Why stopifnot is not working as expected?

Question

I want to output number of the fruits user inputs and if it is <0 or if it is greater than number I want it to break. Not sure why it is not working?!

how.many<-function(fruit, number){
  string<-paste("How many",fruit,"?",sep=" ")
  #cat(string)
  #return(scan(string))
  fruit_number<-readline(string)
  stopifnot(fruit_number>number)
  stopifnot(fruit_number<0)
  return(fruit_number)
}

I should output 4 when the number is 4 but it breaks. > how.many("banana",5)

How many banana ?0
Error: fruit_number > number is not TRUE
> how.many("banana",5)
How many banana ?9
Error: fruit_number < 0 is not TRUE
> how.many("banana",5)
How many banana ?4
Error: fruit_number > number is not TRUE

Answer 1

Your tests are reversed. You need to specify a TRUE value to not stop (as it says, stopifnot).

how.many<-function(fruit, number){
    string<-paste("How many",fruit,"?",sep=" ")
    #cat(string)
    #return(scan(string))
    fruit_number<-readline(string)
    stopifnot(fruit_number<=number)
    stopifnot(fruit_number>=0)
    return(fruit_number)
}

> how.many("banana", 5)
How many banana ?9
Error: fruit_number <= number is not TRUE
> how.many("banana", 5)
How many banana ?4
[1] "4"

Answer 2

stopifnot actually means what it says: "stop if not(...)". And (4 > 5) is not true. So it stops. You want the opposite signs.