C- mystery value at the end of array

Question

I am trying to insert an integer at the end of an array. However, a random value(47) shows up at the last position when I am trying to execute the code. 47 doesn't change even if I change the value to be inserted. Output is this.

Can anyone tell me why it is 47? And is it a garbage value? My code is as follows:-

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

int main()
{
  int upper=6; 
  int a[upper];
  int i;
  for(i=0;i<upper;i++)
  {
    scanf("%d",&a[i]);
  }
  printf("The array before insertion is \n");
  for(i=0;i<upper;i++)
  {
    printf("%d \n",a[i]);
  }
  printf("\n The array after insertion is \n");
  upper=upper+1;
  a[upper]=66;
  for(i=0;i<upper;i++)
  {
    printf("%d \n",a[i]);
  }
  return 0;
}

Answer 1

you declare an array with size upper (6) but here:

a[upper]=66;

you try to access the 8th location of the array (since you did before that upper= upper + 1)- a location you don't own. What you are doing is UB and therefore it can print 47 or anything else can happen