what text string should be used to prompt the user? (character string)
type
the class of object expected to be returned - "logical", "numeric", "integer", "character". If the user input does not match this return, the prompt is repeated
bounds
the lower and upper bounds of number to be returned. Ignored if type is "logical" or "character"
na.allow
if TRUE, allows the user to input "NA" for any type, which is returned as NA