Accumulating different sized column vectors stored as a cell array into a matrix padded with NaNs

Question

Imagine I have a series of different sized column vectors inside an array and want to group them into a matrix by padding the empty spaces with NaN. How can I d

Answer 1

If you want speed the cell data structure is your enemy. For this example I will assume you have this vectors stored in a structure called vector_holder:

elements = fieldnames(vector_holder);

% Per Dan request
maximum_size = max(structfun(@max, vector_holder));

% maximum_size is the maximum length of all your separate arrays
matrix = NaN(length(elements), maximum_size);

for i = 1:length(elements)
    current_length = length(vector.holder(element{i}));
    matrix(i, 1:current_length) = vector.holder(element{i});
end

Many Matlab functions are slower when dealing with cell variables. In addition, a cell matrix with N double-precision elements requires more memory than a double-precision matrix with N elements.

Answer 2

You can just slightly tweak that answer you found to work for columns:

tcell = {[1,2,3]', [1,2,3,4,5]', [1,2,3,4,5,6]', [1]', []'};                      %\\ ignore this comment, it's just for formatting in SO
maxSize = max(cellfun(@numel,tcell));    
fcn = @(x) [x; nan(maxSize-numel(x),1)]; 
cmat = cellfun(fcn,tcell,'UniformOutput',false);  
cmat = horzcat(cmat{:}) 

cmat =

     1     1     1     1   NaN
     2     2     2   NaN   NaN
     3     3     3   NaN   NaN
   NaN     4     4   NaN   NaN
   NaN     5     5   NaN   NaN
   NaN   NaN     6   NaN   NaN

Or you could tweak this as an alternative:

cell2mat(cellfun(@(x)cat(1,x,NaN(maxSize-length(x),1)),tcell,'UniformOutput',false))