triTrainingG {ssc}

Tri-training generic method

Description

Tri-training is a semi-supervised learning algorithm with a co-training style. This algorithm trains three classifiers with the same learning scheme from a reduced set of labeled examples. For each iteration, an unlabeled example is labeled for a classifier if the other two classifiers agree on the labeling proposed.

Usage

triTrainingG(y, gen.learner, gen.pred)

Arguments

y

A vector with the labels of training instances. In this vector the unlabeled instances are specified with the value NA.

gen.learner

A function for training three supervised base classifiers. This function needs two parameters, indexes and cls, where indexes indicates the instances to use and cls specifies the classes of those instances.

gen.pred

A function for predicting the probabilities per classes. This function must be two parameters, model and indexes, where the model is a classifier trained with gen.learner function and indexes indicates the instances to predict.

Details

TriTrainingG can be helpful in those cases where the method selected as base classifier needs a learner and pred functions with other specifications. For more information about the general triTraining method, please see the triTraining function. Essentially, the triTraining function is a wrapper of the triTrainingG function.

Value

A list object of class "triTrainingG" containing:

model

The final three base classifiers trained using the enlarged labeled set.

model.index

List of three vectors of indexes related to the training instances used per each classifier. These indexes are relative to the y argument.

instances.index

The indexes of all training instances used to train the three models. These indexes include the initial labeled instances and the newly labeled instances. These indexes are relative to the y argument.

model.index.map

List of three vectors with the same information in model.index but the indexes are relative to instances.index vector.

Examples

library(ssc)

## Load Wine data set
data(wine)

cls <- which(colnames(wine) == "Wine")
x <- wine[, -cls] # instances without classes
y <- wine[, cls] # the classes
x <- scale(x) # scale the attributes

## Prepare data
set.seed(20)
# Use 50% of instances for training
tra.idx <- sample(x = length(y), size = ceiling(length(y) * 0.5))
xtrain <- x[tra.idx,] # training instances
ytrain <- y[tra.idx]  # classes of training instances
# Use 70% of train instances as unlabeled set
tra.na.idx <- sample(x = length(tra.idx), size = ceiling(length(tra.idx) * 0.7))
ytrain[tra.na.idx] <- NA # remove class information of unlabeled instances

# Use the other 50% of instances for inductive testing
tst.idx <- setdiff(1:length(y), tra.idx)
xitest <- x[tst.idx,] # testing instances
yitest <- y[tst.idx] # classes of testing instances

## Example: Training from a set of instances with 1-NN (knn3) as base classifier.
gen.learner <- function(indexes, cls)
  caret::knn3(x = xtrain[indexes, ], y = cls, k = 1)
gen.pred <- function(model, indexes)
  predict(model, xtrain[indexes, ]) 

# Train
set.seed(1)
md1 <- triTrainingG(y = ytrain, gen.learner, gen.pred)

# Predict testing instances using the three classifiers
pred <- lapply(
  X = md1$model, 
  FUN = function(m) predict(m, xitest, type = "class")
)
# Combine the predictions
cls1 <- triTrainingCombine(pred)
table(cls1, yitest)

## Example: Training from a distance matrix with 1-NN (oneNN) as base classifier.
dtrain <- as.matrix(proxy::dist(x = xtrain, method = "euclidean", by_rows = TRUE))
gen.learner <- function(indexes, cls) {
  m <- ssc::oneNN(y = cls)
  attr(m, "tra.idxs") <- indexes
  m
}

gen.pred <- function(model, indexes)  {
  tra.idxs <- attr(model, "tra.idxs")
  d <- dtrain[indexes, tra.idxs]
  prob <- predict(model, d, distance.weighting = "none")
  prob
}

# Train
set.seed(1)
md2 <- triTrainingG(y = ytrain, gen.learner, gen.pred)

# Predict
ditest <- proxy::dist(x = xitest, y = xtrain[md2$instances.index,],
                      method = "euclidean", by_rows = TRUE)

# Predict testing instances using the three classifiers
pred <- mapply(
  FUN = function(m, indexes){
    D <- ditest[, indexes]
    predict(m, D, type = "class")
  },
  m = md2$model,
  indexes = md2$model.index.map,
  SIMPLIFY = FALSE
)
# Combine the predictions
cls2 <- triTrainingCombine(pred)
table(cls2, yitest)