What is the correct way to ask for user input in an R program?

Question

The my program below(which is in two parts) works if I run them separately – that is, if I paste the first part into the R Console, run it and then paste the second and run it. However, that is not how I want it. I want to run the whole program at once. If I do that it shows the following error in my console :

1: 
Read 0 items
1: 
Read 0 items
Error in while ((n <= 0) | (acr <= 0) | (acr >= 1)) { : 
  argument is of length zero

I have tried to identify the problem but I have not been able find the root cause. I would be more than glad, if someone could come to my aid.

#**FIRST PART OF THE PROGRAM**

n <- -2
acr <- -2
while((n<=0) | (acr<=0) | (acr>=1)) {
   print("enter a  positive integer and the average cancellation rate between 0 and 1  
                you want")
   try(n <- scan(what=integer(), nmax=1), silent=TRUE)
   try(acr <- scan(what=double(), nmax=1), silent=TRUE)
}


#**SECOND PART OF THE PROGRAM**

bygrace <- read.table("C:\\MyRfolder\\bygrace.txt", header=FALSE)
r <- nrow(bygrace)
c <- ncol(bygrace)
copybygrace <- array(bygrace, dim=c(r, c))
copybygrace <- bygrace[-((n+1):r), ]
write.table(copybygrace,file="C:\\MyRfolder\\copybygrace.txt", sep="\t")
copybygrace <- read.table("C:\\MyRfolder\\copybygrace.txt", header=TRUE)

Answer 1

@Marek is very right. A few more remarks :

• In general, you shouldn't be using scan() but readline() for this.
• I'd split the code so it becomes clear what serves to read in n, and what serves to read in acr.
• think about whether you want to return to the prompt when people just press enter, or whether you want to reask the question until they fill in some correct value.
• you can use the power of grepl() to check whether the input is the right format.

To include the correct controls and catch all possible mistakes, the following construct is a lot cleaner and won't break your code when copied to the console :

while(n < 1 ){
  n <- readline("enter a positive integer: ")
  n <- ifelse(grepl("\\D",n),-1,as.integer(n))
  if(is.na(n)){break}  # breaks when hit enter
}

This shows how to terminate the question when people don't fill in anything. The grepl construct exludes any character that is not a digit, including the dot.

while(is.na(acr) | acr <= 0 | acr >= 1 ){
  acr <- readline("and the average cancellation rate between 0 and 1 :")
  acr <- ifelse(grepl("[^0-9.]",acr),-1,as.numeric(acr))
}

This shows how to re-ask the question when people don't fill in anything. The grepl excludes any character that is not a digit or a dot.