I got the error:

Error in yall$[$test$]$: invalid subscript type 'list'

Traceback:

$1.$ mean$(($kmod$fitted $-$ yall$[$test$])^2$

Below is my code:

## import libraries needed

library$($testthat$)$

library$($digest$)$

library$($stringr$)$

library$($MASS$)$

library$($kknn$)$

n $=$ nrow$($Boston$)$ #number of observatons$/$rows$,$ i$.$e$.$ sample size

xall $=$ Boston$dis #our input variable

yall $=10*$Boston$nox #our output variable

# create data frame df that consists only of our input and output variables

df $=$ data.frame$($xall$,$yall$)$ #simple data frame for convenience

dim$($df$)$ #checking the dimension of the dataframe df $($note there are $506$ obs$)$

set.seed$(123)$

tr $=$ sample$($x$=1$:$506,$ size$=400,$ replace$=$FALSE$)$

head$($tr$)$

# Create train and test data frames

train $<-$ df$[$tr$,]$

test $<-$ df$[-$tr$,]$

#loop over values of k$,$ fit on train, predict on test

kvec $=1$:$350$

nk$=$length$($kvec$)$

outRMSE $=$ rep$(0,$nk$)$ #pre$-$allocating RMSE for out$-$of$-$sample data

inRMSE $=$ rep$(0,$nk$)$ #pre$-$allocating RMSE for in$-$sample data

# Loop over the values of k

for $($i in kvec$)\{$

# Fit the kknn model using the training dataset

kmod $=$ kknn$($yall ~ xall, train$=$train, test$=$test, k$=123,$ kernel$=$"rectangular"$)$

kmodtr $=$ kknn$($yall ~ xall, train$=$train, test$=$train, k$=123,$ kernel$=$"rectangular"$)$

# Compute the out$-$of$-$sample and in$-$sample RMSEs

outRMSE$[$i$]<-$ sqrt$($mean$(($kmod$fitted $-$ yall$[$test$])^2))$

inRMSE$[$i$]<-$ sqrt$($mean$(($kmodtr$$.$fitted $-$ yall$[$train$])^2))$

$\}$

# Print the first few values of the out$-$of$-$sample and in$-$sample RMSEs

print$($outRMSE$)$

print$($inRMSE$)$