ddply:如何将列名作为参数传递? [英] ddply: how do I pass column names as parameters?

问题描述

我有一个数据框，其中的列名称是根据参数生成的-因此我不知道它们的确切值.我想将这些字段也作为参数传递给ddply. 我想答案很明显，但是有人可以帮我打开灯.

I have a data frame where the column names are generated based on parameters - so I don't know their exact values. I want to pass these fields to ddply also as parameters. I guess the answer is obvious, but can someone please turn the light on for me.

下面的示例使用虹膜数据集，它给出了我想做的事情的想法，以及我所付出的努力的意外结果.第一个示例的结果是我要实现的虹膜1，但是通过将列名作为参数传递(如我的虹膜2所做的那样)，这并没有达到我想要的结果.

Example below using the iris data set that gives the idea of what I want to do, and the unintended result of my effort. The results of first example, iris1 is what I want to achieve, but by passing the column names in as parameters, as in my iris2 effort, that doesn't give me the intended results.

iris1 <- ddply(iris, .(Species), transform, pw_first = Petal.Width[1], 
              pw_last = Petal.Width[length(Petal.Width)])
myCol <- 'Petal.Width'
iris2 <- ddply(iris, .(Species), transform, pw_first = myCol[1], 
               pw_last = myCol[length(myCol)])

head(iris1)
#   Sepal.Length Sepal.Width Petal.Length Petal.Width Species pw_first pw_last
# 1          5.1         3.5          1.4         0.2  setosa      0.2     0.2
# 2          4.9         3.0          1.4         0.2  setosa      0.2     0.2
# 3          4.7         3.2          1.3         0.2  setosa      0.2     0.2
# 4          4.6         3.1          1.5         0.2  setosa      0.2     0.2
# 5          5.0         3.6          1.4         0.2  setosa      0.2     0.2
# 6          5.4         3.9          1.7         0.4  setosa      0.2     0.2

head(iris2)
#   Sepal.Length Sepal.Width Petal.Length Petal.Width Species    pw_first     pw_last
# 1          5.1         3.5          1.4         0.2  setosa Petal.Width Petal.Width
# 2          4.9         3.0          1.4         0.2  setosa Petal.Width Petal.Width
# 3          4.7         3.2          1.3         0.2  setosa Petal.Width Petal.Width
# 4          4.6         3.1          1.5         0.2  setosa Petal.Width Petal.Width
# 5          5.0         3.6          1.4         0.2  setosa Petal.Width Petal.Width
# 6          5.4         3.9          1.7         0.4  setosa Petal.Width Petal.Width

推荐答案

colName<-"Petal.Width"

iris1 <- ddply(iris, .(Species), function (x) {
               pw.first=x[1,colName]
               pw.last=x[length(x[,1]),colName]
               result=cbind(x,pw.first,pw.last)
               return(result)})

unique(iris1$pw.first)
[1] 0.2 1.4 2.5

unique(iris1$pw.last)
[1] 0.2 1.3 1.8

如果只需要种类，然后是pw.first和pw.last，只需从cbind中删除x.

If you only want the species, and pw.first and pw.last, simple remove the x from cbind.

这篇关于ddply:如何将列名作为参数传递?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！