rlist question - returns a blank list

[PeterHui]

Is there a reason why this returns list() but contains nothing in it??

library(rlist)
library(tidyverse)

data(chickwts)

list(chickwts) %>%
list.select(feed,weight) %>%
list.filter(.$weight<=200)

[AlexisW]

list() means an empty list: nothing matched the condition.

If I understand correctly, list.filter() can only test a condition on an entire element, not on the vector inside this element:

y <- list(list(weight=1))
list.filter(y, weight == 1)
# [[1]]
# [[1]]$weight
# [1] 1
# 

y <- list(list(weight=1:2))
list.filter(y, weight == 1)
# list()

To do processing inside the element, you need list.map(). Here are two versions of what you want:

list(chickwts) %>%
  list.select(feed, weight) %>%
  list.map(Filter(function(x){x <= 200}, weight))
# [[1]]
#  [1] 179 160 136 168 108 124 143 140 181 141 148 169 193 199 171 158 153

list(chickwts) %>%
  list.select(feed, weight) %>%
  list.map(weight[weight <= 200])
# [[1]]
#  [1] 179 160 136 168 108 124 143 140 181 141 148 169 193 199 171 158 153

[PeterHui]

Thank you! this is very helpful