cellfun with two arrays of indices - matlab

Question

I have one big cell with N by 1 dimension. Each row is either a string or a double. A string is a variable name and the sequential doubles are its values until the next string (another variable name). For example:
data = {
var_name1;
val1;
val2;
val3;
val4;
val5;
var_name2;
val1;
val2;
var_name3;
val1;
val2;
val3;
val4;
val5;
val6;
val7}
and so on. I want to separate the data cell into three cells; {var_name and it's 5 values}, {var_name and it's 2 values}, {var_name and it's 7 values}. I try not to loop as much as possible and have found that vectorization along with cellfun works really well. Is it possible? The data cell has close to million rows.

Answer 1

I believe the following should do what you're after. The main pieces are to use cumsum to work out which name each row corresponds to, and then accumarray to build up lists per name.
% Make some data
data = {'a'; 1; 2; 3;
'b'; 4; 5;
'c'; 6; 7; 8; 9;
'd';
'e'; 10; 11; 12};
% Which elements are the names?
isName = cellfun(#ischar, data);
% Use CUMSUM to work out for each row, which name it corresponds to
whichName = cumsum(isName);
% Pick out only the values from 'data', and filter 'whichName'
% for just the values
justVals = data(~isName);
whichName = whichName(~isName);
% Use ACCUMARRAY to build up lists per name. Note that the function
% used by ACCUMARRAY must return something scalar from a column of
% values, so we return a scalar cell containing a row-vector
% of those values
listPerName = accumarray(whichName, cell2mat(justVals), [], #(x) {x.'});
% All that remains is to prepend the name to each cell. This ends
% up with each row of output being a cell like {'a', [1 2 3]}.
% It's simple to make the output be {'a', 1, 2, 3} by adding
% a call to NUM2CELL on 'v' in the anonymous function.
nameAndVals = cellfun(#(n, v) [{n}, v], data(isName), listPerName, ...
'UniformOutput', false);

Answer 2

cellfun is for applying a function to each element of a cell.
When you pass multiple arguments to cellfun like that, it takes the ith argument of data, indx_first, and indx_last, and uses each of them in the anonymous function. Substituting those variables in, your function evaluates to x(y : z), for each element x in data. In other words, you're doing data{i}(y : z), i.e., indexing the actual elements of the cell array, rather than indexing the cell array itself. I don't think that's what you want. Really you want data{y : z}, for each (y, z) pair given by corresponding elements in indx_first and indx_last, right?
If that's indeed the case, I don't see a vectorized way to solve your problem, because each of the "variables" has different size. But you do know how many variables you have, which is the size of indx_first. So I'd pre-allocate and then loop, like so:
>> vars = cell(length(indx_first), 2);
>> for i = 1:length(vars)
vars{i, 1} = data{indx_first(i) - 1}; % store variable name in first column
vars{i, 2} = [data{indx_first(i) : indx_last(i)}]; % store data in last column
end
At the end of this, you'll have a cell array with 2 columns. The first column in each row is the name of the variable. The second is the actual data. I.e.
{'var_name1', [val1 val2 val3 val4 val5];
'var_name2', [val1 val2];
.
.
.