Trouble with saving data to and reading from a file in c

Question

As per the heading really the code for saving the data to a file seems to work fine as the program runs and the file exists. However in the second part of code the program compiles but stops with error when running. Before I post code wanted to say thanks to the posters as I have learned a lot from this site but is my first time posting.

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

int const MAX_SIZE = 1000000;
FILE *randomdata;
char outputFilename[] = "array.dat"; 

int main(void)
{
    int i;

    randomdata = fopen(outputFilename, "w");  
    for (i = 0; i < MAX_SIZE; i++)
        fprintf(randomdata, "%lf\n", rand() * 1.0 / RAND_MAX);
    return 0;
}

As can be seen the first piece of code makes a file called array.dat which stores random numbers.

The second piece where I assume there is a logic error somewhere is meant to take in the numbers from array.dat and then figure out the median value.

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

#define MAX_SIZE 1000000

#include <time.h>

int main(void)
{
    int left, right;
    int pivot_index, store_index;
    int i, k;
    double tmp;
    double a[MAX_SIZE];
    double pivot_value;
    double start;
    double end;

    FILE *f;
    char inputFilename[] = "array.dat";

    for (i = 0; i < MAX_SIZE; i++) {
        f = fopen("array.dat", "r");
        fscanf(f,"%lf", &a[i]);
        printf("%lf", &a[i]);
    }

    k = MAX_SIZE / 2; /* the median position */
    left = 0; right = MAX_SIZE;
    start = clock();
    srand(time(0));

    while (left != k) {
        pivot_index = rand() % (right -left) + left;
        pivot_value = a[pivot_index]; /* swap (a[ pivot_index ],a[ right -1]) ; */
        tmp = a[pivot_index];
        a[pivot_index] = a[right - 1];
        a[right - 1] = tmp;
        store_index = left;

        for (i = left; i < right - 1; i++) {
            if (a[i] < pivot_value) { /* swap (a[ store_index ], a[i ]) ; */
                tmp = a[store_index];
                a[store_index] = a[i];
                a[i] = tmp;
                store_index++;
            }
        } /* swap (a[ right -1] , a[ store_index ]) ; */

        tmp = a[right - 1];
        a[right - 1] = a[store_index];
        a[store_index] = tmp;
        pivot_index = store_index;

        if (k <= pivot_index) {
            right = pivot_index;
        } else {
            left = pivot_index + 1;
        }
    }
    end = clock();
    printf("The program took %lf seconds\n", endstart / 1000000);
    printf("Median is %lf\n", a[k - 1]);
    return 0;
}

The problem in the code definitely comes somewhere around the following part I think:

for (i = 0; i < MAX_SIZE; i++) {
    f = fopen("array.dat", "r");
    fscanf(f,"%lf", &a[i]);
    printf ("%lf", &a[i]);
}

Answer 1

there are three things you must know about while using FILE in read mode in c . 1) open the file first by fopen(); 2) check whether the file is open or not. 3)close the file by fclose().