What's the problem with loop execution?

Question

C++ / C#

for (long dd = alen - da - 1; dd < alen; dd++)
    {
      printf("%ld dd, %ld alen\n", dd, alen);
      a[dd] = a[dd + 1];
    }

The program crashes in this cycle, why?
the variable and the condition are fine, and the crash occurs. the pointer is in place, and its size is normal

Answer 1

Here:
a[dd + 1]
array out of bounds on the last iteration of the loop.