Assigning operators into an R variable

问题

I am trying to create a function where users can select the operator they want to use, which result in a different output. But I can't seem to get it to work. I know that we can't assign operators into an R object and then use it as an operator based on the R object name. Is there a way I could do this? Or perhaps a better way to write the function?

test <- function(items, operator = "+"){
bank_alpha <- matrix(ncol=6)
colnames(bank_alpha) <- colnames(bank_alpha, do.NULL = FALSE, prefix = "Q")
colnames(bank_alpha)[6] <- "A"
alphabet <- LETTERS[seq(1:26)]

 for (i in 1:items) {
  item <- c(alphabet[i], alphabet[i operator 1], alphabet[i operator  2], alphabet[i operator  3], alphabet[i operator  4], alphabet[i operator 5])
  bank_alpha <- rbind(bank_alpha, item)
  bank_alpha <- na.omit(bank_alpha)
}
return(bank_alpha)
}

  test(items=4, operator = "-")

回答1:

Check out do.call, which takes the name of a function as an argument. With

operator <- "+"
do.call(operator, list(2,3)

you will get 5 as the result.

In your example:

test <- function(items, operator = "+"){
  bank_alpha <- matrix(ncol=6)
  colnames(bank_alpha) <- colnames(bank_alpha, do.NULL = FALSE, prefix = "Q")
  colnames(bank_alpha)[6] <- "A"
  alphabet <- LETTERS[seq(1:26)]

  for (i in 1:items) {
    item <- c(alphabet[i], alphabet[do.call(operator, list(i,1))], alphabet[do.call(operator, list(i,2))], alphabet[do.call(operator, list(i,3))], alphabet[do.call(operator, list(i,4))], alphabet[do.call(operator, list(i,5))])
    bank_alpha <- rbind(bank_alpha, item)
    bank_alpha <- na.omit(bank_alpha)
  }
  return(bank_alpha)
}

test(items=4, operator = "*")

Beware, "-" doesn't make sense in this case.

回答2:

You can use the get() functionally to dynamically call the operator functions, e.g.:

> get('+')(5, 2)
[1] 7

The get() function takes a string as input and returns a reference to the function corresponding to that string, if it exists.

Usually operators are called with one function argument on the left and one on the right (e.g. 5 + 2).

The + operator function itself however just takes two arguments as input:

> args('+')
function (e1, e2) 
NULL

This is why we can call it as we did above.

回答3:

You can define your own operators in R using the % sign, which would allow you to use the infix notation in your code. See a related question for more details.

test <- function(items, `%op%` = `+`){
    bank_alpha <- matrix(ncol=6)
    colnames(bank_alpha) <- colnames(bank_alpha, do.NULL = FALSE, prefix = "Q")
    colnames(bank_alpha)[6] <- "A"
    alphabet <- LETTERS[seq(1:26)]

    for (i in 1:items) {
       item <- c(alphabet[i], alphabet[i %op% 1], alphabet[i %op% 2], alphabet[i %op% 3], alphabet[i %op% 4], alphabet[i %op% 5])
       bank_alpha <- rbind(bank_alpha, item)
       bank_alpha <- na.omit(bank_alpha)
    }
    return(bank_alpha)
}

test(items=4, `%op%` = `*`)

来源：https://stackoverflow.com/questions/39316116/assigning-operators-into-an-r-variable

标签

function

operators