MFCC Deltas, Delta-Deltas
It's better to copy first frame and last frame values to extend vector sequence beyond boundaries than to assign 0. This could be implemented just by adjusting indexes:

if (index1 &lt; 0)
    index1 = 0
if (index2 &gt; N - 1)
   index2 = N - 1

delta = v[index1] - v[index2]