随机返回数据框中子集的行号

Question

我希望能够从数据集中随机返回行号，其中行是数据的子集。例如，使用数据框

x.f<-data.frame(
     G = c("M","M","M","M","M","M","F","F","F","F","F","F"),
     A = c("1","2","3","1","2","3","1","2","3","1","2","3"),
     E = c("W","W","W","B","B","B","W","W","W","B","B","B"))

我想，比方说，随机给我一个行号，其中G ==“M”和A ==“3”，所以答案将是第3行或第6行。返回的数字必须是位置在原始数据框中。虽然这个例子结构很好（每个可能的组合只出现一次），但实际上不会有这样的结构，例如组合（M，2，W）将随机分布在整个数据框中并且可以发生更多或更少次比其他组合。

Answer 1

请尝试一个：which(((x.f$G == "M") & (x.f$A == 3)))

Answer 2

或许这个：

row.names(subset(x.f, x.f$G == "M" & x.f$A == 3))
[1] "3" "6"

Answer 3

使用Sourabh和sample的答案，您可以尝试：

# create a function using the sample function, which selects one value by chance
foo <- function(G, A, data){
  sample(which(data$G == G & data$A == A), 1)
}

foo("M", 3, x.f)
3

要测试相等性，请使用循环运行该函数1000次：

res <- NULL
for(i in 1:1000){
  res[i] <- foo("M", 3, x.f)
}
hist(res)

似乎是平等分配。

Answer 4

其他任何一个答案都会为您提供符合条件的行列表，但不会随机选择一行。如需完整答案：

sample(which(x.f$G == "M" & x.f$A == 3),1)

或

sample(row.names(subset(x.f, x.f$G == "M" & x.f$A == 3)),1)

或

sample(row.names(x.f[x.f$G=="M" & x.f$A==3,]),1)

一切都会奏效。可能有两到三种其他方法来生成符合一组条件的行索引或名称列表。