按 R 中的因素过滤数据框

我有以下数据框：

sp <- combn(c("sp1","sp2","sp3","sp4"),2)
d <- data.frame(t(sp),"freq"=sample(0:100,6))

和两个因素

x1 <- as.factor(c("sp1","sp2"))
x2 <- as.factor(c("sp3","sp4"))

我需要返回一个包含所有可能组合的数据框x1 and x2和freq来自数据框d与此组合相关。

返回的数据框将如下所示：

data.frame("X1" = c("sp1","sp1","sp2","sp2"),
           "X2" = c("sp3","sp4","sp3","sp4"),
           "freq" = c(4,94,46,74))

我努力了：

sub <- d[d$X1 == x1 & d$X2 == x2,]

但出现错误

Error in Ops.factor(d$X1, x1) : level sets of factors are different

关于如何解决这个问题有什么想法吗？

不要做x1 and x2因素。只需使用向量即可。使用%in%用于逻辑测试。

sp <- combn(c("sp1","sp2","sp3","sp4"),2)
d <- data.frame(t(sp),"freq"=sample(0:100,6))
x1 <- c("sp1","sp2")
x2 <- c("sp3","sp4")
sub <- d[d$X1 %in% x1 & d$X2 %in% x2,]