MATLAB: How to filter without a for loop (first order filter)

filterfor loopMATLABrecursive

Hi for a large vector (361156 points or more) I have this for loop to filter engine speed (RPM), it works fine but it is too slow
for i=1:length(RPM)
   RPMFilt(i) = RPM(i) + RPMFilt(i-1) - RPM(i) * RPMFiltConstant
end
where RPMFiltConstant = 0.98 (heavily filtered)
Too avoid the for loop, I guess that I could use the filter function y = filter(b,a,x) (only the matlab filter function, I don't have the DSP or signal processing tool boxes)
But the problem is I am not sure what I should set b and a to in order to achieve the same result as the equation above with 0.98 filter constant, can anyway explain and show how to calculate what b and a should be for different values of RPMFiltConstant between 0 and 1.
thanks

Best Answer

Your code does not run: What is RPMFilt(i-1) for i=1 ? Please post the real code, which runs and produces the wanted result.
Do you pre-allocate the output?
RPM = rand(1, 361156);
RPMFiltConstant = 0.98;
tic;
RPMFilt    = zeros(size(RPM));
RPMFilt(1) = RPM(1);  % Or what else?

for i = 2:length(RPM)
  RPMFilt(i) = RPM(i) + RPMFilt(i-1) - RPM(i) * RPMFiltConstant;
end
toc
This takes 0.026 sec on my R2016b/64/Win7 system. Not so much. This is a little bit faster:
tic;
b          = 1 - RPMFiltConstant;
RPMFilt    = zeros(size(RPM));
RPMFilt(1) = RPM(1);  % Or what else?
for i = 2:length(RPM)
  RPMFilt(i) = b * RPM(i) + RPMFilt(i-1);
end
toc
And with filter:
    RPMFilt = [RPM(1), filter([0.02, 0], [1, -1], RPM(2:end), RPM(1))];
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