Mathematica: part assignment

问题

I'm trying to implement an algorithm to build a decision tree from a dataset. I wrote a function to calculate the information gain between a subset and a particular partition, then I try all the possible partition and want to choose the "best" partition, in the sense that it's got the lowest entropy. This procedure must be recursive, hence, after the first iteration, it needs to work for every subset of the partition you got in the previous step.

These are the data:

X = {{1, 0, 1, 1}, {1, 1, 1, 1}, {0, 1, 1, 1}, {1, 1, 1, 0}, {1, 1, 0, 0}}

Xfin[0]=X

This is the function: for every subset of the partition, it tries all the possible partitions and calculate the IG. Then it selects the partition with IGMAX:

Partizioneottimale[X_, n_] := 
For[l = 1, l <= Length[Flatten[X[n], n - 1]], l++, 
For[v = 1, v <= m, v++, 
If[IG[X[n][[l]], Partizione[X[n][[l]], v]] == IGMAX[X[n][[l]]], 
 X[n + 1][[l]] := Partizione[X[n][[l]], v]]]]

then I call it:

Partizioneottimale[Xfin, 0]

and it works fine for the first one:

 Xfin[1]

 {{{1, 0, 1, 1}, {1, 1, 1, 1}, {0, 1, 1, 1}, {1, 1, 1, 0}}, {{1, 0, 0, 0}}}

That is the partition with lowest entropy.

But it doesn't work for the next ones:

  Partizioneottimale[Xfin, 1]
  Set delayed::steps : Xfin[1+1] in the part assignment is not a symbol

Has anybody any idea about how to solve this? Thanks

回答1:

without unraveling all your logic a simple fix is this:

Partizioneottimale[X_, n_] := (
  xnp1 = Table[Null, {Length[Flatten[X[n], n - 1]]}] ;
  For[l = 1, l <= Length[Flatten[X[n], n - 1]], l++, 
  For[v = 1, v <= m, v++, 
  If[IG[X[n][[l]], Partizione[X[n][[l]], v]] == IGMAX[X[n][[l]]], 
  xnp1[[l]] = Partizione[X[n][[l]], v]]]] ;
  X[n+1] = xnp1 ; )

来源：https://stackoverflow.com/questions/16500401/mathematica-part-assignment