Counter goes up than down to 1 in recursive function

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Léonard Roussel 2012-7-4

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Hello,

I want my function to return how many 'moving windows' of a particular size can fit in a given length_n-long signal, with a given hop size.

here is my function :

    |  function N_n = numWindows( N_n, currentSample_n, hopSize_n, length_n, width_n )
      % check if we have reached the end of the signal
      if currentSample_n + width_n >= length_n 
          return
      else
          currentSample_n     = currentSample_n + hopSize_n ;
          N_n = N_n + 1;
          % here we call the same function on the rest of the signal
          numWindows( N_n, currentSample_n, hopSize_n, length_n, width_n ); 
      end
      return
      end|

This code returns systematically N_n = 2; When I entered debug mode I saw that N_n was increasing normally to the correct value, then as the function exited the "if"s one after the other, I saw that N_n was decreasing 1 by 1, just as it had increased in the first place. And the function stopped with N_n = 2.

I tried it with a while loop but it was quickly taking a very long time as the signals started to get long (= big length_n input value).

I would be very grateful to any kind of advice on this problem, thank you very much !

Léo.

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AC 2012-7-4

Nevermind, got it, see answer below.

Léonard Roussel 2012-7-4

Oh yeah I just saw it

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AC 2012-7-4

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Ok, I think I got it: I believe you need to recursively output N_n when you are calling numWindows within the function.

I don't know signal processing, so sorry if it doesn't make sense, but if you try:

N_n=numWindows(1,1,2,100,4);

Then you'll get N_n=49 after modification.

Does it sound about right?

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Léonard Roussel 2012-7-4

Yes it DOES work, but I actually don't understand what really changed. Anyway, thank you very much !

AC 2012-7-4

Sure :) What changes is just the storage of your variable, not the computations. When you compute N_n=N_n+1; you do it within the recursive function (except for the first one), but you never actually store it. It's quite tricky, a bit like time travel :)

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