Sorting integer data from file and calculate execution time

// C program to generate random numbers
#include <stdio.h>
#include <stdlib.h>

// Driver program
int main(void)
{
    // This program will create same sequence of
    // random numbers on every program run
    FILE* fptr;

    // creating a file "random.txt" in "write" mode
    fptr = fopen("random.txt", "w"); 
    int i;
    if (fptr == NULL) {
        printf("ERROR");
        exit(1);
    }

    for (i = 0; i < 10000; i++) {

        // to generate number less than 100000
        int val = rand() % 100000; 

        // storing data to file
        fprintf(fptr, "%d ", val); 
    }

    // closing the file
    fclose(fptr); 
    printf("numbers generated successfully !! ");
    return 0;
}

#include <stdio.h>
#include <time.h>
int main()
{

    // data type for calculating time
    clock_t starttime, endtime; 

    // variable for calculating total time of execution
    double totaltime; 
    int i = 0, j, n = 0, min, index;

    // declaring array to store data from file
    int arr[100000];

   
    // declaring file pointer  
    FILE* fptr; 

    // opening the integer file.
    fptr = fopen("random.txt", "r"); 


    // scanning integer from file to array
    while (fscanf(fptr, "%d", &arr[i]) == 1) 
    {

        // for counting the number of elements
        n++; 

        // for incrementing the array index
        i++; 
    }

    // logic for selection sort....
    // starts here...

    // calculating clock when sorting starts..
    starttime = clock(); 
    printf("start time : %f\n", (float)starttime);
    for (i = 0; i < n - 1; i++) {
        min = arr[i];
        for (j = i + 1; j < n; j++) {
            if (arr[j] < min) {
                min = arr[j];
                index = j;
            }
        }

        // swapping the smallest number with 
        // the current arr[i]th value
        int temp = arr[i];
        arr[i] = min;
        arr[index] = temp;
    }
    // selection sort logic ends here

    // calculating clock when sorting  ends
    endtime = clock(); 
    printf("%f\n", (float)endtime);

    totaltime = ((double)(endtime - starttime)) / CLOCKS_PER_SEC;

    // printing the sorted array...
    for (i = 0; i < n; i++)
        printf("%d ", arr[i]);

    printf("\n\nendtime : %f\n", (float)endtime);
    printf("\n\ntotal time of execution = %f", totaltime);

    return 0;
}