Using lapply function within another one avoiding list of lists

Question

Supposing an example in R where we use the lapply function within a same function to create an one-level list:

lapply(1:5, function(x)
{
  r <- sample(4,1)

  i<-lapply(1:r, function (y)
  {
    matrix(rep(1*x*y,3))
  })
  i
})

The given result is a two-level list (list of lists) like this one:

[[1]]
[[1]][[1]]
     [,1]
[1,]    1
[2,]    1
[3,]    1


[[2]]
[[2]][[1]]
     [,1]
[1,]    2
[2,]    2
[3,]    2

[[2]][[2]]
     [,1]
[1,]    4
[2,]    4
[3,]    4

[[2]][[3]]
     [,1]
[1,]    6
[2,]    6
[3,]    6

[[2]][[4]]
     [,1]
[1,]    8
[2,]    8
[3,]    8


[[3]]
[[3]][[1]]
     [,1]
[1,]    3
[2,]    3
[3,]    3

[[3]][[2]]
     [,1]
[1,]    6
[2,]    6
[3,]    6


# ...

Preserving both lapply functions (or similar ones) and matrices with equal dimensions, how we could get an one-level list thus?

[[1]]
     [,1]
[1,]    1
[2,]    1
[3,]    1

[[2]]
     [,1]
[1,]    2
[2,]    2
[3,]    2

[[3]]
     [,1]
[1,]    4
[2,]    4
[3,]    4

# ...

lapply always returns a list. (That's what the l in the name stands for.) Use sapply instead to simplify. — Gregor Thomas
– Gregor Thomas, Commented Jan 10, 2018 at 17:12
If j is the object returned by your code, you may try unlist(j,recursive=FALSE). — nicola
– nicola, Commented Jan 10, 2018 at 17:17
@Gregor, your suggestion doen't keep matrices with the same dimensions. — Nelson A. Morais
– Nelson A. Morais, Commented Jan 10, 2018 at 18:19

Brigadeiro · Accepted Answer · 2018-01-10 17:45:57Z

2

Replace the second lapply with sapply and include the argument simplify = TRUE

answered Jan 10, 2018 at 17:45

Brigadeiro

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3 Comments

Gregor Thomas Over a year ago

simplify = TRUE is the default for sapply, you don't need to include it. And to make this a better answer you should show the full code. (I'm not sure this actually duplicates OP's desired output).

wibeasley Over a year ago

I think that pushes all those nested 1-column matrices in to a wider matrix. I think OP wanted to keep the matrices separate, but just reducing the nesting layers.

Nelson A. Morais Over a year ago

That change doesn't preserve the dimensions of the matrices, like @wibeasley said.

wibeasley · Accepted Answer · 2018-01-10 17:49:02Z

1

I was curious how purrr can best accommodate matrices inside a list. The concluding flatten() line is the equivalent of @nicola's suggestion to use unlist(., recursive=F).

library(magrittr)
purrr::map(1:5, function(x) {
  r <- sample(4,1)

  purrr::map(1:r, function(y) {
    matrix(rep(1*x*y,3))
  }) 
}) %>% 
purrr::flatten() #This line is the equivalent of nicola's `unlist()`

answered Jan 10, 2018 at 17:49

wibeasley

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