MATLAB: Using embedded function block to find differential

embedded matlab functionMATLABsimulink

Hello,
I have a signal and I want to write a code in embedded matlab function block, which could take signal value at the next time step and subtract previous value from it, but the first value has to be zero always. For example, consider the following data with "B" as the main signal, and "dB" would be my required result:
B= 1 3 7 9 11 12
dB= 0 (3-1) (7-3) (9-7) (11-9) (12-11) = 0 2 4 2 2 1
I have written a code to achieve this which is as follows:
  1. function dB=fcn (B)
  2. persistent Bprev;
  3. if isempty (Bprev)
  4. Bprev=B;
  5. end
  6. dB=B-Bprev;
The results from this function, for the same values of B are as follows: # B=1 3 7 9 11 12 # dB= 0 2 6 8 10 11
I think my code is retaining all the previous dB values and adding them to the next dB values. i.e dB= Bnext-Bprev+dBprev, whereas I only want the function to do like this dB= 0 (first value) and then Bnext-Bprev
Can somebody help me in sorting this out.. Will greatly appreciate any help! Thanks

Best Answer

As per your requirement
function dB=fcn(B)
   persistent Bprev;
   if isempty (Bprev)
      Bprev = B;
      dB = 0;
   else
      dB = B-Bprev;
      Bprev = B; % Save current value to use it as previous in next sample time. 
   end
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