# Set up input values
A <- c(1,3,2,4,5,3,4,2,1)
# R doesn't like numbers as keys, so explicitly make them strings
B <- list("1"=2,"2"=1,"3"=4,"4"=2,"5"=3)

# Create result vector, using R's vector arithmetic
# Have to cast A to a character vector
# Usual list indexing doesn't quite work here, so use a method that creates a list then turn that list into a vector using as.numeric()
result <- (A/as.numeric((B[as.character(A)])))^2
# Use R's selective replacement into an array to handle special case
result[result<1] <- 0


print(result)

# output:
# [1] 0.000000 0.000000 4.000000 4.000000 2.777778 0.000000 4.000000 4.000000
# [9] 0.000000