RE: How to vectorize this code?

[Bård Skaflestad]

It's been a while since I checked so I don't know if Octave has adopted the 'repelem' function from MATLAB.  If it has, then this task is accomplished by

 

    ind = repelem(1:numel(DIV), reshape(DIV, 1, []));

Otherwise, you can use a standard CUMSUM incantation:

    ind = zeros([sum(DIV), 1]);  # Preallocate
    ind(cumsum([1 , reshape(DIV(1 : end – 1), 1, [])) = 1; # Put "1" at index jumps
    ind = cumsum(ind);  # Realize those resulting indices

Demo:  DIV = 1 : 5 => [1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5]

Regards,
Bård Skaflestad
SINTEF Digital, Mathematics & Cybernetics

From: Help-octave [mailto:help-octave-bounces+address@hidden On Behalf Of Brian Kaczynski
Sent: onsdag 11. april 2018 13.24
To: address@hidden
Subject: How to vectorize this code?

 

Hello Octavians!

 

I have a long vector DIV (containing about 150,000 elements) which contains integers only from 32 to 64, inclusive.  I want to create a longer vector containing 1's repeated DIV(1) times, 2's repeated DIV(2) times, 3's repeated DIV(3) times, etc.  This is how I'm doing it now:

 

  ind = [];

  for j = 1:length(DIV)

       ind = [ind ; j*ones(DIV(j),1)];

  end

 

I know there are some experts at vectorizing for-loop code like this.  Who can give this a shot?

 

Thanks!

-Brian