如何在R中定義靈活的'函數表達式'

Question

是否可以編寫靈活的函數表達式？ 我想使用輸入參數來控制函數的表達式。

例如

input arg -> function 
c(1,1) -> func1 = function(x) x+1 
c(1,3,2) -> func2 = function(x) x^2+3*x+2 
c(6,8,-1) -> func3 = function(x) 6*x^2+8*x-1 

Answer 1

您可以使用polynom

library(polynom) 
as.polynomial(c(2,3,1)) 
2 + 3*x + x^2 
as.polynomial(c(6,8,1) 
1 + 8*x + 6*x^2 

編輯你當然可以強迫的結果，使用the generic as.function.polynomial . better here you can use , as.polylist`的功能根據係數列表創建許多多項式。例如：

lapply(as.polylist(list(c(2,3,1),c(6,8,1),c(6,8,-1))), 
     as.function) 

[[1]] 
function (x) 
{ 
    w <- 0 
    w <- 1 + x * w 
    w <- 3 + x * w 
    w <- 2 + x * w 
    w 
} 
<environment: 0x00000000113bd778> 

[[2]] 
function (x) 
{ 
    w <- 0 
    w <- 1 + x * w 
    w <- 8 + x * w 
    w <- 6 + x * w 
    w 
} 
<environment: 0x0000000011524168> 

[[3]] 
function (x) 
{ 
    w <- 0 
    w <- -1 + x * w 
    w <- 8 + x * w 
    w <- 6 + x * w 
    w 
} 
<environment: 0x0000000011527f28> 

Answer 2

makepoly <- function(b) 
{ 
    p <- rev(seq_along(b) - 1) 
    function(x) 
    { 
     xp <- outer(x, p, '^') 
     rowSums(xp * rep(b, each=length(x))) 
    } 
} 

# x^2 + 2x + 3 
f <- makepoly(1:3) 
f(0:4) 
[1] 3 6 11 18 27 

Answer 3

目前尚不清楚你想怎麼一般是從OP。對於多項式的特定情況下，你可以這樣做：

f = function(x, coeffs) { 
    sum(outer(x, seq_along(coeffs) - 1, `^`) * coeffs) 
} 

f(2, c(1,2,3)) # 1 + 2*x + 3*x^2, with x = 2 
#[1] 17 

Answer 4

我看這是使職能的願望，我認爲agstudy/EDDI反應可能會做到這一點，但我想從頭開始嘗試它可能是有益的：

poly.maker <- function(coefs) { func <- function(x){} #empty func in x 
      body(func) <- parse(text= paste(seq_along(coefs),"*x^", 
            (length(coefs)-1):0,collapse="+")) 
      return(func) } 
func2 <- poly.maker(c(1,2,3)) # return a function 
func2(3) # now test it out 
#[1] 18 

注意，我需要交換爲了與OP的要求，我只獲得比@dickoa不同的結果後發現同意。這似乎不太笨重：

poly.make2 <- function(coefs) { func <- function(x){} 
     body(func) <- bquote(sum(.(coefs)*x^.((length(coefs)-1):0))) 
     return(func) } 
func <- poly.make2(c(1,2,5)) 
func 
#function (x) 
#sum(c(1, 2, 5) * x^c(2L, 1L, 0L)) 
#<environment: 0x29023d508> 
func(3) 
#[1] 20 

Answer 5

這是我擔此重任

create_poly <- function(coef) 
paste(rev(coef), 
     paste("x", seq_along(coef) - 1, sep = "^"), 
     sep = "*", collapse = " + ") 


make_polyfun <- function(input) { 
    myfun <- paste("function(x)", create_poly(input)) 
    eval(parse(text = myfun)) 
} 

隨着例如OP給了我們：

make_polyfun(c(1, 1)) 
## function(x) 1*x^0 + 1*x^1 
## <environment: 0x243a540> 

make_polyfun(c(1, 3, 2)) 
## function(x) 2*x^0 + 3*x^1 + 1*x^2 
## <environment: 0x1bd46e0> 

make_polyfun(c(6, 8, 1)) 
## function(x) 1*x^0 + 8*x^1 + 6*x^2 
## <environment: 0x22a59c0> 

Answer 6

一個內膽：

polymaker2 <- function(coefs) 
{ 
    eval(parse(text=paste0("function(x) sum(x^(",length(coefs)-1,":0) * ",capture.output(dput(coefs)),")"))) 
} 

矢量化形式：

polymaker3 <- function(coefs) 
{ 
    eval(parse(text=paste0("function(x) colSums(t(outer(x, ",length(coefs)-1,":0, `^`))*",capture.output(dput(coefs)),")"))) 
}