How can I create a MxN tall array, out of tall column vectors without known and not equal row numbers?

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Eva-Maria Weiss
Eva-Maria Weiss 2019-8-29
评论： Eva-Maria Weiss 2019-8-29
Hi there,
again I'm struggeling with tall arrays and want to ask you for help.
I have a data set consisting of voltage values. Data was collected on a few hundreds of electrodes. So voltage values are stored row-wise per electrodes, that are columnwise ordered.
Now, in my program the crossings of a threshold are calculated by looping through the columns relusting in tall column vectors. Depending on the number of detected crossings the number of values differs. I need the values evaluated (using gather in the end) for further processing, but I want to do that for the crossings of all electrodes all together, I would like to store each tall column vector in one tall variable.
But I don't know how. Since the number of values differ, horzcat is not possible. I thought I could preallocate by a zero tall array, with the highest possible number of values as row number, but I guess I cannot index that to store a tall column vector with unknown number of rows.
I appreciate any ideas and hints.
Thank you so mch in advance
Eva
ds = datastore(fullfile(filepath,filename));
names = ds.VariableNames;
len = length(names); %first column contains time
%len = number of electrodes + 1
for col = 2:len
nam = names{1,col};
ds.SelectedVariableNames = {nam};
dat = table2array(tall(ds));
crossings = goBinning(dat, parameter, filterCoeff);
cr = gather(crossings);
positions = getLoc(dat,crossings); %searching for precise location by max slope in spike
end

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回答（1 个）

Guillaume
Guillaume 2019-8-29
Instead of horizontally concatenating the results, which as you say is not possible since they don't have the same number of rows, why not vertically concatenate them with a second column indicating which column each row corresponds to:
ds = datastore(fullfile(filepath,filename));
t = tall(ds);
allcrossings = [];
for col = 2:width(t) %much simpler way to iterate over the columns
coldata = t.(col);
crossings = goBinning(coldata, paramter, filterCoeff);
colvalue = crossings; %duplicate tall array
colvalue(:) = col; %and set to column index
allcrossings = [allcrossings; colvalue, crossings];
end
allcrossings = gather(allcrossings);
No idea if in practice it's more efficient than multiple gather.

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