ssc (version 2.1-0)

setredG: SETRED generic method

Description

SETRED is a variant of the self-training classification method (selfTraining) with a different addition mechanism. The SETRED classifier is initially trained with a reduced set of labeled examples. Then it is iteratively retrained with its own most confident predictions over the unlabeled examples. SETRED uses an amending scheme to avoid the introduction of noisy examples into the enlarged labeled set. For each iteration, the mislabeled examples are identified using the local information provided by the neighborhood graph.

Usage

setredG(y, D, gen.learner, gen.pred, theta = 0.1, max.iter = 50,
  perc.full = 0.7)

Arguments

y

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

D

A distance matrix between all the training instances. This matrix is used to construct the neighborhood graph.

gen.learner

A function for training a supervised base classifier. 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.

theta

Rejection threshold to test the critical region. Default is 0.1.

max.iter

Maximum number of iterations to execute the self-labeling process. Default is 50.

perc.full

A number between 0 and 1. If the percentage of new labeled examples reaches this value the self-training process is stopped. Default is 0.7.

Value

A list object of class "setredG" containing:

model

The final base classifier trained using the enlarged labeled set.

instances.index

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

Details

SetredG 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 setred method, please see setred function. Essentially, setred function is a wrapper of setredG function.

Examples

# NOT RUN {
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

# Compute distances between training instances
D <- as.matrix(proxy::dist(x = xtrain, method = "euclidean", by_rows = TRUE))

## 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, ]) 

md1 <- setredG(y = ytrain, D, gen.learner, gen.pred)

cls1 <- predict(md1$model, xitest, type = "class")
table(cls1, yitest)

## Example: Training from a distance matrix with 1-NN (oneNN) as base classifier
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 <- D[indexes, tra.idxs]
  prob <- predict(model, d, distance.weighting = "none")
  prob
}

md2 <- setredG(y = ytrain, D, gen.learner, gen.pred)
ditest <- proxy::dist(x = xitest, y = xtrain[md2$instances.index,],
                      method = "euclidean", by_rows = TRUE)
cls2 <- predict(md2$model, ditest, type = "class")
table(cls2, yitest)
# }