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; }