Compile and run this code in C

#include <stdio.h>

int main()
{
  int a[] = {10, 20, 30, 40, 50};
  int index = 2;
  int i;

  a[index++] = index = index + 2;
  for(i = 0; i <= 4; i++)
    printf("%d\n", a[i]);
}

Output : 10 20 4 40 50

Now for the same logic in Java

class Check
{

  public static void main(String[] ar)
  {
    int a[] = {10, 20, 30, 40, 50};
    int index = 2;

    a[index++] = index = index + 2;
    for(int i = 0; i <= 4; i++)
      System.out.println(a[i]);
  }
}

Output : 10 20 5 40 50

Why is there output difference in both languages, output is understandable for Java but I cannot understand output in C

One more thing, if we apply the prefix ++ operator, we get the same result in both languages, why?