我有一个由字母表的前20个字母组成的向量，我想创建一个包含6列的 Dataframe ，其中包含6个字母的所有组合。
我已经生成了每一种可能的排列，使用子集函数，我可以减少观测的总数，以消除重复，但我的代码效率非常低，有没有办法直接杀死并生成组合，而不是从排列开始，然后从那里开始？
参见以下代码：

a = c("a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", 
      "t")
a = data.frame(a)
a = as.vector(a[[1]])

permutations = data.frame((expand.grid(a,a,a,a,a,a,
                                       stringsAsFactors = FALSE)))

permutations = subset(permutations, Var1!=Var2)
permutations = subset(permutations, Var1!=Var3)
permutations = subset(permutations, Var1!=Var4)
permutations = subset(permutations, Var1!=Var5)
permutations = subset(permutations, Var1!=Var6)
permutations = subset(permutations, Var2!=Var3)
permutations = subset(permutations, Var2!=Var4)
permutations = subset(permutations, Var2!=Var5)
permutations = subset(permutations, Var2!=Var6)
permutations = subset(permutations, Var3!=Var4)
permutations = subset(permutations, Var3!=Var5)
permutations = subset(permutations, Var3!=Var6)
permutations = subset(permutations, Var4!=Var5)
permutations = subset(permutations, Var4!=Var6)
permutations = subset(permutations, Var5!=Var6)

combinations = data.frame(permutations[!duplicated(t(apply(
  permutations[c("Var1", 
            "Var2",
            "Var3",
            "Var4",
            "Var5",
            "Var6")], 1, sort))), ])

子集背后的想法是在执行组合代码之前减少观察的数量，以保存计算时间，但我有一种感觉，有一种更好的方法来实现这一点