Nystroem approximation is now correct (and simplified).
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@ -1,8 +1,7 @@
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source("05_d_svd_mca_code.R")
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source("04_validation_croisee_code.R")
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source("15_loocv_code.R")
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source("19_nystroem_approximation_code.R")
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datasetHousing.nakr <-
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dataset.housing <-
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function()
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{
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dat <- read.csv(file="data/housing.csv", header=TRUE)
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@ -44,68 +43,24 @@ function()
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return(r)
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}
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# test.tbl <- table(c(4,2,4,2,1,1,1))
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# all(intersperse(test.tbl) == c(1, 2, 4, 1, 2, 4, 1))
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intersperse <-
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function(tbl)
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{
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n <- sum(tbl)
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values <- as.numeric(names(tbl))
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r <- numeric(n)
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i <- 1
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while(i <= n)
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{
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for(j in 1:length(tbl))
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{
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if(tbl[j] != 0)
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{
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r[i] <- values[j]
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i <- i + 1
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tbl[j] <- tbl[j] - 1
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}
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}
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}
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return(r)
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}
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print("load dataset")
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dat <- dataset.housing()
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# sample the landmarks from the clusters of individuals
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# obtained after correspondence analysis
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landmarks.by.ca.clst <-
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function(cam, X, nbLandmarks)
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{
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if(nbLandmarks > nrow(X))
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{
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stop("The number of landmarks must be less than the number of training samples.")
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}
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landmarks <- numeric(nbLandmarks)
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clst <- cam$clsti$cluster[as.numeric(rownames(X))]
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clst.tbl <- table(clst)
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nb.by.clst <- table((intersperse(clst.tbl))[1:nbLandmarks])
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clst.id <- as.numeric(names(nb.by.clst))
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set.seed(1123)
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clst <- sample(clst)
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offset <- 0
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for (i in 1:length(nb.by.clst))
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{
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k <- as.numeric(nb.by.clst[i])
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landmarks[(offset+1):(offset+k)] <- as.numeric(names((clst[clst==clst.id[i]])[1:k]))
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offset <- offset+k
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}
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return(landmarks)
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}
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print("Nystroem approx ridge regression")
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nakrm <- nakr(dat$entr$X, dat$entr$Y)
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nakrm.yh <- predict(nakrm, dat$test$X)
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nakrm.mae <- mean(abs(nakrm.yh - dat$test$Y))
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print(paste("MAE for Nystroem: ", nakrm.mae))
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hous.dat.nakr <- datasetHousing.nakr()
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X.entr <- hous.dat.nakr$entr$X
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Y.entr <- hous.dat.nakr$entr$Y
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X.test <- hous.dat.nakr$test$X
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Y.test <- hous.dat.nakr$test$Y
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hous.dat.ca <- datasetHousing.mca()
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hous.cam <- mca(hous.dat.ca)
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nb.landmarks <- round(sqrt(nrow(X.entr)))
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landmarks <- landmarks.by.ca.clst(hous.cam, X.entr, nb.landmarks)
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# nakrm <- kfold.nakr(X.entr, Y.entr, landmarks=landmarks)
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# nakrm.yh <- predict(nakrm, X.test)
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# nakrm.mae <- mean(abs(nakrm.yh - Y.test))
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# nakrm.yh.train <- predict(nakrm, X.entr)
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# rev(order(abs(nakrm.yh.train - Y.entr)))[1:20]
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# hist(Y.entr[rev(order(abs(nakrm.yh.train - Y.entr)))[1:200]])
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print("Linear ridge regression")
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rm <- ridge(dat$entr$X, dat$entr$Y)
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rm.yh <- predict(rm, dat$test$X)
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rm.mae <- mean(abs(rm.yh - dat$test$Y))
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print(paste("MAE for Ridge: ", rm.mae))
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print("Random forest")
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library(randomForest)
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rfm <- randomForest(dat$entr$X, dat$entr$Y)
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rfm.yh <- predict(rfm, dat$test$X)
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rfm.mae <- mean(abs(rfm.yh - dat$test$Y))
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print(paste("MAE for Random Forest: ", rfm.mae))
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@ -3,6 +3,7 @@
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```{r, include=FALSE}
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source("01_intro_code.R", local = knitr::knit_global())
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source("04_validation_croisee_code.R", local = knitr::knit_global())
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source("15_loocv_code.R", local = knitr::knit_global())
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source("19_nystroem_approximation_code.R", local = knitr::knit_global())
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```
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@ -103,7 +104,7 @@ $$
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\end{aligned}
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$$
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Ainsi, nous pouvons introduire une matrice $\boldsymbol\Phi$ de dimension $m$ telle que $\mathbf{K} \approx \boldsymbol\Phi \boldsymbol\Phi^T$.
