# How Could Classification Trees Be So Fast on Categorical Variables?

# How Could Classification Trees Be So Fast on Categorical Variables?

A tutorial on how to create classification trees and calculating probability in Java.

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I think that over the past months, I have been saying non-correct things about classification with categorical covariates. Because I never took time to look at it carefuly, consider some simulated dataset, with a logistic regression:

```
> n=1e3
> set.seed(1)
> X1=runif(n)
> q=quantile(X1,(0:26)/26)
> q[1]=0
> X2=cut(X1,q,labels=LETTERS[1:26])
> p=exp(-.1+qnorm(2*(abs(.5-X1))))/(1+exp(-.1+qnorm(2*(abs(.5-X1)))))
> Y=rbinom(n,size=1,p)
> df=data.frame(X1=X1,X2=X2,p=p,Y=Y)
```

Here, we use some continuous covariate, except that is considered as not-observed. Instead, we have a categorical covariate with 26 categories. The (theoretical) relationship between the covariate and the probability is given below:

```
> vx1=seq(0,1,by=.001)
> vp=exp(-.1+qnorm(2*(abs(.5-vx1))))/(1+exp(-.1+qnorm(2*(abs(.5-vx1)))))
> plot(vx1,vp,type="l")
```

And the empirical probability, for each modality is:

If we run a classification tree, we get:

```
> library(rpart)
> tree=rpart(Y~X2,data=df)
> library(rpart.plot)
> prp(tree, type=2, extra=1)
```

To be more specific, the output is here:

```
> tree
1) root 1000 249.90000 0.4900000
2) X2=F,G,H,I,J,K,L,M,N,O,P,Q,R 499 105.3 0.302
4) X2=J,K,L,M,N,O,P,Q,R 346 65.12 0.25144 *
5) X2=F,G,H,I 153 37.22876 0.4183007 *
3) X2=A,B,C,D,E,S,T,U,V,W,X,Y,Z 501 109.61 0.67
6) X2=B,C,D,E,S,T,U,V,W,X 385 90.38 0.623 *
7) X2=A,Y,Z 116 14.50862 0.8534483 *
```

Note that it takes less than a second to get that output. So clearly, we did not look for all combinations between modalities. For the first node, there are like possible groups, i.e.

`> 67108864`

It is big… not huge, but too big to try all combinations, since that’s only the first node, and we have to do it again on the two leaves, etc. Antoine (aka @ly_antoine) told me – while we were having a coffee after lunch today – the trick to get a fast algorithm, on categories. And as usual, the idea is very clever…

First, we need a function to compute Gini index:

```
> gini=function(y,classe){
+ T=table(y,classe)
+ nx=apply(T,2,sum)
+ n=sum(T)
+ pxy=T/matrix(rep(nx,each=2),nrow=2)
+ omega=matrix(rep(nx,each=2),nrow=2)/n
+ g=-sum(omega*pxy*(1-pxy))
+ return(g)}
```

For the first node, the idea is very simple:

Compute empirical averages:

`> cond_prob=aggregate(df$Y,by=list(df$X2),mean)`

Then sort those values:

Based on that ordering, consider

`> Group_Letters=cond_prob[order(cond_prob$x),2]`

Then consider (only) possible partitions,

against

```
> v_gini=rep(NA,26)
> for(v in 1:26){
+ CLASSE=df$X2 %in% Group_Letters[1:v]
+ v_gini[v]=gini(y=df$Y,classe=CLASSE)
+ }
```

If we plot them, we get:

`> plot(1:26,v_gini,type="b)`

As for continuous variables, we seek for the maximum value, and then, we have our two groups:

```
> sort(Group_Letters[1:which.max(v_gini)])
[1] F G H I J K L M N O P Q R
```

That’s exactly what we got with the tree function in R:

```
1) root 1000 249.90000 0.4900000
2) X2=F,G,H,I,J,K,L,M,N,O,P,Q,R 499 105.30 0.30
```

Now, consider the leaf on the left (for instance):

`> sub_df=df[df$X2 %in% sort(Group_Letters[1:which.max(v_gini)]),]`

Then use the same algorithm as before: sort the conditional means:

```
> cond_prob=aggregate(sub_df$Y,by=
+ list(sub_df$X2),mean)
> s_Group_Letters=cond_prob[order(cond_prob$x),2]
```

Then compute Gini indices based on groups obtained from that ordering:

```
> v_gini=rep(NA,length(sub_Group_Letters))
> for(v in 1:length(sub_Group_Letters)){
+ CLASSE=sub_df$X2 %in% s_Group_Letters[1:v]
+ v_gini[v]=gini(y=sub_df$Y,classe=CLASSE)
+ }
```

If we plot it, we get our two groups:

```
> plot(1:length(s_Group_Letters),v_gini,type="b")
```

And the first group is here:

```
> sort(sub_Group_Letters[1:which.max(v_gini)])
[1] J K L M N O P Q R
```

Again, that’s exactly what we got with the R function:

```
1) root 1000 249.90000 0.4900000
2) X2=F,G,H,I,J,K,L,M,N,O,P,Q,R 499 105.30 0.30
4) X2=J,K,L,M,N,O,P,Q,R 346 65.12 0.25144 *
```

Clever, isn’t?

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