Extracting last (non NaN) 200 columns from matrix with varying number of NaNs ending the rows

Question

I have pupil size data from an eye tracking experiment. In the experiment, to start each trial the participant looks in the centre of the screen for 1000 ms (prefixation period), then the trial begins. If they look away or blink, the 1000 ms period restarts. So, each trial has a different length prefixation period. When I create a matrix (with each row being a different trial, and each column being a pupil size sample over time) it creates the matrix with the number of columns based on the trial with the longest prefixation period, and then adds varying numbers of NaNs to the end of each other row. I need to extract the last 200 samples (columns) of each trial, but these are not the last 200 columns of the matrix because of the additional NaNs that are added.

At the moment I have this:

Row1 = PreFixBase(1,:); % extract the first row
Row1(isnan(Row1)) = []; %get rid of the NaNs
Row1Base = Row1(end-200+1:end); %extract the last 200 samples / columns

which I do for each row separately and then paste them back together. It works, but is really inefficient (I have 324 rows / trials) and I'm sure there must be a more concise way of doing this, but haven't been able to find the answer.

Any help appreciated.

Amy

rahnema1 rahnema1 · Accepted Answer · 2017-07-13T05:17:20

You can use cumsum + logical indexing to extract the desired elements

Base = PreFixBase.';                            % transpose the matrix
S=cumsum(~isnan(Base),1,'reverse');             % number last non NaN columns from 1 to 200 (from end to begin)
Result = reshape(Base(S> 0 & S<=200),200,[]).'; % extract data and reshape to the correct size

Extracting last (non NaN) 200 columns from matrix with varying number of NaNs ending the rows

1 Answers