r-使用for中的2个变量进行迭代

Question

R中是否有一个结构，我们可以同时在R中使用两个变量进行迭代？像这样

for(i in list1 and j in list2)

list1和list2可以是任何可迭代的。

Answer 1

If the contents of your for loop can be written as some kind of a function then you can use mapply.

a <- 1:10
b <- LETTERS[1:10]

a
[1]  1  2  3  4  5  6  7  8  9 10
b
[1] "A" "B" "C" "D" "E" "F" "G" "H" "I" "J"

mapply(paste, a, b)
[1] "1 A"  "2 B"  "3 C"  "4 D"  "5 E"  "6 F"  "7 G"  "8 H"  "9 I"  "10 J"

Sure you will have to replace "paste" with a function that takes 2 elements (one from each list) as input. Also using more than 2 lists/vectors is ok.

Answer 2

还有 foreach 包：

library(foreach)
foreach(i = letters, j = LETTERS) %do% {
  paste(i,j)
}

[[1]]
[1] "a A"

[[2]]
[1] "b B"

[[3]]
[1] "c C"
etc...

这还具有易于并行化的优点：只需将 %do% 更改为 %dopar%，注册一个并行后端，然后摇滚。

Answer 3

通常，迭代多个变量（相同长度）最好使用单个索引来实现，该索引充当引用列表中元素（变量）的索引：

var_list <- list(
  var1 = 1:10,          # 1, ..., 10
  var2 = letters[17:26] # q, ..., z
)

for (i in 1:length(var_list$var1)) {

  # Process each of var1 and var2
  print(paste(
    'var1:', var_list$var1[i],
    '; var2:', var_list$var2[i]
  ))

}