R: sapply 有效，但 lapply 无效

Question

我试图更好地理解 lapply 和 sapply 之间的区别。在下面的示例中，为什么后者有效，而前者无效？

# Data

  data <- data.frame("A" = rnorm(500, 0, 1), 
                     "B" = rnorm(500, 0, 1), 
                     "C" = sample(c("a", "b" , "c"), 500, replace = T))

# Give the mean of all numeric elements in the dataframe. 

  lapply(data[lapply(data, is.numeric)], mean) # Doesn't work

  Error in `[.default`(data, lapply(data, is.numeric)) : 
  invalid subscript type 'list'

  lapply(data[sapply(data, is.numeric)], mean) # Works

Answer 1

lapply 默认返回一个列表:

来自文档:

lapply returns a list of the same length as X, each element of which is the result of applying FUN to the corresponding element of X.

sapply 默认返回一个向量:

来自文档:

sapply is a user-friendly version and wrapper of lapply by default returning a vector.

因此，当您像这样 data[sapply(data, is.numeric)] 对 data.frame 进行切片/子集时，您需要传递一个元素向量，否则它将无法工作。这就是为什么 sapply 有效而 lapply 无效的原因。