and exercises with pointers instead of array indexing.Good possibilities include getline, atoi, itoa, and their variants, reverse and strindex and getop

#include <stdio.h>

/*getline: read a line into s, return length */
int getline(char *s, int lim){

   int c;
   char *t = s;

   while(--lim > 0 && (c =getchar()) != EOF && c != '\n')
         *s++ = c;
   if(c == '\n')
        *s++ = c;
   *s = '\0';
   return s - t;
}

#include <ctype.h>

/* atoi:convert s to integer; pointer version   */
int atoi(char *s){
   int n, sign;
   for(; isspace(*s); s++)    /* skip white space */
         ;
   sign = (*s == '-') ? -1 : 1;
   if(*s == '+' || *s == '-')  /* skip sign */
          s++;
   for(n = 0; isdigit(*s); s++)
      n =10 *n + *s - '0';
   return sign * n;
}

void reverse(char *);

/* itoa : convert n to characters in s; pointer version */
void itoa(int n, char *s){
   int sign;
   char *t = s;        /* save pointer to s */
   if((sign = n) < 0)  /* record sign */
       n = -n;         /* make n positive   */
   do{                     /* generate digits in reverse order    */
     *s++ = n %10 + '0';     /* get next digit */
   } while((n /= 10) > 0);   /*delete it */
   if(sign < 0)
      *s++ = '-';
   *s = '\0';
   reverse(t);
}

#include <string.h>

/*reverse: reverse string s in place      */
void reverse(char *s){
   int c;
   char *t;

   for(t = s + (strlen(s) - 1); s < t; s++, t--){
         c = *s;
         *s = *t;
         *t = c;
   }
}

/*strindex: return index of t in s, -1 if none   */
int strindex(char *s, char *t){
    char *b = s;    /* beginning of string s */
    char *p , *r;
  
    for(; *s != '\0'; s++){
       for(p=s, r=t; *r != '\0' && *p == *r; p++, r++)
              ;
       if(r > t && *r == '\0')
            return s - b;
    }
    return -1;
}

#include <ctype.h>

/* atof: convert string s to double;pointer version   */
double atof(char *s){
     double val, power;
     int sign;

     for(; isspace(*s);s++)   /* skip white space */
             ;
     sign = (*s == '-') ? -1 : 1;
     if(*s == '+' || *s == '-')
          s++;
     for(val = 0.0; isdigit(*s); s++)
        val = 10.0 * val + (*s - '0');
     if (*s == '.')
          s++;
     for(power = 1.0 ; isdigit(*s);s++){
        val = 10.0 * val + (*s - '0');
        power *= 10.0;
     }
     return sign *val /power;
}

#include <stdio.h>
#include <ctype.h>

#define    NUMBER    '0'  /*signal that a number was found */

int getch(void);
void ungetch(int);

/*getop: get next operator or numeric operand; pointer version */
int getop(char *s){
    int c;

    while((*s =c =getch()) == ' ' || c == '\t')
             ;
    *(s + 1) = '\0';
    if (!isdigit(c) && c != '.')
         return c;              /* not a number */
    if(isdigit(c))         /*collect integer part */
       while(isdigit(*++s =c=getch()))
               ;
    if(c == '.')       /* collect fraction part */
       while(isdigit(*++s = c =getch()))
                ;
    *s = '\0';
    if (c != EOF)
          ungetch(c);
    return NUMBER;
}