Question

I don’t know how to do it. I have some matrices with repeated values and I’d like to sort them and get a matrix of sorted indices. For example:

a = [1 4 3 10 8 2];
b = sort(a);

% This doesn't work but I wish it did - that's what I'm looking for.
% idx = find(a==b);  idx = [1 6 3 2 5 4];

[v idx] = ismember(b,a);

However, when there are repeated values and NaN this produces errors. Try this:

a = [1 NaN 4 2 10 8 2];
b=sort(a);
[v, i] = ismember(b,a);

Gives [1 7 7 3 6 5 0] which is valid but I need it to be [ 1 4 7 3 6 5 0].

I can deal with this later but it’s going to be much more elegant if it were to return the above result.

Answer 1

sort has a two output value version that gives you exactly the indices you’re looking for:

a = [1 4 3 10 8 2];
[b, idx] = sort(a);

Outputs

idx =
     1     6     3     2     5     4

This works correctly with NaNs as well:

a = [1 NaN 4 2 10 8 2];
[b, idx] =sort(a);
>> idx
idx =
     1     4     7     3     6     5     2
>> b
b =
     1     2     2     4     8    10   NaN