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Ainsi, nous pouvons introduire une matrice $\boldsymbol\Phi$ de rang $m$ telle que $\mathbf{K} \approx \boldsymbol\Phi \boldsymbol\Phi^T$.
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$$
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\boldsymbol\Phi =
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\left[
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@ -162,45 +163,7 @@ $$
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\end{aligned}
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$$
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## Régression ridge à noyau et approximation de Nyström
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Dans le cadre de la régression ridge à noyau (voir un précédent module), nous notons : $\mathbf{G} = \mathbf{K} + \lambda\mathbf{I_n}$. Les coefficients du modèle ridge sont alors donnés par : $\boldsymbol\alpha_\lambda = \mathbf{G}^{-1} \mathbf{y}$. Nous cherchons à calculer efficacement $\mathbf{G}^{-1}$ à partir d'une approximation Nyström de rang $m$ de $\mathbf{K} \approx \mathbf{L}\mathbf{L}^T$. Pour ce faire, nous utilisons une forme de l'identité de Woodbury :
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$$
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\left(\mathbf{A} + \mathbf{U}\mathbf{C}\mathbf{V}\right)^{-1} = \mathbf{A}^{-1} - \mathbf{A}^{-1}\mathbf{U}\left(\mathbf{C}^{-1} + \mathbf{V}\mathbf{A}^{-1}\mathbf{U}\right)^{-1}\mathbf{V} \mathbf{A}^{-1}
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$$
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Nous obtenons ainsi :
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$$
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\begin{aligned}
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& \mathbf{G}^{-1} \\
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= \{& \text{Définition de $\mathbf{G}$} \} \\
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& \left(\lambda\mathbf{I_n} + \mathbf{K}\right)^{-1} \\
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\approx \{& \text{Approximation de Nyström} \} \\
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& \left(\lambda\mathbf{I_n} + \mathbf{L}\mathbf{L}^T\right)^{-1} \\
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= \{& \text{Identité de Woodbury} \} \\
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& \lambda^{-1}\mathbf{I_n} - \lambda^{-1}\mathbf{L}\left(\mathbf{I_m}+\lambda^{-1}\mathbf{L}^T\mathbf{L}\right)^{-1}\lambda^{-1}\mathbf{L}^T \\
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= \{& \text{Algèbre linéaire : si $\mathbf{A}$ et $\mathbf{B}$ sont des matrices carrés inversibles alors $(\mathbf{A}\mathbf{B})^{-1} = \mathbf{B}^{-1}\mathbf{A}^{-1}$} \} \\
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& \lambda^{-1}\mathbf{I_n} - \lambda^{-1}\mathbf{L}\left(\lambda\mathbf{I_m}+\mathbf{L}^T\mathbf{L}\right)^{-1}\mathbf{L}^T \\
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\end{aligned}
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$$
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Montrons comment calculer efficacement la prédiction $\hat{y}_{new}$ d'une nouvelle observation $\mathbf{x_{new}}$. Nous avons montré plus haut que l'approximation de Nyström pouvait s'écrire $\mathbf{K} \approx \mathbf{C} \mathbf{K_{11}}^{-1} \mathbf{C}^T$. Donc la ligne de l'approximation du noyau $\mathbf{K}$ qui correspond à l'observation $\mathbf{x_j}$ du jeu d'entrainement (dont la version non approximée est $[k(\mathbf{x_j},\mathbf{x_1}),\dots,k(\mathbf{x_j},\mathbf{x_n})]$) s'obtient par combinaison linéaire des lignes de $\mathbf{K_{11}}^{-1} \mathbf{C}^T$. Les $m$ coefficients de cette combinaison linéaire sont $(k(\mathbf{x_j},\mathbf{x_1}),\dots,k(\mathbf{x_j},\mathbf{x_m}))$.
