Sort numbers from a .txt file in C - c

Question

I made a sorting algorithm that works alright when the numbers I need to sort are in an array inside the code, like this:
int sort[] = {16,8,23,4,42,15};
But I need the code to sort the numbers from a .txt file, I do know the size of the file (so no need of a sizeof to know how many numbers you need to sort) but the problem is that the numbers in the file are not separated by commas, only spaces, and I don't know how to make my code operate with this list of numbers.
My code is like this, and like I said, it works when the sorting array of numbers is inside the code separated by comas:
int main(){
int temp, size;
int sort[] = {16,8,23,4,42,15};
size = sizeof(sort) / sizeof(int);
for(int j = 0; j < size; j++){
for(int i = 0; i < size; i++){
if(sort[i] > sort[i+1]){
temp = sort[i];
sort[i] = sort[i+1];
sort[i+1] = temp;
}
}
}
for(int p = 0; p < size; p++){
printf("%d ", sort[p]);
}
}
And I also know that to open a file in C the code is something like this:
FILE* f;
f = fopen("1000.txt", "r");
if(f == 0){
printf("Database unavaible or corrupted\n\n");
exit(1);
}
But I don't know what do next, how do I get this file with unsorted numbers not separated by commas and make my code sort and print them?

Answer 1

You can use fscanf to read the number from file then store these numbers in an array. Finally, you can using your sorting function to sort the this array.
The code below is an example for reading the numbers from the file:
#include <stdio.h>
#define MAX_NUM 10
int main() {
FILE * fp = fopen("input.txt", "r");
if(!fp) {return -1;}
int array[MAX_NUM] = {0}; // you can use array[size] if you know exactly how many numbers in the file
int i = 0;
while(i < MAX_NUM && fscanf(fp, "%d", &array[i]) == 1) {
printf("a[%d] = %d\n", i, array[i]);
i++;
}
// sort the array here as you did in your code
return 0;
}
OT, your code has a mistake in for loop:
for(int i = 0; i < size; i++){
if(sort[i] > sort[i+1]){...}
...
}
when i = size - 1, sort[i+1] will become sort[size] that is out of the array sort, because the maximum index that you can access is size-1 not size. It should change to:
for(int j = 0; j < size; j++) {
for(int i = 0; i < size-j-1; i++) {...}
}

Answer 2

To read the numbers you can use fscanf but you still have the problem of memory allocation.
Imagine the file has 20 elements but the MAX_NUM is 4096, you would be taking 4086*4 bytes of memory unnecessarily.
On the other hand the file can have 8000 elements but you can only store 4096.
What you can do is dynamic memory allocation.
int *array = (int*)malloc(10*sizeof(int));
unsigned numbers_counter = 0;
while(fscanf(f,"%i",&array[numbers_counter++])==1){
if(numbers_counter>=sizeof(array)){
if((array = (int*) realloc(array,sizeof(int)*sizeof(int)*10)) == NULL)
exit(1);
}
}
What I do in this segment is allocate a space of 10 int on memory, then, when the array is full (if(numbers_counter>=sizeof(array))) the array for a bigger set of numbers.
If you have until 10 numbers, it only occupies 10*4 bytes, the from 11 to 100 it occupies 100*4, etc.