# How to get index of each points in data?

1 view (last 30 days)
ha ha on 18 Nov 2017
Edited: KL on 18 Nov 2017
Let's say:
A= [ 111 111 111 22 %index 1
999 999 999 11 %index 2
555 555 555 44 %index 3
222 222 222 33] %index 4
If I want to reorder the rows in matrix A by sorting the elements of column 4 in ascending order, I will use this code:
sort_cell=arrayfun(@(c) sortrows(A,c),,'uniform',0);
% generate a cell by sorting matrix A in ascending of column 4
sort_A=sort_cell{:};
% convert cell "sort_cell_A" to matrix
The result will as follows:
sort_A= [ 999 999 999 11
111 111 111 22
222 222 222 33
555 555 555 44 ]
My question is: How can I get back the index matrix from "sort_A" matrix? I hope the result will be:
index=[2
1
4
3]

KL on 18 Nov 2017
Edited: KL on 18 Nov 2017
Simply use sortrows,
>> [B,index] = sortrows(A,4)
B =
999 999 999 11
111 111 111 22
222 222 222 33
555 555 555 44
index =
2
1
4
3
and below link has all you need:

Are Mjaavatten on 18 Nov 2017
>> [~,ix] = sort(A(:,4))
ix =
2
1
4
3
>> sort_A = A(ix,:)
sort_A =
999 999 999 11
111 111 111 22
222 222 222 33
555 555 555 44

Stephen Cobeldick on 18 Nov 2017
Edited: Stephen Cobeldick on 18 Nov 2017
Your code would be much simpler if you called sort on the required column:
>> [~,idx] = sort(A(:,4))
idx =
2
1
4
3
>> B = A(idx,:)
B =
999 999 999 11
111 111 111 22
222 222 222 33
555 555 555 44
To get back the original order, call sort again on the indices:
>> [~,idy] = sort(idx)
idy =
2
1
4
3
>> C = B(idy,:)
C =
111 111 111 22
999 999 999 11
555 555 555 44
222 222 222 33