Converting arbitrary R expressions to PMML

The pmmlTransformations R package can be used to transform data and add new features to be used in predictive PMML models.
In this blog post, we will focus on FunctionXform, a function introduced in version 1.3.0 of pmmlTransformations, and present a few examples of using it to create new data features.

How it works

Transformations in the pmmlTransformations package work in the following manner: given a WrapData object and a transformation name, the code calculates data for a new feature and creates a new WrapData object. This new object is then passed in as the data argument when training an R model with a compatible R package. When PMML is produced with pmml::pmml(), the transformation is inserted into the LocalTransformations node as a DerivedField. Any original fields used by transformations are added to the appropriate nodes in the resulting PMML file.
While other transformations in the package transform only one field, FunctionXform makes it possible to use multiple data fields and functions to produce a new feature.
Note that while FunctionXform is part of the pmmlTransformations package, the code to produce PMML from R is in the pmml package. The following examples require both packages to be installed to work.
To make tables more readable in this blog post, we are using the kable function (part of knitr).

Single numeric field

Using the iris dataset as an example, let’s construct a new numeric feature by transforming one variable.
First, load the required libraries:

library(pmml)
library(pmmlTransformations)
library(knitr)

Then load the data and display the first 3 lines:

data(iris)
kable(head(iris,3))

Sepal.Length	Sepal.Width	Petal.Length	Petal.Width	Species
5.1	3.5	1.4	0.2	setosa
4.9	3.0	1.4	0.2	setosa
4.7	3.2	1.3	0.2	setosa

Create the irisBox wrapper object with WrapData:

irisBox <- WrapData(iris)

irisBox contains the data and transform information that will be used to produce PMML later. The original data is in irisBox$data. Any new features created with a transformation are added as columns to this data frame.

kable(head(irisBox$data,3))

Sepal.Length	Sepal.Width	Petal.Length	Petal.Width	Species
5.1	3.5	1.4	0.2	setosa
4.9	3.0	1.4	0.2	setosa
4.7	3.2	1.3	0.2	setosa

Transform and field information is in irisBox$fieldData. The fieldData data frame contains information on every field in the dataset, as well as every transform used. The functionXform column contains expressions used in the FunctionXform transform. Here we’ll show only a few of the columns:

kable(irisBox$fieldData[,1:5])

	type	dataType	origFieldName	sampleMin	sampleMax
Sepal.Length	original	numeric	NA	NA	NA
Sepal.Width	original	numeric	NA	NA	NA
Petal.Length	original	numeric	NA	NA	NA
Petal.Width	original	numeric	NA	NA	NA
Species	original	factor	NA	NA	NA

Now add a new feature, Sepal.Length.Sqrt, using FunctionXform:

irisBox <- FunctionXform(irisBox,origFieldName="Sepal.Length",
                         newFieldName="Sepal.Length.Sqrt",
                         formulaText="sqrt(Sepal.Length)")

The new feature is calculated and added as a column to the irisBox$data data frame:

kable(head(irisBox$data,3))

Sepal.Length	Sepal.Width	Petal.Length	Petal.Width	Species	Sepal.Length.Sqrt
5.1	3.5	1.4	0.2	setosa	2.258318
4.9	3.0	1.4	0.2	setosa	2.213594
4.7	3.2	1.3	0.2	setosa	2.167948

irisBox$fieldData now contains a new row with the transformation expression in the functionXform column:

kable(irisBox$fieldData[6,c(1:3,14)])

	type	dataType	origFieldName	functionXform
Sepal.Length.Sqrt	derived	numeric	Sepal.Length	sqrt(Sepal.Length)

Construct a linear model to predict Petal.Width using this new feature, and convert it to PMML:

fit <- lm(Petal.Width ~ Sepal.Length.Sqrt, data=irisBox$data)
fit_pmml <- pmml(fit, transform=irisBox)

Since the model predicts Petal.Width using a variable based on Sepal.Length, Sepal.Length will be added to the DataDictionary and MiningSchema nodes in the resulting PMML. We can take a look at the relevant parts of the output like so:

fit_pmml[[2]] #Data Dictionary node
#> <DataDictionary numberOfFields="2">
#>  <DataField name="Petal.Width" optype="continuous" dataType="double"/>
#>  <DataField name="Sepal.Length" optype="continuous" dataType="double"/>
#> </DataDictionary>
fit_pmml[[3]][[1]] #Mining Schema node
#> <MiningSchema>
#>  <MiningField name="Petal.Width" usageType="predicted"/>
#>  <MiningField name="Sepal.Length" usageType="active"/>
#> </MiningSchema>

The LocalTransformations node contains Sepal.Length.Sqrt as a derived field:

fit_pmml[[3]][[3]]
#> <LocalTransformations>
#>  <DerivedField name="Sepal.Length.Sqrt" dataType="double" optype="continuous">
#> <Apply function="sqrt">
#>   <FieldRef field="Sepal.Length"/>
#> </Apply> 
#>  </DerivedField>
#> </LocalTransformations>

The PMML model can now be deployed and consumed. For any input data, the new Sepal.Length.Sqrt feature will be created when the data is scored against the model.

