Nothing

```
## BssWssFast: compute the ratio of between-group to within group sum of squares
## arguments: X is a matrix where columns are variables, rows are observations
## return values: a vector of sorted (in descending order) ratio's and their ranks
## to get the top N variables: X[, sortedvec$ix[1:N]]
BssWssFast <- function (X, givenClassArr, numClass = 2) {
## create indicator variables for each class
## assume class numbers start from 0
classVec <- matrix (0, numClass, length (givenClassArr))
for (k in 1: numClass) {
temp <- rep(0, length(givenClassArr))
temp [givenClassArr == (k-1)] <- 1
classVec[k, ] <- temp
}
## compute BSS/WSS for each variable
classMeanArr <- rep(0, numClass)
ratio <- rep (0, ncol(X))
for (j in 1: ncol(X)) {
## compute mean across all samples
overallMean <- sum (X[, j])/length(X[, j])
## compute class mean for variable j
for (k in 1: numClass) {
classMeanArr[k] <- sum(classVec[k, ] * X[, j])/sum(classVec[k, ])
}
## to re-write classMeanArr as a vector containing class mean for each entry
classMeanVec <- classMeanArr[givenClassArr+1]
## compute BSS and WSS
bss <- sum( (classMeanVec - overallMean)^2)
wss <- sum( (X[, j] - classMeanVec)^2)
ratio[j] <- bss/wss
}
sort (ratio, decreasing=TRUE, index = TRUE)
}
```

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