# Learning Tutorial

In this tutorial we set up a simple linear chain CRF, such as the one used for part-of-speech tagging, named-entity recognition, or noun phrase chunking. It will be our running example in this tutorial, but most of the things we’ll discuss generalize all across Factorie.

package cc.factorie.tutorial
object Tutorial60Learning extends App {
import cc.factorie._
import variable._
import cc.factorie.app.nlp._
import cc.factorie.app.chain._
import cc.factorie.optimize.{ SynchronizedOptimizerOnlineTrainer, Trainer, SampleRankTrainer }
import cc.factorie.infer.{ GibbsSampler, InferByBPChain }

implicit val random = new scala.util.Random(0)



First we create domains representing the labels and features.

  object LabelDomain extends CategoricalDomain[String]
class Label(val token: Token, s: String) extends LabeledCategoricalVariable(s) {
def domain = LabelDomain
}
object FeaturesDomain extends CategoricalVectorDomain[String]
class Features(val token: Token) extends BinaryFeatureVectorVariable[String] {
def domain = FeaturesDomain
}



The ChainModel class implements a default model for linear chains. It by default implements all the factor templates one expects from a linear chain model, with the exception that the (label, label, features) template is optional.

As we saw in the Model tutorial, a model in Factorie is an object that can take some variables and return some factors, which know how to score assignments of those variables. So models store weightsSet, and things like that.

To construct a chain model you need to give it a few things. First, the domains of the labels and the token features. Then, functions that can take a label to its feature vector, a label to its token, and a token to its label. These functions are used by the ChainModel class to generate factors.

  object model extends ChainModel[Label, Features, Token](
LabelDomain,
FeaturesDomain,
l => l.token.attr[Features],
l => l.token,
t => t.attr[Label])

// The Document class implements documents as sequences of sentences and tokens.
val document = new Document("The quick brown fox jumped over the lazy dog.")
val tokenizer = new app.nlp.segment.DeterministicTokenizer
tokenizer.process(document)
val segmenter = new app.nlp.segment.DeterministicSentenceSegmenter
segmenter.process(document)
assertStringEquals(document.tokenCount, "10")
assertStringEquals(document.sentenceCount, "1")

// Let's assign all tokens the same label for the sake of simplicity
document.tokens.foreach(t => t.attr += new Label(t, "A"))
// Let's also have another possible Label value to make things interesting
LabelDomain.index("B")
// Let's also initialize features for all tokens
document.tokens.foreach(t => {
val features = t.attr += new Features(t)
// One feature for the token's string value
features += "W=" + t.string.toLowerCase
// And one feature for its capitalization
features += "IsCapitalized=" + t.string(0).isUpper.toString
})



Now that we have one sentence and a model we can start to do inference in it.

Inference in Factorie is abstracted in objects that implement the Infer trait, which has one method, infer, that takes a sequence of variables, a model, and an optional Summary, and returns a Summary object with the result of inference.

A Summary object has two essential pieces of information in it: it allows you to compute the normalization constant of the model (useful for learning), and it can return marginal distributions over subsets of variables in your model. It also knows how to turn these marginals into expected sufficient statistics for learning.

The most commonly used Infer objects are those in the BP package.

  val summary = InferByBPChain.infer(document.tokens.toSeq.map(_.attr[Label]), model)
assertStringEquals(summary.logZ, "6.931471805599453")
"Proportions(0.49999999999999994,0.49999999999999994)")



Aside from InferByBPChainSum, which knows how to run forward-backward on chain models Factorie has MaximizeByBPChain, which runs viterbi, InferByBPTreeSum, which runs BP on trees, and InferByBPLoopy which runs loopy belief propagation.

For more inference tools see the Inference tutorial.

One of the main uses of inference is in learning.

The learning infrastructure in Factorie is made of three components which can be mixed and matched at will.

An Example abstracts over a piece of data and knows how to compute objectives and gradients.

GradientOptimizers know how to update a model’s weightsSet given gradients and values.

In the middle between Examples and GradientOptimizers we have Trainers, which control when the gradients are evaluated, where they are stored, the degree of parallelism, etc.

The most common example one uses is the LikelihoodExample, which computes the value and the gradient for maximum likelihood training. Here’s how to construct one for this sentence using Factorie’s BP inferencer.

  val example = new optimize.LikelihoodExample(document.tokens.toSeq.map(_.attr[Label]), model, InferByBPChain)



In this tutorial let’s use the AdaGrad optimizer, which is efficient and has per-coordinate learning rates but is not regularized

  val optimizer0 = new optimize.AdaGrad()



To learn a model the simplest way is to call Trainer.onlineTrain. This will optimize the parameters with AdaGrad and parameter averaging using SGD.

  Trainer.onlineTrain(model.parameters, Seq(example), optimizer = optimizer0)

// Factorie also supports batch learning. Note that regularization is built into the optimizer
val optimizer1 = new optimize.LBFGS with optimize.L2Regularization
optimizer1.variance = 10000.0
Trainer.batchTrain(model.parameters, Seq(example), optimizer = optimizer1)



Factorie also supports other batch trainers. The ParallelBatchTrainer keeps a per-thread gradient vector and aggregates all gradients before sending them to the optimizer, while the SynchronizedBatchTrainer keeps a single gradient vector and locks it.

Also note that all online optimizers can be used with the batch trainers, but not the other way around.

One can also not call Trainer.onlineTrain or Trainer.batchTrain and create a Trainer explicitly. A Trainer is a policy object which is responsible for deciding which gradients are evaluated, when, and in which threads, and when these gradients are given to the optimizers.

All trainers inherit from Trainer. To create a trainer, simply instantiate it.

  val trainer = new SynchronizedOptimizerOnlineTrainer(model.parameters, optimizer0)


To use the trainer we can either call .trainFromExamples or call things in a loop:

  trainer.trainFromExamples(Seq(example))
// or
while (!trainer.isConverged) {
trainer.processExamples(Seq(example))
}


Some trainers might require some tear down after training for optimal usage. See the implementation of Trainer.train for an example of how to do so.

Factorie also has many other optimizers. Usually AdaGrad with ParameterAveraging is a good bet, but often one wants something more specific.

There are two main regularized online optimizers in Factorie: AdaGradRDA, which uses regularized dual averaging and supports l1 and l2 regularization, and L2RegularizedConstantRate, which supports only l2 regularization. Neither of these optimizers support parameter averaging.

  // Now we can run inference and see that we have learned
val summary2 = InferByBPChain(document.tokens.map(_.attr[Label]).toIndexedSeq, model)
assertStringEquals(summary2.logZ, "48.63607808729122")
"Proportions(0.9999308678897892,6.913211020966629E-5)")



Factorie also has support for more efficient learning algorithms than traditional inference-based batch and online methods.

The main such method is SampleRank, which runs a sampler and updates the weightsSet while the sampler explores the posterior to make the model’s predictions match an arbitrary loss function.

  val sampler = new GibbsSampler(model, HammingObjective)
val sampleRankExamples = document.tokens.toSeq.map(t => new optimize.SampleRankExample(t.attr[Label], sampler))
Trainer.onlineTrain(model.parameters, sampleRankExamples, optimizer = optimizer0)
// SampleRank comes with its own trainer, however, for ease of use
val trainer2 = new SampleRankTrainer(model.parameters, sampler, optimizer0)
trainer2.processContexts(document.tokens.toSeq.map(_.attr[Label]))
}


Finally, there are many other useful examples in Factorie. The LinearMulticlassExample implements generalized linear models for many classification loss functions, for example, and the DominationLossExample knows how to do learning to rank.