问题描述
假设我有一个数据框:
mydf <- data.frame(colA = c(1,20), colB = c("a", "ab"), colC = c(T, F))
现在假设我想对数据框上的每一行应用一个函数.此函数使用列 C 的布尔值.当使用 apply 时，每个非字符串都将转换为列中存在的最大长度的字符串:
Now suppose I want to apply a function to each row on the data frame. This function uses the boolean value of column C. When using apply, every non-string is converted to a string of the maximum length present in the column:
> apply(mydf, 1, '[', 3)
[1] " TRUE" "FALSE"
字符串 " TRUE" 不再可解释为逻辑.
The string " TRUE" is no longer interpretable as a logical.
> ifelse(apply(mydf, 1, '[', 3), 1, 2)
[1] NA 2
我可以用 gsub(" ", "", x) 解决这个问题，但我敢打赌有更好的方法.当 apply 可以直接将逻辑转换为字符串时，为什么它有这种行为?是否有类似 apply 的函数没有上述行为?
I could solve this with a gsub(" ", "", x), but I'd bet there is a better way. Why does apply have this behavior when it could just directly convert the logicals to strings? Is there an apply-like function which does not have the above behavior?
推荐答案
当您调用 apply 时，您的数据框被转换为字符矩阵.出现空格是因为每个元素都转换为列中最宽元素的宽度.
When you called apply, your data frame was converted to a character matrix. The spaces appear because each element is converted to the width of the widest element in the column.
您可以使用 for 类似循环的 sapply 调用来实现
You can do it with a for loop-like sapply call
> ( s <- sapply(seq(nrow(mydf)), function(i) mydf[i, 3]) )
# [1] TRUE FALSE
> class(s)
# [1] "logical"
对您使用 apply 所做的事情的解决方法是
A workaround to what you are doing with apply would be
> as.logical(gsub("\\s+", "", apply(mydf, 1, `[`, 3)))
# [1] TRUE FALSE
但请注意，这两者完全相同
But note that these are both exactly the same as
> mydf[,3]
# [1] TRUE FALSE
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