How to separate specific data from an array in a new file?

Hello,

I have a doubt on how to separate different data from an array. For example: Each matrix has different number of rows and columns. In the first column of each matrix, the numbers 1, 2, and 3 always appear. How can I do to separate the data in a new file ??? containing only the data of the lines that have the number 1, that have the number 2 and that have the number 3 ???

Thanks and grateful for your help!

Answer 1

[dpb]

ia=ismember(data(:,1),[1:3]);
writematrix(data(ix,:))