If I have a 2100 x 16 matrix, how can I specify for a certain value to go in to rows 1:84, then another into 85:169, and repeat this 25 times in some kind of loop?

Hi,

Sorry for the random question. I'm trying to simulate data to follow a certain format of output file. I already have a script to generate simulated survey data. Basically, I want to add in subject numbers without doing it manually. So, if subject 1 has data from rows 1:84, subject 2 from rows 85:169, down until subject 25 who ends at row 2100... I want to have a column that has "1" for the first 84 rows, then 2 for the next 84 rows, etc.

I'm sure there's a straightforward way to do this with a for loop, I just can't seem to figure it out.

Thanks for any help!

Answer 1

[Stephen23]

Don't bother with a loop, in MATLAB vectorized code is neater and faster:

>> A = 1:4; % the group enumeration
>> N = 3;   % the number of rows in each group
>> reshape(repmat(A,N,1),[],1)
ans =
   1
   1
   1
   2
   2
   2
   3
   3
   3
   4
   4
   4

You can then simply allocate those values to any column of your matrix, here are three options (you did not specify how you want this to be allocated):

X(:,1) = reshape(repmat(A,N,1),[],1); % into existing first column
X(:,end+1) = reshape(repmat(A,N,1),[],1); % into a new trailing column
X = [reshape(repmat(A,N,1),[],1), X]; % into a new first column

[Chelsea]

Wow, that is a lot more clean looking than the loop I was trying to create :P Thanks!!

[Stephen23]

My pleasure. It takes a while to learn a tool like MATLAB, but it is faster and more enjoyable when you learn the efficient ways of doing things.