根据另一列提取值 [英] extracting value based on another column

问题描述

我有一个可以散出矩阵的函数，例如:

I have a function that spits out a matrix, such as:

      x freq
1 FALSE   40
2  TRUE    6

但是当没有FALSE值时，我得到

but when there are no FALSE values, I get

     x freq
1 TRUE   46

当x = TRUE时，我想提取频率值. 如果总是同时存在FALSE和TRUE值，我可以做

I want to extract the freq value when x=TRUE. If there are are always both FALSE and TRUE values, I can do

> matrix [2,2]
[1] 6

但是无论是否存在FALSE值，我都希望能够提取TRUE值.有人知道我该怎么做吗?预先感谢！

But I would like to be able to extract the TRUE value whether or not there are FALSE values. Does anyone know how I can do that? Thanks in advance!

推荐答案

正如@Justin所说，您可能正在使用data.frame而不是matrix.一切都好.使用上面的示例，如果您的data.frame看起来如下:

As @Justin said, you might be working with a data.frame instead of a matrix. All the better. Using your example above, if your data.frame looks as follows:

df <- data.frame(x=c(FALSE,TRUE), freq=c(40, 6))
> df
      x freq
1 FALSE   40
2  TRUE    6

无论是否存在FALSE值，以下内容都会为您提供所需的信息.

The following will get you what you want irrespective of whether there are FALSE values or not.

df$freq[df$x==TRUE]
[1] 6

编辑:正如@DWin所指出的，您可以使用df$x是合乎逻辑的事实来进一步简化:

EDIT: As @DWin points out, you can simplify further by using the fact that df$x is logical:

> df$freq[df$x]
[1] 6
> df$freq[!df$x]
[1] 40

例如:

> df2 <- data.frame(x=TRUE, freq=46)
> df2
     x freq
1 TRUE   46

仍然有效:

> df2$freq[df2$x==TRUE]
[1] 46

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