Multiple input fields

It is also possible to create new features by combining several fields. Using the same iris dataset, let’s create a new field using squares of Sepal.Length and Petal.Length:

irisBox <- WrapData(iris)
irisBox <- FunctionXform(irisBox,origFieldName="Sepal.Length,Petal.Length",
                         newFieldName="Squared.Length.Ratio",
                         formulaText="(Sepal.Length / Petal.Length)^2")

As before, the new field is added as a column to the irisBox$data data frame:

kable(head(irisBox$data,3))

Sepal.Length	Sepal.Width	Petal.Length	Petal.Width	Species	Squared.Length.Ratio
5.1	3.5	1.4	0.2	setosa	13.27041
4.9	3.0	1.4	0.2	setosa	12.25000
4.7	3.2	1.3	0.2	setosa	13.07101

Fit a linear model for Petal.Length using this new feature, and convert it to PMML:

fit <- lm(Petal.Width ~ Squared.Length.Ratio, data=irisBox$data)
fit_pmml <- pmml(fit, transform=irisBox)

The PMML will contain Sepal.Length and Petal.Length in the DataDictionary and MiningSchema, since these were used in FormulaXform:

fit_pmml[[2]] #Data Dictionary node
#> <DataDictionary numberOfFields="3">
#>  <DataField name="Petal.Width" optype="continuous" dataType="double"/>
#>  <DataField name="Sepal.Length" optype="continuous" dataType="double"/>
#>  <DataField name="Petal.Length" optype="continuous" dataType="double"/>
#> </DataDictionary>
fit_pmml[[3]][[1]] #Mining Schema node
#> <MiningSchema>
#>  <MiningField name="Petal.Width" usageType="predicted"/>
#>  <MiningField name="Sepal.Length" usageType="active"/>
#>  <MiningField name="Petal.Length" usageType="active"/>
#> </MiningSchema>

The Local.Transformations node contains Squared.Length.Ratio as a derived field:

fit_pmml[[3]][[3]]
#> <LocalTransformations>
#>  <DerivedField name="Squared.Length.Ratio" dataType="double" optype="continuous">
#> <Apply function="pow">
#>   <Apply function="/">
#>     <FieldRef field="Sepal.Length"/>
#>     <FieldRef field="Petal.Length"/>
#>   </Apply>
#>   <Constant dataType="double">2</Constant>
#> </Apply> 
#>  </DerivedField>
#> </LocalTransformations>

PMML for arbitrary functions

The function functionToPMML (part of the pmml package) makes it possible to convert an R expression into PMML directly, without creating a model or calculating values. This can be useful for debugging.
As long as the expression passed to the function is a valid R expression (e.g., no unbalanced parentheses), it can contain arbitrary function names not defined in R. Constants in the expression passed to FunctionXform are always assumed to be of type double. Variables in the expression are always assumed to be field names, and not substituted. That is, even if x has a value in the R environment, the resulting expression will still use x.

functionToPMML("1 + 2")
#> <Apply function="+">
#>   <Constant dataType="double">1</Constant>
#>   <Constant dataType="double">2</Constant>
#> </Apply>

x <- 3
functionToPMML("foo(bar(x * y))")
#> <Apply function="foo">
#>   <Apply function="bar">
#>     <Apply function="*">
#>       <FieldRef field="x"/>
#>       <FieldRef field="y"/>
#>     </Apply>
#>   </Apply>
#> </Apply>

functionToPMML("if(a<2) {x+3} else if (a>3) {'four'} else {5}")
#> <Apply function="if">
#>   <Apply function="lessThan">
#>     <FieldRef field="a"/>
#>     <Constant dataType="double">2</Constant>
#>   </Apply>
#>   <Apply function="+">
#>     <FieldRef field="x"/>
#>     <Constant dataType="double">3</Constant>
#>   </Apply>
#>   <Apply function="if">
#>     <Apply function="greaterThan">
#>       <FieldRef field="a"/>
#>       <Constant dataType="double">3</Constant>
#>     </Apply>
#>     <Constant dataType="string">four</Constant>
#>     <Constant dataType="double">5</Constant>
#>   </Apply>
#> </Apply>

Conclusion

functionXform makes it possible to easily create new features for PMML models with R.
The pmmlTransformations functionXform vignette contains additional examples, including transforming categorical data, using transformed features in another transform, unsupported functions, and notes on limitations of the function.

References

• pmmlTransformations on CRAN
• PMML 4.2.1 official specification
• In-depth discussion of pmmlTransformations