Obtaining the index of elements that belong to group

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NA 2020-9-8

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https://ww2.mathworks.cn/matlabcentral/answers/590764-obtaining-the-index-of-elements-that-belong-to-group

评论： Adam Danz 2020-9-9

I have 4 groups

A =[1;2;3];
B = [4;5];
C = [6;7];
D = [8,9];

I put all of these value together

all_val = [1;2;3;4;5;6;7;8;9];

From all_val I choose 2 elements.

idx = datasample(all_val, 2,'Replace',false);

I want to know the chosen idx is belong to which groups.

I do not know how to write this code.

Result should be something like:

i=1                          i=2                      
third index of A             first index of B    
second index of c            second index of D

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Answer 1

Adam Danz 2020-9-8

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https://ww2.mathworks.cn/matlabcentral/answers/590764-obtaining-the-index-of-elements-that-belong-to-group#answer_491713

编辑：Adam Danz 2020-9-8

在 MATLAB Online 中打开

See the second output of datasample to get the selected index.

[y,idx] = datasample(___)

The problem you're having is that you don't know which rows of all_val corresponds to which groups. When you create all_val you need to keep track of the group IDs.

Demo:

A =[1;2;3];
B = [4;5];
C = [6;7];
D = [8;9];
abcd = {A;B;C;D};
all_vals = cell2mat(abcd); % <-- Create all_vals this way
n = cellfun(@numel,abcd); 
g = repelem(1:numel(abcd),1,n)'; 
g(:,2) = cell2mat(arrayfun(@(n){(1:n)'},n));

g is a nx2 grouping variable where the first column indicates which variable (A,B,C,D) each value in all_vals comes from. The second column indicates the index within that variable.

This is where the 2nd output to datasample comes into play.

[y, idx] = datasample(all_val, 2,'Replace',false);

If idx is [2,8] then the selected variable comes from

g(idx,:)
%     ans =
%          1     2   % 1st variable (A), 2nd row
%          4     1   % 4th variable (D), 1st row  

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NA 2020-9-9

在 MATLAB Online 中打开

Thank you for your suggestion. I want to use them as labels.

Suppose I have the additional data.

e = [1,2; 2,3; 3,1];
v = (1:max(max(3)))';
%% first grouping
A =[1;2;3];
B = [4;5;6];
C = [7;8;9];
D = [10;11;12];
%% second grouping
los = {A;C}; 
def = {B;D}; 

Rest of the code

abcd = {A;B;C;D};
all_vals = cell2mat(abcd); % <-- Create all_vals this way
n = cellfun(@numel,abcd); 
g = repelem({'A','B','C','D'},1,n)';
g(:,2) = repelem({'los','def','los','def'},1,n)';

I want to add another column to g like this

g =
  12×3 cell array
    {'A'}    {'los'}    {[1,2]}
    {'A'}    {'los'}    {[2,3]}
    {'A'}    {'los'}    {[3,1]}
    {'B'}    {'def'}    {[1]}
    {'B'}    {'def'}    {[2]}
    {'B'}    {'def'}    {[3]}
    {'C'}    {'los'}    {[1,2]}
    {'C'}    {'los'}    {[2,3]}
    {'C'}    {'los'}    {[2,3]}
    {'D'}    {'def'}    {[1]}
    {'D'}    {'def'}    {[2]}
    {'D'}    {'def'}    {[3]}

I mean

    first element of los should be ---> e(1)=(1,2)
    second element of los should be ---> e(2)=(2,3)
    third element of los should be ---> e(3)=(3,1)
    
    first element of def should be ---> v(1)=1
    second element of def should be ---> v(2)=2
    third element of def should be ---> v(3)=3
    