purrr: element-wise length comparison of multiple lists?-CodePudding

I have three lists with 2 elements each. How do I check whether every element has the same length in every list? Preferably using purrr. Thank you!

list.a = list(a = 1, b = c(1, 2))
list.b = list(a = 2, b = c(1, 2))
list.c = list(a = 3, b = c(1, 2, 3))

Should return T, T, F.

CodePudding user response：

Not entirely sure on the requirement but here's 2 potentially usefully snippets.

map_lgl(transpose(list(list.a, list.b, list.c)), ~ var(lengths(.x))==0)

  a     b 
 TRUE FALSE

or for a more general output you can manipulate easier

map_dfr(list(list.a, list.b, list.c), ~map(.x, length))

      a     b
1     1     2
2     1     2
3     1     3

CodePudding user response：

For any two lists:

all(lengths(list.a)==lengths(list.b))

To check if all lists are equal:

same_length <- function (x, y) all(lengths(x) == lengths(y))
Reduce(f, list(list.a, list.b, list.c))

If you want to use purrr:

same_length <- function (x, y) all(lengths(x) == lengths(y))
purrr::reduce(list(list.a, list.b, list.c), f)

CodePudding user response：

each element has the same length in every list? - This would be a single TRUE or FALSE. Based on your expected output and task I think you want to compare for specific length of list elements.

master_list <- list(list.a, list.b, list.c)
map_lgl(master_list, ~ all(lengths(.x) == 1:2))
[1]  TRUE  TRUE FALSE