问题描述
我希望(1)按一个变量(State)对数据进行分组，(2)在每个组中找到另一个变量(Employees)的最小值行，以及 (3) 提取整行.
I wish to (1) group data by one variable (State), (2) within each group find the row of minimum value of another variable (Employees), and (3) extract the entire row.
(1) 和 (2) 是简单的单行，我觉得 (3) 也应该是，但我无法理解.
(1) and (2) are easy one-liners, and I feel like (3) should be too, but I can't get it.
这是一个示例数据集:
> data
State Company Employees
1 AK A 82
2 AK B 104
3 AK C 37
4 AK D 24
5 RI E 19
6 RI F 118
7 RI G 88
8 RI H 42
data <- structure(list(State = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L,
2L), .Label = c("AK", "RI"), class = "factor"), Company = structure(1:8, .Label = c("A",
"B", "C", "D", "E", "F", "G", "H"), class = "factor"), Employees = c(82L,
104L, 37L, 24L, 19L, 118L, 88L, 42L)), .Names = c("State", "Company",
"Employees"), class = "data.frame", row.names = c(NA, -8L))
按组计算min很容易，使用aggregate:
Calculate min by group is easy, using aggregate:
> aggregate(Employees ~ State, data, function(x) min(x))
State Employees
1 AK 24
2 RI 19
...或data.table:
> library(data.table)
> DT <- data.table(data)
> DT[ , list(Employees = min(Employees)), by = State]
State Employees
1: AK 24
2: RI 19
但是如何提取与这些 min 值对应的整行，即结果中还包括 Company ?
But how do I extract the entire row corresponding to these min values, i.e. also including Company in the result?
推荐答案
稍微优雅一点:
library(data.table)
DT[ , .SD[which.min(Employees)], by = State]
State Company Employees
1: AK D 24
2: RI E 19
比使用 .SD 略逊一筹，但要快一些(对于多组数据):
Slighly less elegant than using .SD, but a bit faster (for data with many groups):
DT[DT[ , .I[which.min(Employees)], by = State]$V1]
另外，如果您的数据集有多个相同的最小值并且您'我想对所有这些进行子集化.
Also, just replace the expression which.min(Employees) with Employees == min(Employees), if your data set has multiple identical min values and you'd like to subset all of them.
另请参见 对应于 max 的子集行使用 data.table 分组值.
这篇关于按组提取对应于变量最小值的行的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！