A Quicksort for files
/*
C: The Complete Reference, 4th Ed. (Paperback)
by Herbert Schildt
ISBN: 0072121246
Publisher: McGraw-Hill Osborne Media; 4 edition (April 26, 2000)
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define NUM_ELEMENTS 4 /* This is an arbitrary number
that should be determined
dynamically for each list. */
struct address {
char name[30];
char street[40];
char city[20];
char state[3];
char zip[11];
} ainfo;
struct address addrs[NUM_ELEMENTS] = {
"A. Alexander", "101 1st St", "Olney", "Ga", "55555",
"B. Bertrand", "22 2nd Ave", "Oakland", "Pa", "34232",
"C. Carlisle", "33 3rd Blvd", "Ava", "Or", "92000",
"D. Dodger", "4 Fourth Dr", "Fresno", "Mi", "45678"
};
void quick_disk(FILE *fp, int count);
void qs_disk(FILE *fp, int left, int right);
void swap_all_fields(FILE *fp, long i, long j);
char *get_zip(FILE *fp, long rec);
int main(void)
{
FILE *fp;
/* first, create a file to sort */
if((fp=fopen("mlist", "wb"))==NULL) {
printf("Cannot open file for write.\n");
exit(1);
}
printf("Writing unsorted data to disk.\n");
fwrite(addrs, sizeof(addrs), 1, fp);
fclose(fp);
/* now, sort the file */
if((fp=fopen("mlist", "rb+"))==NULL) {
printf("Cannot open file for read/write.\n");
exit(1);
}
printf("Sorting disk file.\n");
quick_disk(fp, NUM_ELEMENTS);
fclose(fp);
printf("List sorted.\n");
return 0;
}
/* A Quicksort for files. */
void quick_disk(FILE *fp, int count)
{
qs_disk(fp, 0, count-1);
}
void qs_disk(FILE *fp, int left, int right)
{
long int i, j;
char x[100];
i = left; j = right;
strcpy(x, get_zip(fp, (long)(i+j)/2)); /* get the middle zip */
do {
while((strcmp(get_zip(fp,i),x) < 0) && (i < right)) i++;
while((strcmp(get_zip(fp,j),x) > 0) && (j > left)) j--;
if(i <= j) {
swap_all_fields(fp, i, j);
i++; j--;
}
} while(i <= j);
if(left < j) qs_disk(fp, left, (int) j);
if(i < right) qs_disk(fp, (int) i, right);
}
void swap_all_fields(FILE *fp, long i, long j)
{
char a[sizeof(ainfo)], b[sizeof(ainfo)];
/* first read in record i and j */
fseek(fp, sizeof(ainfo)*i, SEEK_SET);
fread(a, sizeof(ainfo), 1, fp);
fseek(fp, sizeof(ainfo)*j, SEEK_SET);
fread(b, sizeof(ainfo), 1, fp);
/* then write them back in opposite slots */
fseek(fp, sizeof(ainfo)*j, SEEK_SET);
fwrite(a, sizeof(ainfo), 1, fp);
fseek(fp, sizeof(ainfo)*i, SEEK_SET);
fwrite(b, sizeof(ainfo), 1, fp);
}
/* Return a pointer to the zip code */
char *get_zip(FILE *fp, long rec)
{
struct address *p;
p = &ainfo;
fseek(fp, rec*sizeof(ainfo), SEEK_SET);
fread(p, sizeof(ainfo), 1, fp);
return ainfo.zip;
}
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