how to find indices of duplicates in array and save results on a structure

Hi, I want to know how to find indices of duplicaes in a given array and save results on a structure.

For example, array A is given as below:

A=[1;5;2;5;3;3;4;1;2;1];

so there are duplicates of values: 1,2,3,5 , and I want to find the indices of duplicates directly without using 'find'

I manually wrote down on structure as below, but how can I write the code to save the indices of duplicates on a structure?

So those are my two questions.. 1) how to find indices of duplicates in array , and 2) how to save results on a structure

Many thanks:)

Answer 1

[Stephen23]

A = [1;5;2;5;3;3;4;1;2;1];
[U,~,X] = unique(A);
V = 1:numel(A);
C = accumarray(X,V(:),[],@(v){v});
S = struct('value',num2cell(U),'indices',C)

S = 5×1 struct array with fields:
    value
    indices

Checking:

S.value

ans = 1

ans = 2

ans = 3

ans = 4

ans = 5

S.indices

ans = 3×1

1 8 10

ans = 2×1

3 9

ans = 2×1

5 6

ans = 7

ans = 2×1

2 4