Looping over multiple lists with base R - r

Question

In python we can do this..
numbers = [1, 2, 3]
characters = ['foo', 'bar', 'baz']
for item in zip(numbers, characters):
print(item[0], item[1])
(1, 'foo')
(2, 'bar')
(3, 'baz')
We can also unpack the tuple rather than using the index.
for num, char in zip(numbers, characters):
print(num, char)
(1, 'foo')
(2, 'bar')
(3, 'baz')
How can we do the same using base R?

Answer 1

To do something like this in an R-native way, you'd use the idea of a data frame. A data frame has multiple variables which can be of different types, and each row is an observation of each variable.
d <- data.frame(numbers = c(1, 2, 3),
characters = c('foo', 'bar', 'baz'))
d
## numbers characters
## 1 1 foo
## 2 2 bar
## 3 3 baz
You then access each row using matrix notation, where leaving an index blank includes everything.
d[1,]
## numbers characters
## 1 1 foo
You can then loop over the rows of the data frame to do whatever you want to do, presumably you actually want to do something more interesting than printing.
for(i in seq_len(nrow(d))) {
print(d[i,])
}
## numbers characters
## 1 1 foo
## numbers characters
## 2 2 bar
## numbers characters
## 3 3 baz

Answer 2

For another option, how about mapply, which is the closest analog to zip I can think of in R. Here I'm using the c function to make a new vector, but you could use any function you'd like:
numbers<- c(1, 2, 3)
characters<- c('foo', 'bar', 'baz')
mapply(c,numbers, characters, SIMPLIFY = FALSE)
[[1]]
[1] "1" "foo"
[[2]]
[1] "2" "bar"
[[3]]
[1] "3" "baz"
Which way is of most use depends on what you want to do with your output, but as the other answers mention, a dataframe is the most natural approach in R (and pandas dataframe probably in python).

Answer 3

To index a vector in R, where the vector is variable x would be x[1]. This would return the first element of the vector. R element numbering starts at 1 in contrast to Python which starts at 0.
For this problem it would be:
x = seq(1,10)
j = seq(11,20)
for (i in 1:length(x)){
print (c(x[i],j[i]))
}

Answer 4

Many functions in R are vectorized and don't require loops:
numbers = c(1, 2, 3)
characters = c('foo', 'bar', 'baz')
myList <- list(numbers, characters)
myDF <- data.frame(numbers,characters, stringsAsFactors = F)
print(myList)
print(myDF)

Answer 5

This is the conceptual equivalent:
for (item in Map(list,numbers,characters)){ # though most of the time you would actually do all your work inside Map
print(item[c(1,2)])
}
# [[1]]
# [1] 1
#
# [[2]]
# [1] "a"
#
# [[1]]
# [1] 2
#
# [[2]]
# [1] "b"
#
# [[1]]
# [1] 3
#
# [[2]]
# [1] "c"
#
# [[1]]
# [1] 4
#
# [[2]]
# [1] "d"
#
# [[1]]
# [1] 5
#
# [[2]]
# [1] "e"
Though most of the time you would actually do all your work inside Map and do something like this:
Map(function(nu,ch){print(data.frame(nu,ch))},numbers,characters)
This is the closest I could get to a clone:
zip <- function(...){ Map(list,...)}
print2 <- function(...){do.call(cat,c(list(...),"\n"))}
for (item in zip(numbers,characters)){
print2(item[[1]],item[[2]])
}
# 1 a
# 2 b
# 3 c
# 4 d
# 5 e
to be able to call items by their names (still works with indices):
zip <- function(...){
names <- sapply(substitute(list(...))[-1],deparse)
Map(function(...){setNames(list(...),names)}, ...)
}
for (item in zip(numbers,characters)){
print2(item[["numbers"]],item[["characters"]])
}

Answer 6

The tidyverse solution would be to use purrr::map2 function. Ex:
numbers <- c(1, 2, 3)
characters <- c('foo', 'bar', 'baz')
map2(numbers, characters, ~paste0(.x, ',', .y))
#[[1]]
#[1] "1,foo"
#[[2]]
#[1] "2,bar"
#[[3]]
#[1] "3,baz"
See API here

Answer 7

Other scalable alternatives: Store the vectors in the list and iterate over.
vect1 <- c(1, 2, 3)
vect1 <- c('foo', 'bar', 'baz')
vect2 <- c('a', 'b', 'c')
idx_list <- list(vect1, vect2)
idx_vect <- c(1:length(idx_list[[1]]))
for(i in idx_vect){
x <- idx_list[[1]][i]
j <- idx_list[[2]][i]
print(c(i, x, j))
}