knr/5.7.1.c

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

   #define MAXLINES 5000     /* max #lines to be sorted */

   char *lineptr[MAXLINES];  /* pointers to text lines */

   int readlines(char *lineptr[], int nlines);
   void writelines(char *lineptr[], int nlines);

   void qsort(char *lineptr[], int left, int right);

   /* sort input lines */
   int main()
   {
       int nlines;     /* number of input lines read */

       if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
           qsort(lineptr, 0, nlines-1);
           writelines(lineptr, nlines);
           return 0;
       } else {
           printf("error: input too big to sort\n");
           return 1;
       }
   }

   #define MAXLEN 1000  /* max length of any input line */


   int getline(char line[], int maxline);


   /* getline:  read a line into s, return length  */
   int getline(char s[],int lim)
   {
       int c, i;

       for (i=0; i < lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)
           s[i] = c;
       if (c == '\n') {
           s[i] = c;
           ++i;
       }
       s[i] = '\0';
       return i;
   }

   char *alloc(int);

   /* readlines:  read input 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 = alloc(len) == NULL)
               return -1;
           else {
               line[len-1] = '\0';  /* delete newline */
               strcpy(p, line);
               lineptr[nlines++] = p;
           }
       return nlines;
   }

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

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

   /* writelines:  write output lines */
   void writelines(char *lineptr[], int nlines)
   {
       while (nlines-- > 0)
           printf("%s\n", *lineptr++);
   }

   /* qsort:  sort v[left]...v[right] into increasing order */
   void qsort(char *v[], int left, int right)
   {
       int i, last;
       void swap(char *v[], int i, int j);

       if (left >= right)  /* do nothing if array contains */
           return;         /* fewer than two elements */
       swap(v, left, (left + right)/2);
       last = left;
       for (i = left+1; i <= right; i++)
           if (strcmp(v[i], v[left]) < 0)
               swap(v, ++last, i);
       swap(v, left, last);
       qsort(v, left, last-1);
       qsort(v, last+1, right);
   }
   /* swap:  interchange v[i] and v[j] */
   void swap(char *v[], int i, int j)
   {
       char *temp;

       temp = v[i];
       v[i] = v[j];
       v[j] = temp;
   }