使用R选择列中组内的前N个值 [英] Selecting top N values within a group in a column using R

问题描述

我需要从R中的以下数据框中为每个组的[yearmonth]值选择前两个值.我已经按照count和yearmonth对数据进行了排序.如何在随后的数据中实现呢?

I need to select top two values for each group[yearmonth] value from the following data frame in R. I have already sorted the data by count and yearmonth.How can I achieve that in following data?

 yearmonth  name    count
1   201310  Dovas   5
2   201310  Indulgd 2
3   201310  Justina 1
4   201310  Jolita  1
5   201311  Shahrukh Sheikh 1
6   201311  Dovas   29
7   201311  Justina 13
8   201311  Lina    8
9   201312  sUPERED 7
10  201312  John Hansen 7
11  201312  Lina D. 6
12  201312  joanna1st   5

推荐答案

或使用data.table(来自@jazzurro帖子的mydf).一些选项是

Or using data.table (mydf from @jazzurro's post). Some options are

  library(data.table)
  setDT(mydf)[order(yearmonth,-count), .SD[1:2], by=yearmonth]

或

   setDT(mydf)[mydf[order(yearmonth, -count), .I[1:2], by=yearmonth]$V1,]

或

   setorder(setkey(setDT(mydf), yearmonth), yearmonth, -count)[
                                          ,.SD[1:2], by=yearmonth]
  #    yearmonth        name count
  #1:    201310       Dovas     5
  #2:    201310     Indulgd     2
  #3:    201311       Dovas    29
  #4:    201311     Justina    13
  #5:    201312     sUPERED     7
  #6:    201312 John Hansen     7

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