Since my variable changes size on every loop iteration, how do I preallocate memory?

MATLAB says that my variable activity_dff changes size on every loop iteration and for this reason I should consider preallocating. I do know the size of the array before the for loop begins (I do know it'll always have 1 row and n columns, where n is equal to the number of rows of another variable Activity_smooth), so I should probably preallocate it and then shrink it afterwards by doing: X = X(1:n,:);

%% Calculate dff
fd = prctile(Activity_smooth,10); %baseline, f0

% Calculating Activity(k)-fd/fd for photometry
for k = 1:length(Activity_smooth) 
   Activity_dff(k) = (Activity_smooth(k)-fd)/fd;
end

Activity_dff_ds = group_z_project_vector(Activity_dff,2560); %to see in epoch 

end

Is the following code correct? And how do I make sure it's faster than the previous one?

%% Calculate dff
fd = prctile(Activity_smooth,10); %baseline, f0

% Calculating Activity(k)-fd/fd for photometry
Activity_dff = zeros(1,1000000);
for k = 1:length(Activity_smooth) 
   Activity_dff(k) = (Activity_smooth(k)-fd)/fd;
end
Activity_dff = Activity_dff(:,1:length(Activity_smooth));

Activity_dff_ds = group_z_project_vector(Activity_dff,2560); %to see in epoch 

end

Answer 1

[Image Analyst]

You can do this:

%% Calculate dff
fd = prctile(Activity_smooth,10); %baseline, f0

% Calculating Activity(k)-fd/fd for photometry
Activity_dff = zeros(1, length(Activity_smooth));
for k = 1:length(Activity_smooth) 
   Activity_dff(k) = (Activity_smooth(k)-fd)/fd;
end
Activity_dff = Activity_dff(1:length(Activity_smooth));

Activity_dff_ds = group_z_project_vector(Activity_dff,2560); %to see in epoch 

or even better, get rid of the loop and other unneeded stuff:

%% Calculate dff
fd = prctile(Activity_smooth,10); %baseline, f0
Activity_dff = Activity_smooth-fd ./ fd;
Activity_dff_ds = group_z_project_vector(Activity_dff,2560); %to see in epoch 

Answer 2

[Ameer Hamza]

From you code, it appear that Activity_dff is of same size as Activity_smooth

% Calculating Activity(k)-fd/fd for photometry
Activity_dff = zeros(size(Activity_smooth)); % <---- pre-allocation
for k = 1:length(Activity_smooth) 
   Activity_dff(k) = (Activity_smooth(k)-fd)/fd;
end
Activity_dff_ds = group_z_project_vector(Activity_dff,2560); %to see in epoch