C语言中浮点数和整数型强制转换

在数组arr中为什么不能输出相邻两个数字的平均数呢

#include
int main()
{
    int i,n;
    int arr1[20] = {1,1,2};
    float arr2[19];
    float average;
    printf("1\n1\n");
    for (i = 2; i <20; i++)
    {
        arr1[i] = arr1[i - 1] + arr1[i - 2];
        printf("%d\n",arr1[i]);
        n=arr1[i - 1] + arr1[i - 2];
        average=(float)n/2;
        arr2[i-2]=average;
        printf("%f\n",arr2);
    }
    return 0;
}