CODEWITHSUNDEEP
matlabwolfram-mathematica
clartaq
clartaq

Reputation: 5372

Does Mathematica have a function equivalent to Matlab's "unique" function

Is there a Mathematica function that provides results equivalent to the unique() function in MATLAB? I realize I could use the Union[] function to obtain the unique elements of a list, but I would like something equivalent to the three-result version of the function that provides index arrays that map between the input array and the array of unique values.

If there is nothing built in, is there an implementation of that function available somewhere? Does someone here know how to build it?

Upvotes: 7

Views: 3247

Answers (3)

earnric
earnric

Reputation: 534

There's a simple way:

a={1,2,3,4,5,5,5,4,3,2}

  {1,2,3,4,5,5,5,4,3,2}

uniques = DeleteDuplicates[a]

  {1,2,3,4,5}

Upvotes: 2

Cambise Homa
Cambise Homa

Reputation: 11

Try Length[Union[x]]. If x=[1,0,1,1,1], then you'll get Length[Union[x]] = 2.

Upvotes: 1

Timo
Timo

Reputation: 4326

You can easily build similar functionality yourself with Mathematica's Position[]. E.g. given a list of numbers you could do the following:

In[1] := A = {1, 5, 2, 3, 7, 3, 2, 8, 6, 5, 9, 2, 1};
In[2] := {#, Flatten[Position[A, #]]} & /@ Union[A]
Out[2]:= {{1, {1, 13}}, {2, {3, 7, 12}}, {3, {4, 6}}, {5, {2, 10}}, {6, {9}}, {7, {5}}, {8, {8}}, {9, {11}}}

to get the list of unique elements and the indices of where they appear in the original list. To replicate exactly the functionality of Matlab's Unique(), especially for

[b,m,n] = unique(A)

you need

b = Union[A];
m = Last[Position[A, #]] & /@ b // Flatten;
n = Position[b, #] & /@ A // Flatten;

which now provide the desired behavior

In[1] := A[[#]] & /@ m == b
Out[1]:= True

In[2] := b[[#]] & /@ n == A
Out[2]:= True

Upvotes: 8

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