用户指定的函数和R中的操作符

I want to use a user-specified function and apply the function to a list of values. I envision that the user will give a 'formula' as a character string containing the names of variable and operators, e.g. "a * b %% c - d / e ^ f + g %/% h".

我希望使用用户指定的函数，并将该函数应用到值列表。我设想用户将给出一个“公式”作为一个字符串，其中包含变量和操作符的名称，例如。“a * b % % c f + g - d / e ^ % / % h”。

The following toy example works

下面的玩具示例是有效的

prmlist <- list(a=1:10, b=21:30, c=31:40, d=4, e=5, f=6, g=7, h=8) 
with(prmlist, a * b %% c - d / e ^ f + g %/% h)

The problem starts when I want to use this approach within a function. To do that I must get the 'formula' specified by the user inside the function. A character string seems the obvious route. The question is how to evaluate it inside the function. do.call() doesn't seem to be suited because the operators are each really a function. I hoped something simple like

当我想在函数中使用这种方法时，问题就出现了。要做到这一点，我必须得到用户在函数中指定的“公式”。字符串似乎是明显的路径。问题是如何在函数中求值。call()似乎不适合，因为操作符实际上都是函数。我希望有一些简单的东西

my.formula <- "a * b %% c - d / e ^ f + g %/% h"
with(prmlist, eval(my.formula))

would work but it doesn't.

可能行得通，但行不通。

1 个解决方案

#1

You can envoke your operators using substitute() instead:

您可以使用substitute()替代您的操作符:

my.formula <- substitute(a * b %% c - d / e ^ f + g %/% h)
with(prmlist, eval(my.formula)) 
 [1]  20.99974  43.99974  68.99974  95.99974 124.99974 155.99974 188.99974
 [8] 223.99974 260.99974 299.99974

Update: If the command is a string you can use parse:

更新:如果命令是字符串，可以使用parse:

myCmd <- "a * b %% c - d / e ^ f + g %/% h"
with(prmlist, eval( parse(text=myCmd) ))
 [1]  20.99974  43.99974  68.99974  95.99974 124.99974 155.99974 188.99974
 [8] 223.99974 260.99974 299.99974

#1