Sample rows of a list of matrices based on numbers from another list

Question

I have a list of matrices from which I want to randomly draw rows based on the numbers of another list. Here is the list of matrices:

x <- list(`1` = matrix(1:20, nrow=10), `2` = matrix(1:20, nrow=10))

Here is the list of numbers

y <- list(`1` = 2, `2` = 3) #for `1` I want to draw 2 rows and for `2` I want to draw 3 rows

The final list will look like this:

$`1`
      [,1] [,2]
 [1,]    1   11
 [2,]    6   16

$`1`
      [,1] [,2]
 [1,]    1   11
 [2,]    7   17 
 [3,]    9   19

How to achieve this in base R? Thanks for any help!

Answer 1

We can use Map in base R - loop over the corresponding list elements of 'x' and 'y' sample the rows of matrixes in 'x' based on the values in 'y'

Map(function(u, v) u[sample(seq_len(nrow(u)), v),], x, y)
$`1`
     [,1] [,2]
[1,]    9   19
[2,]    6   16

$`2`
     [,1] [,2]
[1,]    3   13
[2,]    8   18
[3,]    5   15

---

Or use map2 from purrr

library(purrr)
map2(x, y,  ~ .x[sample(seq_len(nrow(.x)), .y), ])

If we convert to tibble, then slice_sample can be used as well

library(dplyr)
library(tibble)
map2(x, y,  ~ .x %>% 
   as.data.frame %>%
   as_tibble %>% 
   slice_sample(n = .y))
$`1`
# A tibble: 2 × 2
     V1    V2
  <int> <int>
1     4    14
2     7    17

$`2`
# A tibble: 3 × 2
     V1    V2
  <int> <int>
1     8    18
2     6    16
3     9    19