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Ainsi, l'approximation d'une "nouvelle ligne" du noyau $\mathbf{K}$ formée des similarités d'une nouvelle observation $\mathbf{x_{new}}$ avec les observations $(\mathbf{x_1},\dots,\mathbf{x_n})$ du jeu d'entrainement est :
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$$[k(\mathbf{x_{new}},\mathbf{x_1}),\dots,k(\mathbf{x_{new}},\mathbf{x_n})] \approx [k(\mathbf{x_{new}},\mathbf{x_1}),\dots,k(\mathbf{x_{new}},\mathbf{x_m})] \mathbf{K_{11}}^{-1} \mathbf{C}^T$$
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Enfin, la prédiction associée à $\mathbf{x_{new}}$ à partir de la valeur approximée du noyau est :
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$$
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\begin{aligned}
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& \hat{y}_{new} \\
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\approx \{& \text{voir raisonnement ci-dessus, et } \hat{y} = \mathbf{K} \boldsymbol\alpha_\lambda \} \\
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& \left[ k(\mathbf{x_{new}},\mathbf{x_1}),\dots,k(\mathbf{x_{new}},\mathbf{x_m}) \right] \left( \mathbf{K_{11}}^{-1} \mathbf{C}^T \boldsymbol\alpha_\lambda \right) \\
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= \{& \text{introduction de $\boldsymbol\beta_\lambda = \mathbf{K_{11}}^{-1} \mathbf{C}^T \boldsymbol\alpha_\lambda$} \} \\
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& \left[ k(\mathbf{x_{new}},\mathbf{x_1}),\dots,k(\mathbf{x_{new}},\mathbf{x_m}) \right] \boldsymbol\beta_\lambda \\
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\end{aligned}
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$$
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$\boldsymbol\beta_\lambda$ peut être calculé une seule fois à la fin de l'entrainement pour être ensuite réutilisé pour chaque prédiction.
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Nous voyons que la matrice $\mathbf{L}$ correspond exactement à la matrice $\boldsymbol\Phi$ dont les lignes sont les nouvelles variables $m$-dimensionnelles telles que $\boldsymbol\Phi \boldsymbol\Phi^T$ soit une approximation de rang-$m$ de $\mathbf{K}$. Ainsi, pour opérer une régression ridge à noyau avec approximation de Nyström, il suffit de faire une régression ridge sur ces variables transformées.
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## Exemple sur un jeu de données synthétique
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@ -214,7 +177,7 @@ Nous reprenons le jeu de données synthétique utilisé depuis le premier module
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entr <- splitres$entr
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test <- splitres$test
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nakrm <- nakr(entr$X,entr$Y, nb.landmarks=25)
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nakrm <- nakr(entr$X,entr$Y,nb.landmarks=25)
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yh <- predict(nakrm,test$X)
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plt(test,f)
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points(test$X, yh, pch=4)
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@ -1,80 +1,38 @@
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# compute the gaussian kernel between each row of X1 and each row of X2
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# should be done more efficiently (C code, threads)
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gausskernel.nakr <-
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function(X1, X2, sigma2)
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{
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n1 <- dim(X1)[1]
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n2 <- dim(X2)[1]
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K <- matrix(nrow = n1, ncol = n2)
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for(i in 1:n1)
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for(j in 1:n2)
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K[i,j] <- sum((X1[i,] - X2[j,])^2)
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K <- exp(-1*K/sigma2)
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}
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# Nystroem Approximation Kernel Ridge Regression
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nakr <-
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function(X, y, sigma2=NULL, lambda=1E-8, landmarks=NULL, nb.landmarks=NULL)
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function(X, y, sigma2=NULL, lambdas=NULL, nb.landmarks=NULL)
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{
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set.seed(1123)
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X <- as.matrix(X)
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n <- nrow(X)
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p <- ncol(X)
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if(is.null(sigma2)) { sigma2 <- p }
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ldm <- landmarks.nakr(X, landmarks, nb.landmarks)
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if(is.null(nb.landmarks)) { nb.landmarks <- 15*n^(1/3) }
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X <- scale(X)
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y <- scale(y)
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C <- gausskernel.nakr(X, as.matrix(X[ldm$idx,]), sigma2)
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K11 <- C[ldm$idx,]
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svdK11 <- svd(K11)
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# K11 often ill-formed -> drop small sv
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ks <- which(svdK11$d < 1E-12)
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if (length(ks)>0) {k <- ks[1]} else {k <- length(svdK11$d)}
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US <- svdK11$u[,1:k] %*% diag(1 / sqrt(svdK11$d[1:k]))
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L <- C %*% US
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Ginv <- t(L) %*% L
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diag(Ginv) <- diag(Ginv) + lambda
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Ginv <- chol2inv(chol(Ginv))
