Why do I get: Error using sum Invalid data type. First argument must be numeric or logical?

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I have a cell array called "pre_data" with 1 column and 27 rows. Each element in the column contains a cell with 21 colums and a varying number of rows.
I want to scan the columns in the cells of pre_data. For each seperate column, if there are values in a column that are above 3 standard deviations of that column, then I want the row cointaining that value to be removed.
For that I have written the following piece of code:
pre_data_clean = cell(size(pre_data));
% iterate over each cell in pre_data
for i = 1:length(pre_data)
data_pre = pre_data{i}; % get the data in the current cell
means_pre = cellfun(@mean, data_pre, 'UniformOutput', false); % calculate the means of each column
stds_pre = cellfun(@std, data_pre, 'UniformOutput', false); % calculate the standard deviations of each column
% remove values that are above 3 standard deviations from the mean
for j = 1:size(data_pre{1}, 2) % iterate over each column
for k = 1:length(data_pre) % iterate over each row in each cell
data_pre{k}(:, j) = data_pre{k}(:, j) .* (abs(data_pre{k}(:, j) - means_pre{k}(j)) <= 3*stds_pre{k}(j));
pre_data_clean{i} = data_pre; % save the cleaned data to pre_data_clean
The code seems to me like it works. But when I apply the code for other cell arrays that I have (balls_data, between_data, baskets_data or post_data), similar to "pre_data", then I continue to get error messages like this:
Error using sum
Invalid data type. First argument must be numeric or logical.
Error in mean (line 127)
y = sum(x, dim, flag) ./ mysize(x,dim);
Is this because the cell arrays have empty cells? If so, how can I fix this code?
Thank you!

Answers (1)

Walter Roberson
Walter Roberson on 14 Mar 2023
You generally have cell arrays that contain cell arrays. However, in some places some of the entries are not cell arrays and are instead [] which is an empty double array.
Example fix:
baskets_data(cellfun(@isempty,baskets_data)) = {{}};
lil brain
lil brain on 16 Mar 2023
Thank you!
I have one last question. I just tried editing the code by making it scan for values above 1 SD... but nothing is removed which I think is not what would happen. Is the code performing correctly because it seems odd to me that not one value is above 1 SD.

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