Starting with our data file, calc.txt:

4 * 6
5 + 6
457 - 75
54 / 3
4 + 6

calc.R then looks like:

lines <- readLines(file("calc.txt"))

for (line in lines){
  words <- strsplit(line, " ")[[1]]
  lhs <- as.numeric(words[1])
  operator <- words[2]
  rhs <- as.numeric(words[3])
  
  if (operator == "+"){
    cat(line, "is", lhs + rhs, "\n")
  } else if (operator == "-"){
    cat(line, "is", lhs - rhs, "\n")
  } else if (operator == "*"){
    cat(line, "is", lhs * rhs, "\n")
  } else if (operator == "/"){
    cat(line, "is", lhs / rhs, "\n") 
  }
}
Rscript calc.R

4 * 6 is 24 
5 + 6 is 11 
457 - 75 is 382 
54 / 3 is 18 
4 + 6 is 10