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Question

How do I reduce a list of lists to only contain the values that live up to a certain criteria? I've seen similar questions, and looked at Filter , Reduce , etc. but can't seem to make anything work.

> h = list(A=c(1, 2, 3), B=c(2, 3, 4), C=c(0, 1, 2))
> h
$A
[1] 1 2 3

$B
[1] 2 3 4

$C
[1] 0 1 2 

Desired output:

> MagicFunction(h, element >= 3)
$A
[1] 3

$B
[1] 3 4

Answer 1

You can try,

Filter(length, lapply(h, function(i)i[i>=3]))

#$A
#[1] 3

#$B
#[1] 3 4

If you have elements with NA, then you can use na.omit to remove those, .ie

h = list(A=c(1, 2, 3), B=c(2, 3, 4), C=c(0, 1, 2), D=NA)
Filter(length, lapply(h, function(i)na.omit(i[i>=3])))

#$A
#[1] 3

#$B
#[1] 3 4

An alternative would be,

h = list(A=c(1, 2, 3), B=c(2, 3, 4), C=c(0, 1, 2), D=c(NA, 7))
Filter(length, lapply(h, function(i)(i[i >= 3 & !is.na(i)])))

#$A
#[1] 3

#$B
#[1] 3 4

#$D
#[1] 7

Answer 2

You can also do this with purrr and map . map applies the function across groups, then discard(is_empty) removes lists that contain no values (ie that were filtered out by the function). ~ is used to specify the function that is to be applied to each list and . is a placeholder for each list passing through the function.

library(purrr)

map(h, ~.[. >= 3]) %>% discard(is_empty)

$A
[1] 3

$B
[1] 3 4