mapply基础知识? -如何从两个向量和一个函数创建矩阵 [英] mapply basics? - how to create a matrix from two vectors and a function

问题描述

我正在尝试创建一个data.frame来从中创建图形.我有一个函数和两个向量要用作两个输入.这有点简化，但是基本上我只有:

I am trying create a data.frame from which to create a graph. I have a function and two vectors that I want to use as the two inputs. This is a bit simplified, but basically all I have is:

relGPA <- seq(-1.5,1.5,.2)
avgGPA <- c(-2,0,2)

f <- function(relGPA, avgGPA) 1/(1+exp(sum(relGPA*pred.model$coef[1],avgGPA*pred.model$coef[2])))

我想要的只是一个data.frame，其中avgGPA值具有3列，而relGPA值具有16行，并在单元格中具有结果值.

and all I want is a data.frame with 3 columns for the avgGPA values, and 16 rows for the relGPA values with the resulting values in the cells.

我很抱歉这很基本，但是我向您保证，在没有您帮助的情况下，我试图做到这一点.我尝试按照sapply和mapply手册页上的示例进行操作，但是对于R来说，我有点太新了，无法了解我要做什么.

I apologize for how basic this is, but I assure you I have tried to make this happen without your assistance. I have tried following the examples on the sapply and mapply man pages, but I'm just a little too new to R to see what I'm trying to do.

谢谢！

推荐答案

无法使用所提供的信息进行测试，但这应该可以:

Cannot be tested with the information offered, but this should work:

expGPA  <- outer(relGPA, avgGPA, FUN=f) # See below for way to make this "work"

要生成组合时，另一个有用的功能是expand.grid，这将使您获得长格式":

Another useful function when you want to generate combinations is expand.grid and this would get you the "long form":

expGPA2 <-expand.grid(relGPA, avgGPA)
expGPA2$fn <- apply(expGPA2, 1, f)

长格式是将晶格和ggplot作为更高级别绘图的输入格式所期望的形式.

The long form is what lattice and ggplot will expect as input format for higher level plotting.

也许有必要构造一种更具体的方法，以将列引用传递给djhurio和Sam Swift指出的(通过解决方案)使用Vectorize策略的函数.在apply的情况下，sum函数将如上所述开箱即用，但是除法运算符将无法工作，因此这里是另一个示例，可以将其推广到具有多个参数的更复杂的函数.程序员需要的只是"apply()"-ed函数中适当参数的列号，因为(不幸的)列名没有传递给x参数:

It may be necessary to construct a more specific method for passing column references to the function as pointed out by djhurio and (solved) by Sam Swift with the Vectorize strategy. In the case of apply, the sum function would work out of the box as described above, but the division operator would not, so here is a further example that can be generalized to more complex functions with multiple arguments. All the programmer needs is the number of the column for the appropriate argument in the "apply()"-ed" function, because (unfortunately) the column names are not carried through to the x argument:

> expGPA2$fn <- apply(expGPA2, 1, function(x) x[1]/x[2])
> str(expGPA2)
'data.frame':   48 obs. of  3 variables:
 $ Var1: num  -1.5 -1.3 -1.1 -0.9 -0.7 ...
 $ Var2: num  -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 ...
 $ fn  : num  0.75 0.65 0.55 0.45 0.35 ...
 - attr(*, "out.attrs")=List of 2
  ..$ dim     : int  16 3
  ..$ dimnames:List of 2
  .. ..$ Var1: chr  "Var1=-1.5" "Var1=-1.3" "Var1=-1.1" "Var1=-0.9" ...
  .. ..$ Var2: chr  "Var2=-2" "Var2= 0" "Var2= 2"

Edit2:(2013-01-05)一年后，我意识到，可以通过使主体使用"+"代替sum来对SamSwift的函数进行矢量化处理:

(2013-01-05) Looking at this a year later, I realized that SamSwift's function could be vectorized by making its body use "+" instead of sum:

 1/(1+exp( relGPA*pred.model$coef[1] + avgGPA*pred.model$coef[2]) # all vectorized fns

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