# Gradient Descent method

## Description

Gradient descent is a first-order optimization algorithm. To find a local minimum of a function using gradient descent, one takes steps proportional to the negative of the gradient (or of the approximate gradient) of the function at the current point.In this case, the total cost is nmax*(p+1) simulations (p being the number of input parameters), the finite differences are computed using a step of epsilon=0.01, and the initial input step of descent is delta=0.1.The resulting analysis is the following:

## Source

### Implementation

This basic designer is implemented in following R file (GradientDescent.R):
```#help=First-order local optimization algorithm<br/>http://en.wikipedia.org/wiki/Gradient_descent
#type=Optimization
#output=minimum
#parameters=nmax=100,delta=0.1,epsilon=0.01

# iteration index
i = 0

#' constructor and initializer of R session
init <- function() {
# all parameters are initialy strings, so you have to put as global non-string values
nmax <<- as.integer(nmax)
delta <<- as.numeric(delta)
epsilon <<- as.numeric(epsilon)
}

askfinitedifferences <- function(x) {
xd <- as.array(x);
for (i in 1:length(x)) {
xdi <- as.array(x);
if (xdi[i] + epsilon > 1.0) {
xdi[i] <- xdi[i] - epsilon;
} else {
xdi[i] <- xdi[i] + epsilon;
}
xd <- rbind(xd,xdi,deparse.level = 0)
}
xd
}

gradient <- function(xd,yd) {
d = ncol(xd)
for (i in 1:d) {
grad[i] = (yd[i+1] - yd[1]) / (xd[i+1,i] - xd[1,i])
}
}

#' first design building. All variables are set in [0,1]. d is the dimension, or number of variables
#' @param d number of variables
initDesign <- function(d) {
}

#' iterated design building.
#' @param X data frame of current doe variables (in [0,1])
#' @param Y data frame of current results
#' @return data frame or matrix of next doe step
nextDesign <- function(X,Y) {
if (i>nmax) return();

d = ncol(X)
n = nrow(X)

prevXn = as.matrix(X[(n-d):n,])
prevYn = as.matrix(Y[(n-d):n,1])

if (i > 1)
if (Y[n-d,1] > Y[n-d-d,1]) {
delta <<- delta / 2
prevXn = as.matrix(X[(n-d-d-1):(n-d-1),])
prevYn = as.matrix(Y[(n-d-d-1):(n-d-1),1])
}

xnext = t(prevXn[1,] - (grad * delta))
for (t in 1:d) {
if (xnext[t] > 1.0) {
xnext[t] = 1.0;
}
if (xnext[t] < 0.0) {
xnext[t] = 0.0;
}
}

i <<- i+1
}

#' final analysis. All variables are set in [0,1]. Return HTML string
#' @param X data frame of doe variables (in [0,1])
#' @param Y data frame of  results
#' @return HTML string of analysis
analyseDesign <- function(X,Y) {
m = min(Y)
x = as.matrix(X)[Y==m,]

analyse.files <<- paste("pairs_",i-1,".png",sep="")
resolution <- 600

html=paste(sep="<br/>",paste("<HTML>minimum is ",m),paste(sep="","found at ",paste(collapse="= ",capture.output(x)),"<br/><img src='",analyse.files,"' width='",resolution,"' height='",resolution,"'/></HTML>"))
plotmin=paste("<Plot1D name='min'>",m,"</Plot1D>")

d = dim(X)[2]
if (d == 1) {
plotx=paste("<Plot1D name='argmin'>",paste(x),"</Plot1D>")
} else if (d == 2) {
plotx=paste("<Plot2D name='argmin'>[",paste(collapse=",",x),"]</Plot2D>")
} else {
plotx=paste("<PlotnD name='argmin'>[",paste(collapse=",",x),"]</PlotnD>")
}

png(file=analyse.files,bg="transparent",height=resolution,width = resolution)
red = (as.matrix(Y)-min(Y))/(max(Y)-min(Y))
pairs(X,col=rgb(r=red,g=0,b=1-red),Y=Y[[1]],d=d,panel=panel.vec)
dev.off()

return(paste(html,plotmin,plotx))
}

panel.vec <- function(x, y , col, Y, d, ...) {
#points(x,y,col=col)
for (i in 1:(length(x)/(d+1))) {
n0 = 1+(i-1)*(d+1)
x0 = x[n0]
y0 = y[n0]
for (j in 1:d) {
if (x[n0+j] != x0) {
dx = (Y[n0]-Y[n0+j])/(max(Y)-min(Y))
#break;
}
}
for (j in 1:d) {
if (y[n0+j] != y0) {
dy = (Y[n0]-Y[n0+j])/(max(Y)-min(Y))
#break;
}
}
points(x=x0,y=y0,col=col[n0],pch=20)
lines(x=c(x0,x0+dx),y=c(y0,y0+dy),col=col[n0])
if (exists("x0p")) {
lines(x=c(x0p,x0),y=c(y0p,y0),col=col[n0],lty=3)
}
x0p=x0
y0p=y0
}

}

#' temporary analysis. All variables are set in [0,1]. Return HTML string
#' @param X data frame of doe variables (in [0,1])
#' @param Y data frame of  results
#' @returnType String
#' @return HTML string of analysis
analyseDesignTmp <- function(X,Y) {
analyseDesign(X,Y)
}```

## Installation

• copy in “Promethee_Install_Path/plugins/doe” directory,
• restart Promethee GUI.
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