Finding the set of unique values for another set of unique values

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MJ 2019-1-30

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评论： MJ 2019-1-31

hi everyone,

i have a dataset of which the first column contains buyers ID', the second column has a certin index, i need an output that shows what are the indeces associated with each unique buyer ID, for example: if i have a set like this:

1 3

1 4

2 5

2 4

3 1

3 7

3 9

M = [ 1,3;1,3;1,4;2,5;2,4;3,1;3,7;3,7;3,7;3,9];

i want an output in the form of

1 [3 4]

2 [5 4]

3 [1 7 9]

can anyone please help me?

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Answer 1

Guillaume 2019-1-31

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i am looking for an array type [...] can it be filled with NaN or zeros?

NaN is probably wiser.

M = [ 1,3;1,3;1,4;2,5;2,4;3,1;3,7;3,7;3,7;3,9];
Mdedup = unique(M, 'rows');  %remove duplicate rows.
[id, ~, sub] = unique(Mdedup(:, 1)); %list of unique IDs and corresponding location
maxcount = max(accumarray(sub, 1));  %get max of histogram of IDs = width of matrix to create
result = [id, cell2mat(accumarray(sub, Mdedup(:, 2), [], @(v) {[v.', nan(1, maxcount - numel(v))]}))]

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MJ 2019-1-31

This code works perfect, thank you very much for your time!

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Answer 2

Ollie A 2019-1-30

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This should do the trick:

M = [ 1,2;1,3;1,2;2,4;2,4;2,5]; % Your matrix
u = unique(M(:,1));
for x = 1:length(u)
    N{x} = unique(M(M(:,1)==u(x),2));
end
T = table(u,N'); % Output as table

The output is a table, which displays the data in the way you requested.

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Guillaume 2019-1-30

编辑：Guillaume 2019-1-30

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@Ollie A, get in the habit of preallocating your arrays rather than growing them in a loop, Growing an array in a loop means that at each step of the loop matlab has to reallocate a whole new array and copy over all the previously assigned elements. It's slow. In addition, since you don't preallocate the array, if the variable already exists but is of a different type, you'll get an error. If the variable already exists, is the correct type but larger than length(u), you'll end up with extra elements.

N = cell(size(u));

before the loop would work.

Another way to obtain the same without a loop:

M = [1, 2; 1, 3; 1, 2; 2, 4; 2, 4; 2, 5];
[u, ~, subs] = unique(M(:, 1))
N = accumarray(subs, M(:, 2), [], @(v) {v});
T = table(u, N)

@mohammad,

"is there anyway to separate the 2 and 3?"

What does that even mean? The 2 and 3 are stored as a vector in the particular row of the table. You seem to be confusing storage and display

"what does the 2 is the end refer to?"

This is basic indexing. M(something, 2) always refers to column 2 of M. In this particular case, it's all the elements of column 2 for which column 1 is equal to u(x).

Guillaume 2019-1-31

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"I want to separate the values in the same cell to two different cells"

A table/cell array/matrix is always rectangular. In the case where there's less elements in a row than others what do you want to put in the missing columns?

With your original example, the ouput would be

3 4 x
5 4 x
1 7 9

What should x be?

Also, what type of output are you looking for (matrix? cell array? table?)

MJ 2019-1-31

"what do you want to put in the missing columns?"

can it be filled with NaN or zeros?

"what type of output are you looking for (matrix? cell array? table?)"

i am looking for an array type, i want to use the output numbers in other calculations.

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Answer 3

Stephen23 2019-1-31

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https://ww2.mathworks.cn/matlabcentral/answers/442377-finding-the-set-of-unique-values-for-another-set-of-unique-values#answer_358829

编辑：Stephen23 2019-1-31

>> F = @(v){unique(v,'stable').'};
>> C = accumarray(M(:,1),M(:,2),[],F);
>> C{:}
ans =
   3   4
ans =
   5   4
ans =
   1   7   9

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Stephen23 2019-1-31

编辑：Stephen23 2019-1-31

@mohammad aljarrah: one very easy solution is to download padcat:

https://www.mathworks.com/matlabcentral/fileexchange/22909-padcat

and simply use it like this:

 >> padcat(C{:})
ans =
     3     4   NaN
     5     4   NaN
     1     7     9

MJ 2019-1-31

thank you very much

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