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] != '