How to read user input into the subset command - r

Question

I have some R command like this
subset(
(aggregate(cbind(var1,var2)~Ei+Mi+hours,a, FUN=mean)),
(aggregate(cbind(var1,var2)~Ei+Mi+hours,a, FUN=mean))$Ei == c(1:EXP)
)
I want to do
1) Ask the user to input the var1 and var2
2) Get those variables into the subset command line as shown above and
continue with other things.
Note: for reading the user input I have variables like
c(ax,bx,cx,dx,ex,fx,gx,hx,ix,jx,kx,lx,mx,nx,ox) = c(1:15) and each
variable is mapped to number 1 to 15. So displaying this for user and
asking the user to select any number between 1 to 15 and then
checking the corresponding variable for the entered number and
reading this into the command line is whats the best method, I think.
So how can I implement this?
Regarding the answer:
Just wondering there is one possible scenario like , if the user wants to enter multiple of numbers in one go. [ex: 1,2,3]...than how to read this using readlines as said in the answer below using
v1 <- quote(var1 <- as.numeric(readline('Enter Variable 1: ')))
eavl(v1)
xx <- paste0(letters[1:15], 'x')
xx[v1]
How to read multiple variables in this case?

Answer 1

Here's a rough example of the readline interactive prompt. When v1 is evaluated, the user will be prompted to enter a value. That value is then stored as var1.
> v1 <- quote(var1 <- as.numeric(readline('Enter Variable 1: ')))
> eval(v1)
Enter Variable 1: 1000 ## user enters 1000, for example
> 100 + var1 + 50 ## example to show captured output as object
## [1] 1150
So in your case it might go something like
> v1 <- quote(var1 <- as.numeric(readline('Enter a number from 1 to 15: ')))
> eval(v1)
Enter a number from 1 to 15: 7
> var1
## [1] 7
> xx <- paste0(letters[1:15], 'x')
> xx
## [1] "ax" "bx" "cx" "dx" "ex" "fx" "gx" "hx" "ix" "jx" "kx" "lx" "mx" "nx" "ox"
> xx[var1]
## [1] "gx"
I borrowed this idea for a function from this older SO post. You can return the output invisibly and it will still take in the user values.
input.fun <- function(){
v1 <- readline("var1: ")
v2 <- readline("var2: ")
v3 <- readline("var3: ")
v4 <- readline("var4: ")
v5 <- readline("var5: ")
out <- sapply(c(v1, v2, v3, v4, v5), as.numeric, USE.NAMES = FALSE)
invisible(out)
}
> x <- input.fun()
var1: 7
var2: 4
var3: 8
var4: 5
var5: 2
> x
[1] 7 4 8 5 2
In response to your edit: I'm not sure if this is the standard method for reading multiple numbers in one line, but it works.
> xx <- readline('Enter numbers separated by a space: ')
Enter numbers separated by a space: 4 12 67 9 2
> as.numeric(strsplit(xx, ' ')[[1]])
## [1] 4 12 67 9 2

Answer 2

Here's a possibility using scan()
#sample data
df<-data.frame(
ax=runif(50),
bx=runif(50),
cx=runif(50),
dx=runif(50),
Ei=sample(letters[1:5], 50, replace=T)
)
#get vars
vars<-c(NA,NA)
while(any(is.na(vars))) {
cat(paste("enter var number", sum(!is.na(vars))+1),"\n")
cat(paste(seq_along(names(df)), ":", names(df)), sep="\n")
try(n<-scan(what=integer(), nmax=1), silent=T)
vars[min(which(is.na(vars)))]<-n
}
#--pause
#use vars
subset(aggregate(df[,vars], df[,c("Ei"), drop=F], FUN=mean), Ei=="a")
It's not super robust, but if you copy the first half (before the pause) it will ask you for two variable numbers, and then if you run the second half, it will use those two values. I've adjusted the aggregate and subset to be more appropriate for variable usage which means not using the formula syntax.
I did not do any error checking. That's left as an exercise for the asker.