How to append values to an array in for loop?

rotation_time_by_direction = [];
for i = 1:10:length(direction_table.rodent_RecordingTime)-10
    direction_change = ones(1,10)';
    for j = 1:10
        direction_change(j,1) = abs(direction_table.direction(i,1)-direction_table.direction(i+j,1));
        threshold_degree = 100;
        if any(direction_change > threshold_degree)
            rotation_time_by_direction = cat(1, rotation_time_by_direction, direction_table.rodent_RecordingTime(i));
        end 
    end
end

I am trying append the rodent_RecordingTime values to rotation_time_by_direction array when rotation is more than 100 degrees over next 10 time steps. But I am not sure why the rodent_RecordingTime is appending to the array. I have attached the variables if you want to look at it. Any help is appreciated.

[Stephen23]

"But I am not sure why the rodent_RecordingTime is appending to the array."

cat(1, rotation_time_by_direction, direction_table.rodent_RecordingTime(i))
%                                                  ^^^^^^^^^^^^^^^^^^^^

[Struggling in MATLAB]

Sorry, I didn't get your comment. If you are asking what do I mean by "But I am not sure why the rodent_RecordingTime is appending to the array.", you can consider it redundant.

Answer 1

[dpb]

>> for i=1:10:height(direction_table)-10
dx=abs(direction_table.direction(i+1:i+9)-direction_table.direction(i));
ix=find(dx>100,1);
id=ix+i-1;
if any(ix), disp([direction_table{i,:} dx(ix)]),end
end
            0      -5.1105      -465.77      -72.713       107.33
            4       146.82      -398.22      -228.38       324.64
           12      -126.12      -125.06      -378.62        102.2
           13      -111.11      -120.79      -379.49       289.69
           14       168.73       -118.5      -374.87       215.66
           17       177.23      -118.23      -382.94       352.16
>> 

[Struggling in MATLAB]

Thank you very much. Although 'disp' works but when I try to store it to a variable it shows me only 1 row.

load('direction_table.mat');
for i=1:10:height(direction_table)-10
    dx=abs(direction_table.direction(i+1:i+9)-direction_table.direction(i));
    ix=find(dx>100,1);
    if any(ix)
        x = [direction_table{i,:} dx(ix)];
    end
end

x =

   17.0000  177.2270 -118.2320 -382.9450  352.1560

[dpb]

You didn't subscript x -- writing a variable on the LH side replaces the entire variable with the RHS each time.

[Struggling in MATLAB]

What should I use for subscript if I don't know the height of resulting table beforehand?

[dpb]

Well, the maximum number of rows you can have would be height(direction_table)/10 since that's the index expression for your loop...since you may have some data subsets that contain no matches, there may not be that many "real" answers, but it's the most you could possibly have.

So, allocate for that and then you can remove rows with nothing in them when done --

This is also a case where you would use a second indexing variable inside the if...end clause and only increment it there...

H=height(direction_table)-10;
out=nan(ceil(H/10),5);
j=0;
for i=1:10:
  dx=abs(direction_table.direction(i+1:i+9)-direction_table.direction(i));
  ix=find(dx>100,1);
  id=ix+i-1;
  if any(ix)
    j=j+1;
    out(j,:)=[direction_table{i,:} dx(ix)];
  end
end
out=out(isfinite(out(:,1)),;);

[Walter Roberson]

x(end+1,:) = etc

[Struggling in MATLAB]

Thank you everyone for your comments. I kept on trying with append method, and I think this gives me the result I wanted.

load('direction_table.mat');
rotation_time_by_direction = [];

for i = 1:10:height(direction_table)-10
    direction_change = ones(1,10)';
    for j = 1:10
        direction_change(j,1) = abs(direction_table.direction(i,1)-direction_table.direction(i+j,1));
    end
    threshold_degree = 100;
    if any(direction_change > threshold_degree)
        rotation_time_by_direction = [rotation_time_by_direction; direction_table.rodent_RecordingTime(i)];
    end
end
n_direction_table = direction_table{ismember(direction_table.rodent_RecordingTime,rotation_time_by_direction), :};