How to get index of each points in data?

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),[4],'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]

Answer 1

[KL]

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:

https://de.mathworks.com/help/matlab/ref/sortrows.html

Answer 2

[Are Mjaavatten]

>> [~,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

Answer 3

[Stephen23]

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