### checker if NHS number is valid
nhs_number_validation <- function(nhs_number = NULL){
	require(stringr) # for string manipulation
	### remove spaces in number
	nhs_number <- as.character(nhs_number) # conver to character if a number
	nhs_number <- str_replace_all(nhs_number, " ", "") # remove whitespace with stringr
	### check the length of the number; should be 10
	stopifnot(nchar(nhs_number) ==10)
	# print(nhs_number)
	### create a dataframe that splits into separate numbers
	df <- data.frame("id" = nhs_number)
	df$id <- as.numeric(nhs_number)
	df <- cbind(df, "id" = outer(df$id, 10^c(9:0), function (x , y) x %/% y %% 10 ))
	### copy the first row to the second
	df[2,] <- df[1,]
	### multiply digits 1:9 by 10:2
	multiplier <- c(10, 9, 8, 7, 6, 5, 4, 3, 2)
	df[2,2:10] <- df[2,2:10] * multiplier
	# print(df)
	checker <- sum(df[2, 2:10]) 
	# print(checker)
	integer_part <- checker %/% 11
	remainder <- checker %% 11
	check_digit <- ifelse((11 - remainder) == 11, 0, ifelse((11 - remainder) == 10, NA, (11 - remainder)))
	last_number <- df[1,11]
	# print(c('interger part', integer_part))
	# print(c("remainder", remainder))
	# print(c("last number", last_number))
	# print(c("check digit", check_digit))
	result <- ifelse(check_digit != last_number | is.na(check_digit), "Invalid", "Valid")
	# print(c("result", result))
	return(result)
}