使用未评估但变化的参数创建未评估的函数调用 [英] Creating an unevaluated function call with unevaluated but changing arguments

问题描述

先前的帖子显示了如何使用 quote（）创建一个未评估参数的函数调用：

A previous post shows how to use quote() to create an unevaluated call to a function where the arguments are also unevaluated:

foo <-function(arg1,arg2){
  value <- arg1 + arg2
}

foocall <- call("foo",arg1=quote(x),arg2=quote(y))
foocall
# foo(arg1 = x, arg2 = y)

如何保持这种质量，但允许更改arg1的规范。例如，我的环境中有两个命名对象n和m，有时我想越过一个，有时我想越过另一个。

How can I keep this quality but allow the specification of arg1 to change. E.g, I have two named objects n and m in my environment and sometimes I would like to pass over one and sometimes I would like to pass over the other.

## Named objects
n <- c(2,3)
m <- 3

## Case 1: i would like to pass over n
z <-n
call("foo", arg1=quote(z), arg2=quote(y))

## desired output 
#foo(arg1 = n, arg2=y)

## Case 2: pass over m 
z <- m
call("foo", arg1=quote(z), arg2=quote(y))

## desired output 
#foo(arg1 = m, arg2=y)

我很难正确地表达我的问题，但我会这样说：如何为arg1分配一个变量，该变量可以更改但不求值，而仅对绑定对象的名称赋值？

I have a hard time to properly formulate my question, but I would state it like this: How can I assign to arg1 a variable that can change but doesn't evaluate 'all the way down', but only to the name of the object it is bound to?

推荐答案

这样做

> z <- n

评估右侧，因此符号 z 被分配了 n 的值，而不是符号 n

the right-hand side is evaluated, so the symbol z is assigned the value of n, not the symbol n itself.

> z
[1] 2 3

现在与

> z <- quote(n)

现在 z的值是一个符号，即符号 n 。

Now the value of z is a symbol, namely the symbol n.

> z
n

因此，现在您可以这样做

Therefore now you can do

> call('foo', arg1=z, arg2=quote(y))
foo(arg1 = n, arg2 = y)

，它将按预期工作。

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