How do I loop through a matrix and break it up into distinct sections?

I have a set of data that is a 510x3 matrix in the form of :

[2.000000 56 0.000000

4.000000 5.267480 2

4.500000 6.979411 2

5.000000 8.427968 2

5.500000 9.744838 2

6.000000 11.193396 2

6.500000 12.378578 2

7.000000 13.168700 2

7.500000 14.748944 2

8.000000 15.407379 2

8.500000 15.670753 2

9.000000 16.855936 2

9.500000 17.382685 2

10.000000 19.753050 2 ...]

Every 56 lines of this data is a line that includes the number 55 OR 56 in column 2. I am trying to loop through this matrix and break it up into distinct separate sections marked by these lines that include the number 55/56. My ultimate objective is essentially to reformat this data into a form that utilizes indices positions instead of real world horizontal distances, and this will require several nested loops to read through the data section by section. I am relatively new to programming and coding in this manner, so any tips would be incredibly helpful!

This is my starting point:

% line to read in data from file

j=zeros(3,length(data));
for i=1:length(data)
    if data(i,2)== 55 || 56
        j=data(i,:);
        i=i+1;
    end
end

EDIT: Thank you for the responses! I didn't want to just leave this question open forever with no updates, but I did actually find a python script that someone had made to do the exact thing I'm trying to do (converting data from one software format to another)!

[Dustin]

If I need to provide more information, please don't hesitate to let me know!

Answer 1

[Walter Roberson]

mask = ismember(YourData(:,2), [55 56]) .';  %row vector result
starts = strfind([0 mask], [0 1]);
stops = [starts(2:end)-1, size(YourData,1)];

The data blocks are then

for K = 1 : numel(starts)
    blocks{K} = data(YourData(starts(K):stops(K), :));
end

Answer 2

[Adrián László Szemenyei]

you can use cell arrays for that ( https://www.mathworks.com/help/matlab/cell-arrays.html )

A=randi(60,510,3);%simulate your data
%A(:,2)==55|A(:,2)==56 index of rows where value is 55 or 56
b=[1:size(A,1)]'.*(A(:,2)==55|A(:,2)==56);%get the indices of such rows
indxs=[b(b~=0);size(A,1)];%one can alternatively use the find function also
indx_differences=[indxs(1);diff(indxs)];
my_data=mat2cell(A,indx_differences);%in documentation for mat2cell, rowDist=indx_differences