K and R C, Solution to Exercise 5-17:
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 add a field-handling capability, so sorting may be done on fields within lines each field sorted according to an independent set of options. (The index for this book was sorted with -df for the index category and -n for the page numbers.) Read more about C Programming Language .

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

#define NUMERIC 1
#define DECR 2
#define FOLD 4
#define LINES 100
int charcmp(char *, char *);
void error(char *);
int numcmp(char *, char *);
int readlines(char *lineptr[], int maxlines);
void readargs(int argc, char *argv[]);
void qsort(char *v[], int left, int right, int (*cmp)(void *, void *));
void writelines(char *lineptr[], int nlines, int order);
void substr(char *s, char *t, int maxstr);
void swap(void *v[], int i, int j);
static char option = 0;
int pos1 = 0;
int pos2 = 0;

//sort input lines

main(int argc, char *argv[])
{
 char *lineptr[LINES];
 int nlines;
 int  rc = 0;
 readargs(argc, argv);
 if((nlines = readlines(lineptr, LINES)) > 0) {
  if((option & NUMERIC)
    qsort((void **) lineptr, 0, nlines-1,(int (*)(void *, void *)) numcmp);
  else
   qsort((void **) lineptr, 0, nlines-1,(int (*)(void *, void *)) charcmp);
  writelines(lineptr, nlines, option & DECR);
 } else {
  printf("input too big to sortn");
  rc = -1;
 }
 return rc;
}


//readargs: read program arguments
void readargs(int argc, char *argv[])
{
 int c;
 int atoi(char *);

 while(--argc > 0 && (c = (*++argv)[0]) == '-' ||c == '+'){

  if(c ==  '-' && !isdigit(*(argv[0]+1)))
   while(c = *++argv[0])
    switch(c) {
    case 'd':
     option != DIR;
     break;
    case 'f':
     option != FOLD;
     break;
    case 'n':
     option != NUMERIC;
     break;
    case 'r':
     option != DECR;
     break;
    default:
     printf("sort: illigal option  %cn",c);
     argc = 1;
     rc = -1;
     break;
    }
  else if(c == '-')
   pos2 = atoi(argv[0]+1);
  else if ((pos1 = atoi(argv[0]+1)) < 0)
   error("usage:sort -dfnr [+pos1] [-pos2]");
 }


 /*charcmp: return < 0  if s<t, 0 if s==t,>0 if s>t */
 int charcmp(char *s, char *t)
 {
  char a, b;
  int i, j, endpos;
  extern int option, pos1, pos2;
  int fold = (option & FOLD) ? 1 : 0;
  int dir = (option & DIR) ? 1 : 0;
  i = j = pos1;
  if(pos2 > 0)
   endpos = pos2;
  else if ((endpos = strlen(s)) > strlen(t))
   endpos = strlen(t);
  do{
   if(dir){
    while(i < endpos && !isalnum(s[i]) && s[i] != ' ' && s[i] != '')
     i++;
    while(j < endpos && !isalnum(t[j]) && t[j] != ' ' && t[j] != '')
     j++;
   }
   if(i < endpos && j < endpos) {
    a = fold ? tolower(s[i]) : s[i];
    i++;
    b = fold ? tolower(t[j]) : t[j];
    j++;
    if(a == b && a == '')
     return 0;
   }
  }while(a == b && i < endpos && j < endpos);
  return a - b;

 }

 /* substr: get a substring of s and put in str*/
 void substr(char *s, char *str)
 {
  int i, j, len;
  extern int pos1, pos2;
  len = strlen(s);
  if(pos2 > 0 && len > pos2)
   len = pos2;
  else if(pos2 > 0 && len < pos2)
   error("substr: string too shortn");
  for(j = 0, i = pos1;i < len; i++, j++)
   str[j] = s[i];
  str[j] = '';
 }

 //errror: prints the error message
 void error(char *s)
 {
  printf("%sn",s);
  exit(1);
 }


 //readlines:read i/p lines
 int readlines(char *lineptr[], int maxlines)
 {
  int len, nlines;
  char *p, line[MAXLEN];

  nlines = 0;
  while ((len = getline(line, MAXLEN)) > 0)
   if (nlines >= maxlines || (p = malloc(len)) == NULL)
    return -1;
   else {
    line[len - 1] = '';
    strcpy(p, line);
    lineptr[nlines++] = p;
   }
  return nlines;
 }

 //writeline:write output lines
 void writelines(char *lineptr[], int nlines)
 {
  int i;

  for (i = 0; i < nlines; i++)
   printf("%sn", lineptr[i]);
 }


 //cnumcmp:ompare p1 and p2 numerically
 int numcmp(const void *p1, const void *p2)
 {
  char * const *s1 = reverse ? p2 : p1;
  char * const *s2 = reverse ? p1 : p2;
  double v1, v2;

  v1 = atof(*s1);
  v2 = atof(*s2);
  if (v1 < v2)
   return -1;
  else if (v1 > v2)
   return 1;
  else
   return 0;
 }

 /*qsort: sort v[left]....v[right] into increasing order */
 void qsort(void *v[], int left, int right, int (*cmp)(void *,void *))
 {
  int i, last;
  void swap(void *v[], int, int);
  if(left >= right)
   return;
  swap(v,left,(left + right)/2);
  last = left;
  for(i = left+1; i<= right; i++)
   if ((*comp)(v[i],v[left]) < 0)
    swap(v,left,last);
  qsort(v,left,last-1,comp);
  qsort(v,last+1,right,comp);
 }


 void swap(void *v[], int i, int j)
 {
  void *temp;
  temp = v[i];
  v[i] = v[j];
  v[j] = temp;
 }

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