K and R C, Solution to Exercise 5-15:
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 the option -f to fold upper and l;ower case together, so that case distinctions are not made during sorting; for example, c and C compare equal. Read more about C Programming Language .

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


#define NUMERIC 1
#define DECR 2
#define FOLD 4
#define LINES 100
int charcmp(char *, char *);
int numcmp(char *, char *);
int readlines(char *lineptr[], int maxlines);
void qsort(char *v[], int left, int right, int (*cmp)(void *, void *));
void write lines(char *lineptr[], int nlines, int order);
static char option = 0;
// sort input lines
 main(int argc, char *argv[])
{
 char *lineptr[LINES];
 int nlines;
 int c, rc = 0;
 while(--argc > 0 && (*++argv)[0] == '-')
  while(c = *++argv[0]
                    switch(c) {

                    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;
                    }
 if(argc)
  printf("Usage:sort -dfnr n");
 else{
  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;
}


/*charcmp: return < 0  if s<t, 0 if s==t,>0 if s>t */
int charcmp(char *s, char *t)
{
 for(; tolower(*s) == tolower(*t);s++,t++)
  if(*s == '