# Copyright (C) 2010 Jelmer Ypma. All Rights Reserved.
# This code is published under the Eclipse Public License.
#
# File: lasso.R
# Author: Jelmer Ypma
# Date: 18 April 2010
#
# Example showing how to estimate a LASSO model. Based on
# the example from the Matlab interface to Ipopt. There are
# other packages in R that can estimate LASSO models, e.g.
# using the package glmnet.
library('ipoptr')
# Experiment parameters.
lambda <- 1 # Level of L1 regularization.
n <- 100 # Number of training examples.
e <- 1 # Std. dev. in noise of outputs.
beta <- c( 0, 0, 2, -4, 0, 0, -1, 3 ) # "True" regression coefficients.
# Set the random number generator seed.
ranseed <- 7
set.seed( ranseed )
# CREATE DATA SET.
# Generate the input vectors from the standard normal, and generate the
# responses from the regression with some additional noise. The variable
# "beta" is the set of true regression coefficients.
m <- length(beta) # Number of features.
A <- matrix( rnorm(n*m), nrow=n, ncol=m ) # The n x m matrix of examples.
noise <- rnorm(n, sd=e) # Noise in outputs.
y <- A %*% beta + noise # The outputs.
# DEFINE LASSO FUNCTIONS
# m, lambda, y, A are all defined in the ipoptr_environment
eval_f <- function(x) {
# separate x in two parts
w <- x[ 1:m ] # parameters
u <- x[ (m+1):(2*m) ]
return( sum( (y - A %*% w)^2 )/2 + lambda*sum(u) )
}
# ------------------------------------------------------------------
eval_grad_f <- function(x) {
w <- x[ 1:m ]
return( c( -t(A) %*% (y - A %*% w),
rep(lambda,m) ) )
}
# ------------------------------------------------------------------
eval_g <- function(x) {
# separate x in two parts
w <- x[ 1:m ] # parameters
u <- x[ (m+1):(2*m) ]
return( c( w + u, u - w ) )
}
# ------------------------------------------------------------------
# J = [ I I
# -I I ],
# where I is and identity matrix of size m
eval_jac_g <- function(x) {
# return a vector of 1 and minus 1, since those are the values of the non-zero elements
return( c( rep( 1, 2*m ), rep( c(-1,1), m ) ) )
}
# For m=5, The structure looks like this:
# 1 . . . . 2 . . . .
# . 3 . . . . 4 . . .
# . . 5 . . . . 6 . .
# . . . 7 . . . . 8 .
# . . . . 9 . . . . 10
# 11 . . . . 12 . . . .
# . 13 . . . . 14 . . .
# . . 15 . . . . 16 . .
# . . . 17 . . . . 18 .
# . . . . 19 . . . . 20
eval_jac_g_structure <- lapply( c(1:m,1:m), function(x) { return( c(x,m+x) ) } )
# ------------------------------------------------------------------
# rename lambda so it doesn't cause confusion with lambda in auxdata
eval_h <- function( x, obj_factor, hessian_lambda ) {
H <- t(A) %*% A
H <- unlist( lapply( 1:m, function(i) { H[i,1:i] } ) )
return( obj_factor * H )
}
# For m=5, The structure looks like this:
# 1 . . . . . . . . .
# 2 3 . . . . . . . .
# 4 5 6 . . . . . . .
# 7 8 9 10 . . . . . .
# 11 12 13 14 15 . . . . .
# . . . . . . . . . .
# . . . . . . . . . .
# . . . . . . . . . .
# . . . . . . . . . .
# . . . . . . . . . .
eval_h_structure <- c( lapply( 1:m, function(x) { return( c(1:x) ) } ),
lapply( 1:m, function(x) { return( c() ) } ) )
# ------------------------------------------------------------------
# The starting point.
x0 = c( rep(0, m),
rep(1, m) )
# The constraint functions are bounded from below by zero.
constraint_lb = rep( 0, 2*m )
constraint_ub = rep( Inf, 2*m )
ipoptr_opts <- list( "jac_d_constant" = 'yes',
"hessian_constant" = 'yes',
"mu_strategy" = 'adaptive',
"max_iter" = 100,
"tol" = 1e-8 )
# Set up the auxiliary data.
auxdata <- new.env()
auxdata$m <- m
auxdata$A <- A
auxdata$y <- y
auxdata$lambda <- lambda
# COMPUTE SOLUTION WITH IPOPT.
# Compute the L1-regularized maximum likelihood estimator.
print( ipoptr( x0=x0,
eval_f=eval_f,
eval_grad_f=eval_grad_f,
eval_g=eval_g,
eval_jac_g=eval_jac_g,
eval_jac_g_structure=eval_jac_g_structure,
constraint_lb=constraint_lb,
constraint_ub=constraint_ub,
eval_h=eval_h,
eval_h_structure=eval_h_structure,
opts=ipoptr_opts,
ipoptr_environment=auxdata ) )
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