A function that inserts array into a binary file

Question

My task is to write a function unesi_niz which allows the user to enter array of real numbers (maximum 100) where the entry ends with entering the number -1. The array that is entered in this way should be written to the binary file niz.bin as values of type double. The file must not contain anything other than members of the string (so it must not contain the entered number -1).

Then write a srednja_vrijednost function that calculates the mean value of the numbers in the niz.bin file and returns it. If the file does not exist or is empty, 0 should be returned.

So i started like this:

#include <stdio.h>
#include <stdlib.h>
#define vel 100
int i = 0;
int j = 0;
void unesi_niz() {
  double pomocna;
  double niz[100];

  while (i != 100) {
    scanf("%lf", &pomocna);
    if (pomocna != -1) {
      niz[i] = pomocna;
      i++;
    } else
      break;
  }

  FILE *ulaz = fopen("niz.bin", "w");
  if (!ulaz) {
    printf("Greska pri otvaranju.\n"); //opening fault
  }
  for (j = 0; j < i; j++) {
    fwrite(niz, sizeof(double), j, ulaz);
  }

  fclose(ulaz);
}

double srednja_vrijednost() {
  double suma = 0;

  if (i == 0)
    return 0;
  FILE *ulaz = fopen("niz.bin", "r");
  if (!ulaz) {
    printf("Greska pri otvaranju.\n");//opening fault
    return 0;
  }

  double niz[100];
  fread(niz, sizeof(double), i, ulaz);
  int j;

  for (j = 0; j < i; j++) {
    suma += niz[j];
  }

  fclose(ulaz);
  return suma / i;
}

int main() {
  unesi_niz();
  double n=srednja_vrijednost();
  printf("%g\n", n);
  return 0;
}

My code has several problems. the first is the wrong return value of the function srednja_vrijednost, when I enter the values 5 10 15, the result is 1.6667, which is nonsense, and then many "Profiler errors", my debug console says Error in line 56, main.c file: The program accesses a variable that is not initialized, however I don't see any "Forbidden Action".

Hope some of you can see what I have done wrong :)

Answer 1

Your code fails in unesi_niz at these lines:

  for (j = 0; j < i; j++) {
    fwrite(niz, sizeof(double), j, ulaz);
  }

fwrite takes a pointer to the data, the size of individual elements in bytes and the number of such elements. This means your code writes j elements starting from the first element each time. You probably want to write 1 element each time. Or better yet, you want to write all i elements, since fwrite allows you to write more than one element.

fwrite(nix, sizeof(double), i, ulaz);

As an aside, your "srednja_vrijednost" logic will work, but only because you already know the size of your array in the current process (stored in i). I am not entirely sure what you are trying to do but I suspect you want to be able to read the same file back even after your process exits. For that, you would need some logic to find the size of the array. You can do this either by writing the length as well into the file, or (similar to the input) write an ending -1, or just figure out the size by calculating the file size.