问题描述
我有以下形式的数据：
Id1 Id21 c1 Id22 c2 Id23 c3 Id24 c4
1 20 5 11 9 9 20 32 10
1 40 4 14 9 13 5 36 9
1 43 3 15 3 23 1 39 8
2 47 5 17 8 11 9 10 5
2 5 4 12 8 14 8 28 4
2 6 0 10 2 24 4 23 2
3 . . . . . . . .
3 . . . . . . . .
3
4
.
.
100
100
100
Id1每个都有三个条目Id具有相应的Id2i和ci，i - > [1,4]
，使得id2i总是以递增的顺序，ci对于每个id1总是递减的。
我需要输出：
Id1 with three entries for each Id has corresponding Id2i and ci, i -> [1,4]such that id2i is always in increasing order and ci is always in decreasing order for each id1.I need the output to be :
Id1 Id2 c
1 9 20
1 32 10
1 11 9
1 14 9
1 36 9
2 11 9
2 17 8
2 12 8
2 14 8
2 47 5
.
.
.
100
100
100
100
100 . .
因此，在id1中的每个id的5个条目中，从所有ci中选择前5个c，使得c（输出）是所有ci的最大组。
如何在R中实现？
so that for five entries of each id in id1, top 5 c's are chosen from all the ci, such that c (output) is max group of all the ci.How this can be achieved in R ?
推荐答案
使用：
# using first six rows from your post
require(data.table) # v1.9.5+
ans <- melt(setDT(df), measure = patterns(c("^Id2", "^c[0-9]$"))
value.name = c("Id2", "c"))
ans[order(-c), head(.SD, 5L), by=Id1, .SDcols = -(variable)]
# Id1 Id2 c
# 1: 1 9 20
# 2: 1 32 10
# 3: 1 11 9
# 4: 1 14 9
# 5: 1 36 9
# 6: 2 11 9
# 7: 2 17 8
# 8: 2 12 8
# 9: 2 14 8
# 10: 2 47 5
基本上， melt 可以接受列名列表，将列的每个元素的列分组成单独的列。看看 lapply（...）的结果，看看哪些列组合在一起。
Basically, melt can accept a list of column names to group columns from each element of the list into separate columns. Have a look at the result of lapply(...) to see which columns are combined together.
然后我们按照 c 列排序后按 Id1 分组，并从属于...的数据子集中选出前5行每组。
Then we group by Id1 after ordering by c column in decreasing order and pick the first 5 rows from subset of data belonging to each group.
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