如何在R中排序列表?

问题描述

我有与此类似的列表清单:

I have list of lists similar to this:

a <- list(
  list(day = 5, text = "foo"),
  list(text = "bar", day = 1),
  list(text = "baz", day = 3),
  list(day = 2, text = "quux")
)

字段数未知，并且字段混乱.

with unknown number of fields and the fields my be out of order.

如何根据日期对该列表进行排序?我需要将列表升序排列.我已经搜索过，但是我只找到了如何对向量进行排序.可以对列表进行排序吗?

how can I sort this list based on day? I need the list to be sorted ascending. I've search but I only found how to sort vectors. Is it possible to sort a list?

推荐答案

为了对给定的列表列表" a进行排序，您可以尝试将sapply()与提取运算符[[一起使用，以从中检索数据列表.这些用于对order()的调用:

In order to sort that given "list of lists" a you can try to use sapply()with the extraction operator [[ to retrieve data from the list. These are used in the call to order():

a[order(sapply(a, `[[`, i = "day"))]
#[[1]]
#[[1]]$day
#[1] 1
#
#[[1]]$text
#[1] "bar"
#
#
#[[2]]
#[[2]]$day
#[1] 2
#
#[[2]]$text
#[1] "quux"
# ...

如此评论中所建议，通过使用sapply()中的匿名函数实现:

As suggested in this comment, this can also be achieved by using an anonymous function in sapply():

a[order(sapply(a, function(x) x$day))]

与OP一样，在函数定义中使用时也是如此:

This also works when used in a function definition as the OP did:

sortBy <- function(a, field) a[order(sapply(a, "[[", i = field))]
sortBy(a, "day")

请注意，我们需要将提取运算符[[括在反引号或引号中.

Note that we need to enclose the extraction operator [[ either in backquotes or quotes.

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