K & R C Programs Exercise 5-6.

K and R C, Solution to Exercise 5-6:
K and R C Programs Exercises provides the solution to all the exercises in the C Programming Language (2nd Edition). You can learn and solve K&R C Programs Exercise.
C Program to 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). Read more about C Programming Language .

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

/* getline: get line into s, return length */
int getline(char *s, int lim)
{
char *p;
int c;

p = s;
while (--lim > 0 && (c = getchar()) != EOF && c != 'n')
*p++ = c;
if (c == 'n')
*p++ = c;
*p = '';
return (int)(p - s);
}


int atoi(char *s)
{
int n, sign;

while (isspace(*s))
s++;
sign = (*s == '+' || *s == '-') ? ((*s++ == '+') ? 1 : -1) : 1;
for (n = 0; isdigit(*s); s++)
n = (n * 10) + (*s - '0');
return sign * n;
}

/*The itoa() function converts an integer value into an
ASCII string of digits.*/

char *utoa(unsigned value, char *digits, int base)
{
char *s, *p;

s = "0123456789abcdefghijklmnopqrstuvwxyz";
if (base == 0)
base = 10;
if (digits == NULL || base < 2 || base > 36)
return NULL;
if (value < (unsigned) base) {
digits[0] = s[value];
digits[1] = '';
} else {
for (p = utoa(value / ((unsigned)base), digits, base);
*p;
p++);
utoa( value % ((unsigned)base), p, base);
}
return digits;
}

char *itoa(int value, char *digits, int base)
{
char *d;
unsigned u;

d = digits;
if (base == 0)
base = 10;
if (digits == NULL || base < 2 || base > 36)
return NULL;
if (value < 0) {
*d++ = '-';
u = -((unsigned)value);
} else
u = value;
utoa(u, d, base);
return digits;
}


static void swap(char *a, char *b, size_t n)
{
while (n--) {
*a ^= *b;
*b ^= *a;
*a ^= *b;
a++;
b++;
}
}

void my_memrev(char *s, size_t n)
{
switch (n) {
case 0:
case 1:
break;
case 2:
case 3:
swap(s, s + n - 1, 1);
break;
default:
my_memrev(s, n / 2);
my_memrev(s + ((n + 1) / 2), n / 2);
swap(s, s + ((n + 1) / 2), n / 2);
break;
}
}

void reverse(char *s)
{
char *p;

for (p = s; *p; p++)
;
my_memrev(s, (size_t)(p - s));
}



static char *strchr(char *s, int c)
{
char ch = c;

for ( ; *s != ch; ++s)
if (*s == '')
return NULL;
return s;
}

int strindex(char *s, char *t)
{
char *u, *v, *w;

if (*t == '')
return 0;
for (u = s; (u = strchr(u, *t)) != NULL; ++u) {
for (v = u, w = t; ; )
if (*++w == '')
return (int)(u - s);
else if (*++v != *w)
break;
}
return -1;
}


#define NUMBER '0'

int getop(char *s)
{
int c;

while ((*s = c = getch()) == ' ' || c == 't')
;
*(s + 1) = '';
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 = '';
if (c != EOF)
ungetch(c);
return NUMBER;
}




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