MATLAB and Simulink resources for Arduino, LEGO, and Raspberry Pi

Learn moreOpportunities for recent engineering grads.

Apply Today**New to MATLAB?**

Asked by LifeSux SuperHard
on 12 Jun 2013

This is, lol, I guess another personal exercise (sort of, although it is kind towards solving a bigger problem).

Anyway I'm having trouble figuring out how to find the unique numbers in an array/set.

**Now, I KNOW about the unique function. It's cool. I'm even using it elsewhere. But I am also interested in how it is written, and the unique.m file in the matlab toolbox is basically incomprehensible to me.**

So far I have started out really simple. What I am trying to do is simply look at an array, see if a number is repeated (literally true false atm), then stick that number and it's occurrence boolean into a cell. Here is the code so far:

%What we want to do is fill a cell as follows %C=cell{1,2,N(=Unique values in [S])} %Where %C{1,1,N} = [N_unique] (N_unique => Nq) %C{1,2,N} = [Occurence of N_unique] (=> On(Nq)) %Such that %C{:,:,1} = %[Nq(1)] [Oc(Nq(1))] %... %C{:,:,N} = %[Nq(N)] [Oc(Nq(N))] % %This requires an index N that iterates only once for every number, %whether or not it occurs more than once. %This also requires another index Oc that iterates once for every time %its respective number occurs. Oc must be at least one for every number. %So, the array [267 308 267] should produce %C{:,:,1} = %[267] [2] %C{:,:,2} = %[308] [1]

A = [267 267 308];

N = 1; %Index for each unique number On = 0;

for i = 1:length(A) for j = 1:length(A) if A(i) == A(j) && i~=j %fprintf('%d %d %d %d %d\n', A(i),i, A(j),j,1); %A(i) has occured more than once On = 1; %True for On(A(i)) elseif A(i) ~= A(j) fprintf('%d %d %d %d %d\n', A(i),i, A(j),j, 0); %A(i) has not occured more than once On = 0; %False for On(A(i)) end C{1,1,N} = A(i); C{1,2,N} = On; end end

Now this code is pretty unfinished, obviously. For example, N doesn't even increase. It just sits there the whole time at 1. This is because I can't figure out how to get it to iterate once PER NUMBER, as opposed to, for example, per loop.

Also Oc right now is just true/false, not the actual occurrence rate.

Anyway sorry for wasting time but I'm just really fuckin frustrated and curious about how to do this.

*No products are associated with this question.*

Answer by Roger Stafford
on 12 Jun 2013

Accepted answer

If I were writing a 'unique' function, the first operation would be to sort the set. There are many efficient routines to do that. After that it is a breeze picking out only those elements in the sorted sequence that are unique: pick the first (or last) of each consecutive subsequence of equal elements.

Show 5 older comments

LifeSux SuperHard
on 12 Jun 2013

To first order

B = [N1 N2 N3 ... Nn]; A = sort(B); j = 1; for i = 1:length(A) if i+1>length(A) if A(i-1) ~= A(i) U(j) = A(i); end break elseif A(i) ~= A(i+1) U(j) = A(i); j = j+1; end end

Roger Stafford
on 12 Jun 2013

You are, in effect, storing the last element in each series of equal elements in A, but for a final sequence of the largest element there is no following element to compare them with. That is why you skipped the last one. You should unconditionally store the final element of A in U. In other words, leave out the "if A(i-1) ~= A(i)" part and just do "U(j) = A(i);" at that point.

Here's an easier way to compute U:

A = sort(B); U = A([true,diff(A)~=0]);

Of course you have to work harder to reorder U according to the original order in B. You have to work even harder to come back with the other two returned index vectors from matlab's 'unique' function.

Answer by LifeSux SuperHard
on 12 Jun 2013

B = [1 2 2 3 3 3 4 4 4 4]; A = sort(B); j = 1;

for i = 1:length(A) if i+1> length(A) if A(i)~=A(i-1) O{1,1,j} = A(i); elseif A(i) == A(i-1) O{1,1,j} = A(i); end break elseif A(i)~= A(i+1) O{1,1,j} = A(i); j = j+1; end end

k = 0; for i = 1:length(O) for j = 1:length(A) if O{1,1,i} == A(j) k = k + 1; end O{1,2,i} = k; end k = 0; end

O

LifeSux SuperHard
on 12 Jun 2013

If anyone can think of a better way to do/optimize this code please let me know. It's roughly .01s slower than matlab's own unique function, or matlab's unique is about 6.5 times faster.

## 1 Comment

## ¥agoth

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/78851#comment_168848

I have a similar issue, I need to sort a cell array containing numbers, but unique only works for strings. I have tried :