Nothing
cv.lm <- function(formula, df, nfolds=10)
{
nr <- nrow(df)
meanSE <- 0;
medianSE <- 0
if(nfolds < nr){
split <- ceiling(nr/nfolds)
for(i in 1:nfolds){
s <- sample(1:nr)
l1 <- lm(formula, df[s[split:nr], ])
yp <- predict(l1, df[s[1:split], ])
meanSE <- meanSE + mean((yp - df[s[1:split], 1])^2)
medianSE <- medianSE + median((yp - df[s[1:split], 1])^2)
}
}
else{
nfolds = nr;
for(i in 1:nr){
if(i == 1){
l1 <- lm(formula, df[2:nr, ])
}
else if( i==nr){
l1 <- lm(formula, df[1:(nr-1), ])
}
else{
l1 <- lm(formula, df[c(1:(i-1), (i+1):nr), ])
}
yp <- predict(l1, df[i, ])
meanSE <- meanSE + (yp - df[i, 1])^2
}
medianSE <- meanSE
}
cv <- list(meanSE = meanSE/nfolds, medianSE = medianSE/nfolds)
cv
}
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