R列中的最高日期与sapply [英] Max date in R column with sapply

问题描述

我正在尝试使用sapply获取列中的最大日期，但它返回的是数字而不是日期.任何想法如何解决这个问题?我似乎不知道为什么会这样.

I am trying to use sapply to get the max date in a column but it is returning a number instead of a date. Any idea how to resolve this? I can't seem to figure out why this is occurring..

 mtcars$datecolm = '2015-03-03'
 mtcars$datecolm[1] = '2015-09-09'
  mtcars$datecolm = as.Date(mtcars$datecolm)

    sapply(mtcars, max)   # why is it returning a number instead of a date??
    max(mtcars$datecolm)   # works correctly

请按照我已设置的方式使用sapply ...我知道这适用于apply(mtcars，2，max).

Please use sapply given the way I have set this up... I know this works with apply(mtcars,2,max).

推荐答案

我们需要使用lapply而不是sapply

lapply(mtcars, max)

由于simplify=TRUE的默认参数，

as sapply返回vector，而vector只能容纳一个类.由于存在数字列，因此日期"列(存储为整数)被强制为integer值.但是，如果我们使用simplify = FALSE返回list，我们仍然可以使用sapply.

as sapply returns a vector because of the simplify=TRUE default argument and vector can hold only a single class. As there are numeric columns, the 'Date' column (which is stored as integer) gets coerced to integer value. However, we can still use sapply if we use simplify = FALSE to return a list.

sapply(mtcars, max, simplify = FALSE)

关于apply

apply(mtcars,2,max)

我不建议使用apply，因为它将转换为矩阵并且矩阵只能容纳一个类.在这里，所有的character输出都将作为日期"类被转换为character的.

I would not recommend using apply as this will convert to matrix and matrix can hold only a single class. Here, it will be all character output as the 'Date' class got converted to character.

关于OP在注释中提到的问题，当我们拥有属于不同类的输出时，我们可以使用list或data.frame(因为data.frame是list)作为c，即concatenate返回vector，如上所述，它只能容纳一个类.

Regarding the problem mentioned by OP in the comments, when we have output that belong to different class, we could either use list or data.frame (as data.frame is a list) as c i.e. concatenate returns a vector and as mentioned above, it can hold only a single class.

do.call(rbind, lapply(mtcars, function(x) 
          data.frame(Class=class(x), Max=max(x, na.rm=TRUE))))

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