Exercise 5.6. Rewrite appropriate programs from earlier chapters and exercises with pointers instead of array indexing.

Exercise 5.6. Rewrite appropriate programs from earlier chapters and exercises with pointers
instead of array indexing. Good possibilities include getline (Chapters 1 and 4), atoi, itoa,
and their variants (Chapters 2, 3, and 4), reverse (Chapter 3), and strindex and getop
(Chapter 4).

/* getline */

int getline(char *l, int lim)

{

    int c;

    char *tmp = l;

    for(; --lim > 0 && (c=getchar()) != EOF && c!='\n'; l++)

        *l = c;

    if(c=='\n')

        *l++ = c;

    *l = '\0';

    return l - tmp;

}

/* atoi */

int atoi(char *s)

{

    int number, indigit, hexdig;

    if(*s++ == '0')

        if(*s == 'x' || *s == 'X')

            s++;

    indigit = IN; // in number

    number=0;

    for(;indigit == IN; s++)

    {

        if(*s >= '0' && *s <= '9')

            hexdig = *s - '0';

        else if(*s >= 'a' && *s <= 'f')

            hexdig = *s - 'a'+ 10;

        else if(*s >= 'A' && *s <= 'F')

            hexdig = *s - 'A'+ 10;

        else

            indigit = OUT; // out number

        if(indigit == IN)

            number = number*16 + hexdig;

    }

    return number;

}

/*itoa*/

void itoa(int n, char *s)

{

    int sign;

    char *t = s;

    

    sign = n; //save sign

    do 

    {

        *s++ = abs(n % 10) + '0'; //next number

    } while ((n /= 10) != 0);

    if (sign < 0)

        *s++ = '-';

    *s = '\0';

    reverse(t);

}

/* reverse */

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 pattern in line, -1 if none*/

int strindex(char *s, char *sf)

{

    char *sc = s;

    char *j, *k;

    

    for(; *s != '\0'; s++)

    {

        for(j = s, k = sf; *k != '\0' && *j == *k; j++, k++)

            ;

        if(k > sf && *k == '\0')

            return s - sc ;

    }

    return -1;

}

/* getop: get next character ot numeric operand */

#include <math.h>

int getop(char *s)

{

    int c;

    while((*s = c = getch()) == ' ' || c == '\t')

        ;

    *(s+1) = '\0';

    if(!isdigit(c) && c != '.' && c != '-') // not a number

        return c; 

    if (c == '-')

    {

        if(isdigit(c = getch()) || c == '.')

            *++s = c; //negative numbers

        else

            {

                if(c != EOF)

                    ungetch(c);

                return '-';

            }

    }

    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;

}