R:深拷贝一个函数参数 [英] R: deep copy a function argument

问题描述

考虑下面的代码

i = 3
j = i
i = 4 # j != i

然而，我想要的是

i = 3
f <- function(x, j=i) 
    x * j
i = 4
f(4) # 16, but i want it to be 12

如果您想知道我为什么要这样做，您可以考虑这段代码 - 该应用程序是一个多递减模型.转换矩阵的对角线是该行中其他减量的总和.我想定义我需要的减量，而不是使用这些减量计算其他函数.在这种情况下，我只需要 uxt01 和 uxt10，我想从中生成函数 uxt00 和 uxt11.我想要一些可以扩展到更高维度的东西.

In case you are wondering why I want to do this you could consider this code - the application is a multiple decrements model. The diagonals of a transition matrix are the sum of the other decrements in that row. I would like to define the decrements I need than calculate the other functions using those decrements. In this case, I only need uxt01 and uxt10 and from these I want to produce the functions uxt00 and uxt11. I wanted something that scales to higher dimensions.

Qxt <- matrix(c(uxt00=function(t=0,x=0) 0,
                uxt01=function(t=0,x=0) 0.05,
                uxt10=function(t=0,x=0) 0.07
                uxt11=function(t=0,x=0) 0), 2, 2, byrow=TRUE)

Qxt.diag <- function(Qxt) {
    ndecrements <- length(Qxt[1,])
    for(index in seq(1, N, N+1)) { # 1, 4
        Qxt[[index]] <- function(t=0, x=0, i=index, N=ndecrements) {
            row <- ceiling(index/ndecr)
            row.decrements <- seq( (row - 1)*N + 1, (row)*N) 
            other.decrements <- row.decrements[which(row.decrements != i]
            -sum(unlist(lapply(Qxt.fns[[other.decrements]], 
                function(f) f(t,x))))
        }
    }
    Qxt.fns
}

推荐答案

这可以通过在创建函数后手动为形参 j 分配默认表达式来完成:

This can be done by assigning the default expression for the formal parameter j manually, after creating the function:

i <- 3;
f <- function(x,j) x*j;
f;
## function(x,j) x*j
formals(f);
## $x
##
##
## $j
##
##
formals(f)$j <- i;
f;
## function (x, j = 3)
## x * j
formals(f);
## $x
##
##
## $j
## [1] 3
##
i <- 4;
f(4);
## [1] 12

这是唯一可能的，因为 R 是一种很棒的语言，它为您提供对函数的所有三个特殊属性的完全读/写访问，它们是:

This is only possible because R is an awesome language and provides you complete read/write access to all three special properties of functions, which are:

1. 包含 body 的解析树:body().
2. 形式参数及其默认值(它们本身就是解析树):formals().
3. 封闭环境(用于实现闭包):<代码>环境().

这篇关于R:深拷贝一个函数参数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！