Thread: Qsort help needed

Hey I'm writing this program that is supposed to take a file with random numbers in it, and sort it in ascending order. I can't figure out how to get the qsort right because right now all my program does is print the numbers. Here's my code:

Code:

#include <stdio.h>
#include <stdlib.h>

int compare (const void * a, const void * b);
int compare (const void * a, const void * b)
{
  return ( *(int*)a - *(int*)b );
}

int main(){
 
 char values[500];
 char buffer[500];
 int size = 100;
 int i = 0;
 int n;
 FILE *fp;
 fp = fopen("nums.bin", "r");

 while(fgets(buffer, size, fp) != NULL)
 {
   values[i] = (int)buffer;
   i++;
 }

 qsort(values, 4, sizeof(int), compare);
 for (n=0; n<sizeof(values); n++){
   printf("%d ", values[n]);
 }
 
 return 0;

}

Code:

int compare (const void * a, const void * b);
int compare (const void * a, const void * b)
{
  return ( *(int*)a - *(int*)b );
}

You only need the prototype if the actual function comes after main (or wherever you call it from).

Code:

 char values[500];
 char buffer[500];

You declare these as char arrays, but compare and sort integers. Pick one type (probably ints) and stick with it.

Code:

 fp = fopen("nums.bin", "r");

Check if fopen failed (fp is NULL) before reading from it. If it's NULL, print an error with perror and exit your program.

Code:

 while(fgets(buffer, size, fp) != NULL)
 {
   values[i] = (int)buffer;
   i++;
 }

fgets is for text. If nums.bin is binary, as the file extension suggests, you need to use fread instead of fgets. Also, values[i] = (int)buffer assigns every spot in values to contain the address of buffer. fread can read an integer directly into values[i], and if you use fgets, look into sscanf for converting the line of text to ints.

Code:

 qsort(values, 4, sizeof(int), compare);

You're telling it to only sort 4 elements. Also, again, you're using sizeof(int), but values is a char array.

Code:

 for (n=0; n<sizeof(values); n++){
   printf("%d ", values[n]);
 }

sizeof(values) will not give you what you want if you convert those arrays to ints (it only works for a char array by virtue of the fact that a char always has a size of 1). You need to give it the number of elements, not the number of bytes all the elements take up.