过滤R data.frames [重复]时更新因子级别

Question

这个问题在这里已有答案：

• Drop factor levels in a subsetted data frame 13个答案

我有一个类似于下面的data.frame。我通过删除我不感兴趣的行来预处理它。我的大多数列是'因素'，其'级别'不会更新，因为我过滤data.frame。

我可以看到我在下面做的事情并不理想。在修改data.frame时如何更新因子级别？下面是出现问题的演示。

# generate data
set.seed(2013)
df <- data.frame(site = sample(c("A","B","C"), 50, replace = TRUE),
                 currency = sample(c("USD", "EUR", "GBP", "CNY", "CHF"),50, replace=TRUE, prob=c(10,6,5,6,0.5)),
                 value = ceiling(rnorm(50)*10))

# check counts to see there is one entry where currency =  CHF
count(df, vars="currency")

>currency freq
>1      CHF    1
>2      CNY   13
>3      EUR   16
>4      GBP    6
>5      USD   14


# filter out all entires where site = A, i.e. take subset of df
df <- df[!(df$site=="A"),]

# check counts again to see how this affected the currency frequencies
count(df, vars="currency")

>currency freq
>1      CNY   10
>2      EUR    8
>3      GBP    4
>4      USD   10

# But, the filtered data.frame's levels have not been updated:
levels(df$currency)

>[1] "CHF" "CNY" "EUR" "GBP" "USD"

levels(df$site)

>[1] "A" "B" "C"

期望的产出：

# levels(df$currency) = "CNY" "EUR" "GBP" "USD
# levels(df$site) = "B" "C"

Answer 1

使用droplevels：

> df <- droplevels(df)
> levels(df$currency)
[1] "CNY" "EUR" "GBP" "USD"
> levels(df$site)
[1] "B" "C"