program doesn't read a file properly - c

Question

For homework I have to write a program where I am typing the string of n integer and then the program that prints the string,calculates the arithmetic mean and makes a new file only with even numbers and at the end prints the new file on screen.
And here are 2 programs
#include<stdio.h>
main()
{
int x,n,i;
FILE *p;
p=fopen("podaci.dat","wb");
printf("n=");
scanf("%d",&n);
fwrite(&n,sizeof(int),1,p);
for(i=0;i<n;i++)
{
printf("x=");
scanf("%d",&x);
fwrite(&x,sizeof(int),1,p);
}
fclose(p);
}
#include<stdio.h>
void stampa(int n,int a[])
{
int i;
for(i=0;i<n;i++)
printf("%5d",a[i]);
printf("\n");
}
float ars(int n,int a[])
{
int i,s=0;
float ars=0;
for(i=0;i<n;i++)
s+=a[i];
return 1.0*s/n;
}
main()
{
int i,n;
FILE *p,*u;
u=fopen("niz.dat","wb");
p=fopen("podaci.dat","rb");
fread(&n,sizeof(int),1,p);
printf("n=%d\n",n);
int a[n],m=0;
for(i=0;i<n;i++)
{
fread(&a[i],sizeof(int),1,p);
if(a[i]%2==0)
{
m+=1;
}
fwrite(&m,sizeof(int),1,u);
for(i=0;i<n;i++)
if(a[i]%2==0)
{
fwrite(&a[i],sizeof(int),1,u);
}
stampa(n,a);
printf("ars=%.2f",ars(n,a));
fclose(p);
fclose(u);
}
}
When I type more than 2 numbers the program instead of those numbers reads them as 0 and sometimes it adds numbers.

Answer 1

You cannot declare an array a[n] where n is a variable (number that you don't know at compile time but only once you read the file).
You can ether declare a[N] where N is a number big enough defined in with #DEFINE
#include <stdio.h>
#include ...
#DEFINE N 10000
//...
int a[N];
or you have to allocate a[] dynamically
int * a;
a = (int*)malloc(sizeof(int)*n);
You can read more about this here and here