MATLAB: combine rows with similar values - matlab

Question

I am new to MATLAB and I am trying to combine rows with similar values (I have thousands of rows), for example
1 NaN
1 NaN
1 NaN
2 9
2 26.5
2 21.5
2 18
2 24.5
2 12
2 22.5
3 NaN
3 NaN
3 NaN
3 NaN
4 18.5
4 22
4 35.5
...
...
...
to
1 NaN NaN NaN
2 9 26.5 21.5 18 24.5 12 22.5
3 NaN NaN NaN NaN
4 18.5 22 35.5
can any one please help me with this?

Answer 1

This can't be done with normal arrays. Each row has to have same number of columns, but your desired output isn't so. You can work with cell arrays if you wish.
If cell arrays are an option, the best way to tackle this IMHO would be to use an accumarray/sort/cellfun pipeline. First use accumarray to group all of the values together that belong to the same ID, so the first column in your case. Each group would thus be a cell array. However, a consequence with accumarray is that the values that come in per group are unordered. Therefore, what you'd have to group instead are the locations of the values instead. You'd sort these locations and what is output is a cell array where each cell are a list of indices you'd access in the original data.
You'd then call cellfun as the last step to use the indices access the actual data itself.
Something like this comes to mind, assuming your data is stored in X and it's a two-column array.
ind = (1 : size(X,1)).'; %'
out_ind = accumarray(X(:,1), ind, [], #(x) {sort(x)});
out = cellfun(#(x) X(x,2), out_ind, 'uni', 0);
We thus get:
>> celldisp(out)
out{1} =
NaN
NaN
NaN
out{2} =
9.0000
26.5000
21.5000
18.0000
24.5000
12.0000
22.5000
out{3} =
NaN
NaN
NaN
NaN
out{4} =
18.5000
22.0000
35.5000