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

问题

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)

回答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.

回答2:

It's because when you copy and paste all then scan reads pasted lines as input.

If you copy this tree lines to console

x <- scan(nmax=1)
1
2

x become 1, scan don't wait for your interaction cause it got line to read.

You have to wrap everything in {}:

{
 x <- scan(nmax=1)
 1
 2
}

You have to wrap both parts of your program. To be more clear: when you paste your code to console } should be last sign.

来源：https://stackoverflow.com/questions/5974967/what-is-the-correct-way-to-ask-for-user-input-in-an-r-program

标签

user-input

readline