Vectorize scalar and colon

THIS IS SOMETHING I CAN DO OK WITH A for LOOP, BUT I AM TRYING TO VECTORIZE TO SPEED IT UP:

I ENTER THE FOLLOWING CODE:

p=[.25 .25 .25 .45 .45 .45];
L=length(p);
Nit=10;
preindit=zeros(L,Nit);
l=1:L;
targetit=round(p(l).*Nit);
preindit(l,1:targetit(l))=1;

THIS GIVES THE RESULT:

targetit =

3 3 3 5 5 5

preindit =

1 1 1 0 0 0 0 0 0 0

1 1 1 0 0 0 0 0 0 0

1 1 1 0 0 0 0 0 0 0

1 1 1 0 0 0 0 0 0 0

1 1 1 0 0 0 0 0 0 0

1 1 1 0 0 0 0 0 0 0

WITH THE WARNING

Warning: Colon operands must be real scalars. This warning will become an error in a future release.

BUT WHAT I WAS HOPING FOR WAS:

preindit =

1 1 1 0 0 0 0 0 0 0

1 1 1 0 0 0 0 0 0 0

1 1 1 0 0 0 0 0 0 0

1 1 1 1 1 0 0 0 0 0

1 1 1 1 1 0 0 0 0 0

1 1 1 1 1 0 0 0 0 0

IE MATLAB IS APPLYING THE targetit-value FOR l=1 TO ALL ROWS OF preindit, NOT JUST THE FIRST ROW. MATLAB DOES NOT DO THAT, OR GIVE THE WARNING ABOUT SCALARS, WHEN THE SAME CODE IS EMBEDDED IN A LOOP.

WHAT AM I DOING WRONG WITH THE VECTORIZATION PLEASE?

NB the full thing I am trying to do is on a very large scale, with multiple different values of p and Nit, so simply typing in the array I want is not a sensible option, it has to be done by a loop (slow) or vectorization, I think. The challenge seems to be to get MATLAB to recognize that each possible value of targetit(1:L) is a real scalar.

Answer 1

[Rahul]

Hi @William Sherwin,

I understand that you are trying to vectorize to speed up the code and obtain the desired result mentioned in the question.

You can do that by following this method:

• STEP 1: Creting a matrix of indices
• STEP 2: Then using logical indexing to fill the ones

p = [.25 .25 .25 .45 .45 .45];
L = length(p);
Nit = 10;
preindit = zeros(L, Nit);
targetit = round(p .* Nit);

% STEP 1
indices = repmat(1:Nit, L, 1);
disp(indices);

% STEP 2
preindit(indices <= targetit') = 1;

You can refer to the following documentations to know more:

'repmat' = https://www.mathworks.com/help/releases/R2024a/matlab/ref/repmat.html?searchHighlight=repmat&s_tid=doc_srchtitle

'Array Indexing': https://www.mathworks.com/help/releases/R2024a/matlab/math/array-indexing.html?searchHighlight=logical%20indexing&s_tid=doc_srchtitle

Hope this helps! Thanks.

[William Sherwin]

Thanks Rahul, Bill

Answer 2

[KSSV]

A = zeros(6,10) ;
A(:,1:3) = 1 ;
A(4:6,4:5) = 1;