How to filter rows?

I have a matrix like this

1 2 3 4 5 6 7 8
2 3 4 1 2 3 4 5 
1 2 3 4 1 1 1 1
1 2 3 5 4 2 2 1

I want to separate those rows having first four entries as 1 2 3 4, as in above case I separate out row 1 and row three.

Answer 1

[Aletta Wilbrink]

Probably not the most efficient way, but this works

b = a(a(:,1)==1 & a(:,2) == 2 & a(:,3) == 3 & a(:,4) == 4,:)

Where a is the name of your matrix

[Ammy]

Thank you very much.

[Aletta Wilbrink]

Seeing Guillaume's answer, a better way is

b = a(all(a(:,1:4) == [1 2 3 4],2),:)

Answer 2

[Guillaume]

%R2016b or later:
tokeep = all(A(:, 1:4) == [1 2 3 4], 2)
%earlier versions
tokeep = all(bsxfun(@eq, A(:, 1:4), [1 2 3 4]), 2)

%then
A(tokeep, :)

[Kurt]

(Corrected) How do you make this work if the matrix contains cells?

[Les Beckham]

@Kurt

This question was about extracting elements from a matrix, not from a table containing cell arrays. You should start a new question and provide example data and what you want to do with the data, including any code you have tried (along with the results you expect and what you are getting instead, if applicable).

I suggest reading this first: TUTORIAL: how to ask a question (on Answers) and get a fast answer

[Kurt]

The question is simply "How to filter rows?"

Anyway, I found the answer I needed. To filter rows of cells (not numerics) you can either read your data in as a table or convert your matrix to a table. Then use this one-liner:

output_data = input_data(table2array(input_data(:,col)) == "pattern",:);

where "col" is the column number you are filtering on.

There may be other approaches, but this works for me.

[Les Beckham]

I'm glad you figured it out.