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 ru

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.

Answer 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.