訪問列表中的元素r

Question

我想訪問列表中的某個元素，並將其存儲在data.frame中。如果特定對象沒有該元素，結果應該是NA。

實施例：

list 
$A 
"mark" "peter" "sam" "il" 

$B 
"dd" "pp" "tt" "ff" "jj" "rr" 

$C 
"aa" "aa" "aa" "aa" "aa" "aa" 

結果通過調用第六元件在整個列表應該是：

結果（數據幀。）：

name value 

<NA> A 
rr B 
aa C 

任何建議？

Answer 1

您可以使用sapply和data.frame：

# The list 
l <- list(A = c("mark", "peter", "sam", "il"), 
      B = c("dd", "pp", "tt", "ff", "jj", "rr"), 
      C = c("aa", "aa", "aa", "aa", "aa", "aa")) 

# extract sixth element in each vector 
res <- sapply(l, "[", 6) 

# combine to data frame 
dat <- data.frame(name = res, value = names(res)) 

# name value 
# A <NA> A 
# B rr B 
# C aa C