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Ginv <- L %*% Ginv %*% t(L)
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Ginv <- - Ginv / lambda
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diag(Ginv) <- diag(Ginv) + (1/lambda)
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coef <- Ginv %*% y
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K11inv <- svdK11$v[,1:k] %*% diag(1/svdK11$d[1:k]) %*% t(svdK11$u[,1:k])
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beta <- K11inv %*% t(C) %*% coef
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r <- list(X=X,
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y=y,
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idx.landmarks <- sample(1:n, nb.landmarks, replace = FALSE)
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S <- X[idx.landmarks, ]
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K11 <- gausskernel.nakr(S, S, sigma2)
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C <- gausskernel.nakr(X, S, sigma2)
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svd.K11 <- svd(K11)
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ks <- which(svd.K11$d < 1E-12) # K11 often ill-formed -> drop small sv
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if (length(ks)>0) {k <- ks[1]} else {k <- length(svd.K11$d)}
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W <- svd.K11$u[,1:k] %*% diag(1/sqrt(svd.K11$d[1:k]))
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Phi <- C %*% W
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ridge <- ridge(Phi, y, lambdas)
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r <- list(center.X=attr(X,"scaled:center"),
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scale.X=attr(X,"scaled:scale"),
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center.y=attr(y,"scaled:center"),
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scale.y=attr(y,"scaled:scale"),
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S=S,
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sigma2=sigma2,
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lambda=lambda,
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ldmidx=ldm$idx,
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coef=coef,
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beta=beta
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W=W,
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ridge=ridge
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)
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class(r) <- "nakr"
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return(r)
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}
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landmarks.nakr <-
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function(X, landmarks, nb.landmarks)
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{
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n <- nrow(X)
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if(is.null(landmarks)) {
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if(is.null(nb.landmarks)) { nb.landmarks <- round(sqrt(n)) }
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ldmidx <- sample(1:n, nb.landmarks, replace = FALSE)
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} else {
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ldmidx <- which(rownames(X) %in% as.character(landmarks))
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}
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ldmidx <- sort(ldmidx)
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ldmnms <- as.numeric(rownames(X)[ldmidx])
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return(list(idx=ldmidx, nms=ldmnms))
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}
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predict.nakr <-
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function(o, newdata)
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{
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@ -83,42 +41,30 @@ function(o, newdata)
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UseMethod("predict")
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return(invisible(NULL))
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}
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newdata <- as.matrix(newdata)
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if(ncol(o$X)!=ncol(newdata)) {
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stop("Not the same number of variables btwn fitted nakr object and new data")
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}
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newdata <- scale(newdata,center=attr(o$X,"scaled:center"),
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scale=attr(o$X,"scaled:scale"))
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Ktest <- gausskernel.nakr(newdata, as.matrix(o$X[o$ldmidx,]), o$sigma2)
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yh <- Ktest %*% o$beta
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yh <- (yh * attr(o$y,"scaled:scale")) + attr(o$y,"scaled:center")
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test <- as.matrix(newdata)
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test <- scale(test,center=o$center.X,scale=o$scale.X)
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K.test <- gausskernel.nakr(test, o$S, o$sigma2)
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Phi.test <- K.test %*% o$W
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yh <- predict(o$ridge, Phi.test)
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yh <- yh * o$scale.y + o$center.y
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}
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kfold.nakr <-
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function(X, y, K=5, lambdas=NULL, sigma2=NULL, landmarks=NULL, nb.landmarks=NULL)
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# compute the gaussian kernel between each row of X1 and each row of X2
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# should be done more efficiently (C code, threads)
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gausskernel.nakr <-
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function(X1, X2, sigma2)
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{
