您如何根据最小样本大小对 R 中的数据框进行子集化 [英] How do you subset a data frame in R based on a minimum sample size

问题描述

假设您有一个包含两级因子的数据框，如下所示:

Let's say you have a data frame with two levels of factors that looks like this:

Factor1    Factor2    Value
A          1          0.75
A          1          0.34
A          2          1.21   
A          2          0.75 
A          2          0.53
B          1          0.42
B          2          0.21  
B          2          0.18
B          2          1.42

等

我如何subset这个数据框(df"，如果你愿意的话)基于 Factor1 和 Factor2 的组合(Fact1*Fact2)有超过 2 个观察值的条件?您可以使用 subset 中的 length 参数来执行此操作吗?

How do I subset this data frame ("df", if you will) based on the condition that the combination of Factor1 and Factor2 (Fact1*Fact2) has more than, say, 2 observations? Can you use the length argument in subset to do this?

推荐答案

假设你的 data.frame 叫做 mydf，你可以使用 ave 创建一个逻辑向量来帮助子集:

Assuming your data.frame is called mydf, you can use ave to create a logical vector to help subset:

mydf[with(mydf, as.logical(ave(Factor1, Factor1, Factor2, 
                           FUN = function(x) length(x) > 2))), ]
#   Factor1 Factor2 Value
# 3       A       2  1.21
# 4       A       2  0.75
# 5       A       2  0.53
# 7       B       2  0.21
# 8       B       2  0.18
# 9       B       2  1.42

<小时>

这是 ave 计算您的组合.请注意，ave 返回一个与data.frame 中的行数相同长度的对象(这便于子集化).

---

Here's ave counting up your combinations. Notice that ave returns an object the same length as the number of rows in your data.frame (this makes it convenient for subsetting).

> with(mydf, ave(Factor1, Factor1, Factor2, FUN = length))
[1] "2" "2" "3" "3" "3" "1" "3" "3" "3"

下一步是将该长度与您的阈值进行比较.为此，我们的 FUN 参数需要一个匿名函数.

The next step is to compare that length to your threshold. For that we need an anonymous function for our FUN argument.

> with(mydf, ave(Factor1, Factor1, Factor2, FUN = function(x) length(x) > 2))
[1] "FALSE" "FALSE" "TRUE"  "TRUE"  "TRUE"  "FALSE" "TRUE"  "TRUE"  "TRUE" 

差不多了……但是由于第一项是字符向量，我们的输出也是字符向量.我们想要它 as.logical 这样我们就可以直接使用它进行子集化.

Almost there... but since the first item was a character vector, our output is also a character vector. We want it as.logical so we can directly use it for subsetting.

ave 不适用于 factor 类的对象，在这种情况下，您需要执行以下操作:

ave doesn't work on objects of class factor, in which case you'll need to do something like:

mydf[with(mydf, as.logical(ave(as.character(Factor1), Factor1, Factor2, 
                               FUN = function(x) length(x) > 2))),]

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