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 t开发者_Go百科here is nothing built in, is there an implementation of that function available somewhere? Does someone here know how to build it?

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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

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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}

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