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if(is.null(lambdas)) { lambdas <- 10^seq(-8, 2, by=1) }
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n <- nrow(X)
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folds <- rep_len(1:K, n)
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folds <- sample(folds, n)
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maes <- matrix(data = NA, nrow = K, ncol = length(lambdas))
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colnames(maes) <- lambdas
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lambda_idx <- 1
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ldm <- landmarks.nakr(X, landmarks, nb.landmarks)
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for(lambda in lambdas) {
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for(k in 1:K) {
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fold <- folds == k
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ldmnms2keep <- ldm$nms[! ldm$idx %in% which(fold)]
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nakrm <- nakr(X[!fold,], y[!fold], sigma2, lambda, landmarks=ldmnms2keep)
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pred <- predict(nakrm, X[fold,])
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maes[k,lambda_idx] <- mean(abs(pred - y[fold]))
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print(paste("lbd =", lambda, "; k =", k, "; mae =", maes[k,lambda_idx]))
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}
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lambda_idx <- lambda_idx + 1
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}
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mmaes <- colMeans(maes)
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minmmaes <- min(mmaes)
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bestlambda <- lambdas[which(mmaes == minmmaes)]
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nakrm <- nakr(X, y, sigma2, bestlambda, landmarks=ldm$nms)
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if(is(X1,"vector"))
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X1 <- as.matrix(X1)
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if(is(X2,"vector"))
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X2 <- as.matrix(X2)
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if (!(dim(X1)[2]==dim(X2)[2]))
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stop("X1 and X2 must have the same number of columns")
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n1 <- dim(X1)[1]
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n2 <- dim(X2)[1]
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dotX1 <- rowSums(X1*X1)
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dotX2 <- rowSums(X2*X2)
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res <- X1%*%t(X2)
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for(i in 1:n2) res[,i] <- exp((2*res[,i] - dotX1 - rep(dotX2[i],n1))/sigma2)
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return(res)
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}
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@ -4,7 +4,7 @@ author: "Pierre-Edouard Portier"
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documentclass: book
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geometry: margin=2cm
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fontsize: 12pt
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date: "5 Mar 2023"
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date: "19 Mar 2023"
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toc: true
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classoption: fleqn
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bibliography: intro_to_ml.bib
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26
pad.R
26
pad.R
@ -27,29 +27,3 @@
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# adj = 0, cex = 0.6)
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# points(0, 0, pch = 3)
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# ```
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# source("19_b_nystroem_approximation_housing_experiment_code.R")
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# rdat <- hous.dat.nakr$dat[sample(nrow(hous.dat.nakr$dat), size=2000, replace=FALSE),]
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# X <- rdat[,!(colnames(rdat) %in% c('median_house_value'))]
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# Y <- rdat[,c('median_house_value')]
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# names(Y) <- rownames(X)
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# rsplt <- splitdata(list(X = X, Y = Y), 0.8)
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# X.entr <- rsplt$entr$X
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# Y.entr <- rsplt$entr$Y
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# X.test <- rsplt$test$X
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# Y.test <- rsplt$test$Y
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# source("18_kernel_ridge_regression_code.R")
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# krm <- krr(X.entr, Y.entr)
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# krm.yh <- predict(krm, X.test)
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# krm.mae <- mean(abs(krm.yh - Y.test)) # 35445.1
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# nakrm <- nakr(X.entr, Y.entr, nb.landmarks=1600)
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# nakrm.yh <- predict(nakrm, X.test)
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# nakrm.mae <- mean(abs(nakrm.yh - Y.test)) # 65454.18
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# source("15_loocv_code.R")
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# rm <- ridge(X.entr, Y.entr)
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# rm.yh <- predict(rm, X.test)
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# rm.mae <- mean(abs(rm.yh - Y.test)) # 45786.62
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# library(randomForest)
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# rfm <- randomForest(X.entr, Y.entr)
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# rfm.yh <- predict(rfm, X.test)
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# rfm.mae <- mean(abs(rfm.yh - Y.test)) # 34229.02
|
||||
|
||||
Loading…
Reference in New Issue
Block a user