How to impliment efficient the next expression?

I have the following expression:

a_dot is the rate of change of an angle.So it equals with Da/Dt.I am wondering what should i do for the first value as there is no previous value to calculate the Da.I will calculate it like a_dot(1) = a(1)/(timestep/2). So my question is how to impliment that eficient/vectorized as this calculations are inside a loop with many repeats.I tried the following but ofcurse it didnt work as there is no index = 0 :

 alphaRate=(alpha-(angleNode>1)*angleofattack(angleNode-1))/(obj.timestepPerThitaNode/(1+(angleNode==1)));

 size(angleofattack) = 1x36 double

Any ideas? Thanks in advance.

[Dimitrios]

I managed this one:

 ([angleofattack(1) diff(angleofattack)])/obj.timestepPerThitaNode/(1+(angleofattack(1)==1));

But any other solutions are welcome.