List representation using element position

• To: mathgroup at smc.vnet.net
• Subject: [mg72506] List representation using element position
• From: "Dr. Wolfgang Hintze" <weh at snafu.de>
• Date: Wed, 3 Jan 2007 01:39:12 -0500 (EST)
• Organization: privat

```Hello group,
happy new year to all of you!

This one was put up in a slightly different form by me in March 2006.
It is now more general and it is lossless with respect to information:

Given a list of integers which may repeat, e.g.

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

provide a list of the different values and their respective positions in
the original list. In the example,

LstOut= {
{1,{6,7}},
{2,{2,5}},
{3,{2}},
{4,{3,4,9}},
{5,{8}}
}

Who finds the shortest function doing this task in general?

My solution appears 15 lines below

Thanks.

Best regards,
Wolfgang
1

5

10

fPos[lstIn_] := Module[{f = Flatten /@ (Position[lstIn, #1] & ) /@
Union[lstIn]}, ({#1, f[[#1]]} & ) /@ Range[Length[f]]]

In[15]:=
fPos[lstIn]

Out[15]=
{{1, {6, 7}}, {2, {1, 5}}, {3, {2}}, {4, {3, 4, 9}}, {5, {8}}}

```

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