r - plyr 转换后不返回新变量

Question

我正在尝试学习如何在 R/plyr 中编写函数。我知道有更简单的方法可以完成我在下面展示的内容，但这不是重点。

在下面的示例中，PLYR 不会向我的新数据框返回新变量

library(plyr)
highab <-subset(baseball, ab >= 600)

testfunc1 <-function(x) {
    print(x) #just to show me that the vector does get into the function. Works fine.
    medianAB <- median(x)
    print(medianAB) #just to prove that medianAB was calculated correctly. Works fine   
}


baseball3 <-ddply(highab, .(id), transform, testfunc1(ab))
str(baseball3$medianAB) #No medianAB

我错过了什么明显的东西？

R version 2.12.2 (2011-02-25)
Platform: x86_64-pc-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_CA.UTF-8       LC_NUMERIC=C               LC_TIME=en_CA.UTF-8        LC_COLLATE=en_CA.UTF-8    
 [5] LC_MONETARY=C              LC_MESSAGES=en_CA.UTF-8    LC_PAPER=en_CA.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=en_CA.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] grid      splines   stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] foreign_0.8-42  ggplot2_0.8.9   proto_0.3-9.1   reshape_0.8.4   plyr_1.4.1      rms_3.3-0       Hmisc_3.8-3    
[8] survival_2.36-5 stringr_0.4    

loaded via a namespace (and not attached):
[1] cluster_1.13.3  lattice_0.19-23 tools_2.12.2   

Answer 1

只做两个改变

1. 去掉函数内的打印命令，返回中位数
2. 按照 Joshua 的建议添加 medianAB = testfunc1(ab)

你完成了!

这是带有输出的简化代码

library(plyr)
highab <-subset(baseball, ab >= 600)
baseball3 <-ddply(highab, .(id), transform, medianAB = median(ab))
summary(baseball3$medianAB)

Min. 1st Qu. Median Mean 3rd Qu.
Max. 600.0 612.0 621.5 623.1 631.5 677.0