For-loop和Lapply：相同的函数给出不同的结果

Question

我想在向量列表上迭代一个函数。我正在尝试使用Lapply，但是这会产生不想要的结果，而带有相同参数的for循环则具有正确的结果：

可复制的示例：

library(gtools) # for 'permutations' function
exampleList <- list(c("RETURN", "COMBINATIONS"), c(1,2,3), c("PLEASE WORK") )

所需的输出（for循环返回什么）：

for (i in 1:length(exampleList)) {
  print( permutations(n = length(exampleList[[i]]), r = length(exampleList[[i]]), v = exampleList[[i]]))
}

     [,1]           [,2]          
[1,] "COMBINATIONS" "RETURN"      
[2,] "RETURN"       "COMBINATIONS"
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    1    3    2
[3,]    2    1    3
[4,]    2    3    1
[5,]    3    1    2
[6,]    3    2    1
     [,1]         
[1,] "PLEASE WORK"

Lapply版本当前返回的内容：

lapply(exampleList, permutations, n = length(exampleList), r = length(exampleList))

Error in FUN(X[[i]], ...) : v is either non-atomic or too short

[如果我理解正确，请对每个exampleList [[i]]进行lapply迭代，因此不需要指定'v'参数（请注意，尝试指定它时仍然会出错）。是什么导致我的结果不一致？

Answer 1

[尝试将n和r的值设为length(exampleList)。但是，它应等于列表中每个单独元素的长度。

lapply(exampleList, function(x) 
        gtools::permutations(n = length(x), r = length(x), v = x))


#[[1]]
#     [,1]           [,2]          
#[1,] "COMBINATIONS" "RETURN"      
#[2,] "RETURN"       "COMBINATIONS"

#[[2]]
#     [,1] [,2] [,3]
#[1,]    1    2    3
#[2,]    1    3    2
#[3,]    2    1    3
#[4,]    2    3    1
#[5,]    3    1    2
#[6,]    3    2    1

#[[3]]
#     [,1]         
#[1,] "PLEASE WORK"

您也可以用Map编写此内容

Map(function(x, y) gtools::permutations(n = y, r = y, v = x), 
                   exampleList, lengths(exampleList))