k&r(5.11): Pointers to Functions

/* k&r5.11: Pointers to Functions
   created on Mar 23, 2011
   */
#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(void *lineptr[], int left, int right,
            int (*comp) (void *, void *));
int numcmp(char *, char *);

/* sort input lines */
main(int argc, char *argv[])
{
    int nlines;      /* number of input lines read */
    int numeric = 0;  /* 1 if numberic sort */

    if (argc > 1 && strcmp(argv[1], "-n") == 0)
      numeric = 1;
    if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
        qsort((void**) lineptr, 0, nlines-1,
                    (int (*)(void*, void *)) (numeric ? numcmp : strcmp));
        writelines(lineptr, nlines);
        return 0;
    } else {
        printf("input too big to sort\n");
        return 1;
    }
}

/* qsort: sort v[left]...v[right] into increasing order */
void qsort (void *v[], int left, int right,
            int (*comp)(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, ++last, i);
    swap(v, left, last);
    qsort(v, left, last-1, comp);
    qsort(v, last+1, right, comp);
}

/* swap: interchage tow elements */
void swap(void *v[], int i, int j)
{
    void *temp;
    temp = v[i];
    v[i] = v[j];
    v[j] = temp;
}




#define MAXLEN 1000   /* max length of any input line */
int getline1(char *, int);
char *alloc(int);

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

    nlines = 0;
    while ((len = getline1(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]);

}


int getline1(char *s, int lim)
{
    int i,c;
   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;
}

#define     ALLOCSIZE 10000

static char allocbuf[ALLOCSIZE];
static char *allocp = allocbuf;

char *alloc(int n) /* return pointer to n characters */
{
    if (allocbuf + ALLOCSIZE - allocp >= n) {
        allocp += n;
        return allocp - n;
    } else
      return 0;
}

/* numcmp.c */
#include    "stdlib.h"

/* numcmp: compare s1 and s2 numerically */
int numcmp(char *s1, char *s2)
{
    double v1, v2;

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