在R中创建表达式树 [英] Creating expression tree in R
问题描述
R
中的替代函数以树的形式创建了一个可以解析的语言对象.如何使用list从头开始创建树,否则将其交给eval?
The substitute function in R
creates a language object in the form of a tree that one can parse. How can I create the tree from scratch using list or else to then give it to eval?
# substitute gives a tree representation of the expression
a=1; b=2;
e1 = substitute(a+2*b)
eval(e1) #gives 5 as expected
e1 # is type language
e1[[1]] # this is `+`
e1[[2]] # this is 'a' type symbol
e1[[3]] # this is type language
e1[[3]][[1]] # this is `*` etc....
我想知道如何以编程方式重建e1
对象.理想情况下,我创建了一个包含复杂列表的对象,其中包含正确的对象,也许我在list
对象上调用了as.language
.但是,这不起作用.例如:
I would like to know how I can reconstruct the e1
object programmatically. Ideally I create an object of intricated lists with the correct object in them and maybe I call some as.language
on the list
object. However that does not work. For instance:
# how to construct the tree?
eval(list(as.symbol('+'),1,1)) # does not return 2
eval(as.expression(list(as.symbol('+'),1,1))) # does not return 2
一种方法是先生成字符串"1 + 1",然后对其进行解析,但是当您首先将树放在首位时,生成字符串以再次对其进行解析似乎并不优雅!
One way is to just generate the string '1+1' and then parse it, but it does not seem elegant to generate strings to parse them again when you have the tree in the first place!
eval(parse(text='1+1')) # does return 1, but not elegant if tree is
# large and already in memory
感谢您的帮助!
推荐答案
> plus <- .Primitive("+")
> plus
function (e1, e2) .Primitive("+")
> times=.Primitive("*")
> eval(call("plus", b, call("times",2, b)))
[1] 6
> eval(call("plus", a, call("times",2, b)))
[1] 5
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