How to delete specific elements in a list - r

I have a list:
aa <- list(c(12, 1, 5, 4), c(2, 3, 5), c(6, 4,1))
aa
#[[1]]
#[1] 12 1 5 4
#[[2]]
#[1] 2 3 5
#[[3]]
#[1] 6 4 1
I would like to delete all "4" and "5" in aa and be able to return the following list:
#[[1]]
#[1] 12 1
#[[2]]
#[1] 2 3
#[[3]]
#[1] 6 1
So is there a simple way of doing this? Thank you.


Here you have a simple solution using lapply:
lapply(aa, function(x) {x[!(x %in% c(4, 5))]})
Output:
[[1]]
[1] 12 1
[[2]]
[1] 2 3
[[3]]
[1] 6 1

Related

R arrange list of vectors

Say I have a list of vectors like this:
list.of.vec <- data.frame(id = c(1,2,3,4)) %>%
mutate(a = list(list(c(1,2,3), c(2,3,4)), list(c(2,4,5), c(1,3,4)), list(c(1,2,3), c(4,3,4)), list(c(3,2,3), c(2,3,4))))
how can I arrange a such that in each pair of vectors, the one with the bigger first values goes first? E.g., if there was a list c(1,2,3), c(2,3,4), it should become c(2,3,4), c(1,2,3). And if first values are equal then it looks at the next value.

We can use rrapply
library(rrapply)
library(purrr)
library(dplyr)
list.of.vec <- list.of.vec %>%
mutate(a = map(a, ~ .x[order(-rrapply(.x, f = function(x) x[1],
how = 'unlist'))]))
-output
> list.of.vec
id a
1 1 2, 3, 4, 1, 2, 3
2 2 2, 4, 5, 1, 3, 4
3 3 4, 3, 4, 1, 2, 3
4 4 3, 2, 3, 2, 3, 4
> list.of.vec$a
[[1]]
[[1]][[1]]
[1] 2 3 4
[[1]][[2]]
[1] 1 2 3
[[2]]
[[2]][[1]]
[1] 2 4 5
[[2]][[2]]
[1] 1 3 4
[[3]]
[[3]][[1]]
[1] 4 3 4
[[3]][[2]]
[1] 1 2 3
[[4]]
[[4]][[1]]
[1] 3 2 3
[[4]][[2]]
[1] 2 3 4

You can use -
list.of.vec$a <- lapply(list.of.vec$a, function(x) {
inds <- match(TRUE, x[[1]] != x[[2]])
if(x[[1]][inds] > x[[2]][inds]) x else rev(x)
})
list.of.vec$a
#[[1]]
#[[1]][[1]]
#[1] 2 3 4
#[[1]][[2]]
#[1] 1 2 3
#[[2]]
#[[2]][[1]]
#[1] 2 4 5
#[[2]][[2]]
#[1] 1 3 4
#[[3]]
#[[3]][[1]]
#[1] 4 3 4
#[[3]][[2]]
#[1] 1 2 3
#[[4]]
#[[4]][[1]]
#[1] 3 2 3
#[[4]][[2]]
#[1] 2 3 4
inds would return the index for each list that we need to compare, an index where the value is different in both the vectors. If the first index has a higher value we return the list as it is without any change or else we return the reversed list.

We can try
lapply(
a,
function(x) {
x[order(-sapply(x, `[`, 1))]
}
)
which gives
[[1]]
[[1]][[1]]
[1] 2 3 4
[[1]][[2]]
[1] 1 2 3
[[2]]
[[2]][[1]]
[1] 2 4 5
[[2]][[2]]
[1] 1 3 4
[[3]]
[[3]][[1]]
[1] 4 3 4
[[3]][[2]]
[1] 1 2 3
[[4]]
[[4]][[1]]
[1] 3 2 3
[[4]][[2]]
[1] 2 3 4

Convert each row of dataframe to new list in R

I have below sample input data-
> df <- data.frame(a=c(1,2,9),b=c(3,4,5),c=c(2,6,7))
> df
a b c
1 1 3 2
2 2 4 6
3 9 5 7
I am trying to convert rach row into separate list.
My Attempt-
> apply(df,1,as.list)
The above solution converts each row into sublists. But, I am looking for 3 separate list in this case.
nrow(df) = no. of lists
Desired Output-
> list1
$a
[1] 1
$b
[1] 3
$c
[1] 2
> list2
$a
[1] 2
$b
[1] 4
$c
[1] 6
> list3
$a
[1] 9
$b
[1] 5
$c
[1] 7

You can use by and as.list
out <- by(df, 1:nrow(df), as.list)
out
#1:nrow(df): 1
#$a
#[1] 1
#
#$b
#[1] 3
#$c
#[1] 2
#------------------------------------------------------------------------------
#1:nrow(df): 2
#$a
#[1] 2
#$b
#[1] 4
#$c
#[1] 6
#------------------------------------------------------------------------------
#1:nrow(df): 3
#$a
#[1] 9
#$b
#[1] 5
#$c
#[1] 7
That creates an object of class by. So you may call unclass(out) in the end.

Convert a vector into a specialised list efficiently

I'm looking for a more efficient way to get from my current input to my expected output.
Input
vec <- 1:4
Expected output
[[1]]
[1] 1 2 3 4
[[2]]
[1] 1
[[3]]
[1] 2
[[4]]
[1] 3
[[5]]
[1] 4
Current solution:
lis <- list()
lis[2:5] <- as.list(vec)
lis[[1]] <- vec

We can do
c(list(vec), as.list(vec))
#[[1]]
#[1] 1 2 3 4
#[[2]]
#[1] 1
#[[3]]
#[1] 2
#[[4]]
#[1] 3
#[[5]]
#[1] 4

Indexing table in [R]

I have a vector:
a = c(1, 3, 2, 2, 4, 1, 1, NA)
I want to generate something like:
[["1"]]
1, 6, 7
[["2"]]
3, 4
[["3"]]
2
[["5"]]
5
Mapping from value to index. Any help!

This is a split operation on the indexes of a:
split(seq_along(a), a)
#$`1`
#[1] 1 6 7
#
#$`2`
#[1] 3 4
#
#$`3`
#[1] 2
#
#$`4`
#[1] 5

setNames(lapply(unique(a), function(x) which(x == a)), unique(a))
# $`1`
# [1] 1 6 7
#
# $`3`
# [1] 2
#
# $`2`
# [1] 3 4
#
# $`4`
# [1] 5
#
# $<NA>
# integer(0)
This is slightly shorter using tidyverse
library(tidyverse)
map(unique(a) %>% setNames(.,.), ~which(.x == a))

Here is another option with unstack
unstack(list(i = seq_along(a), a), i ~ a)
#$`1`
#[1] 1 6 7
#$`2`
#[1] 3 4
#$`3`
#[1] 2
#$`4`
#[1] 5

Reverse sort list by max element

I have a list of vectors
l = list(c(1,2),c(3,4),c(2,3),c(7,8),c(5,6))
and would to reverse sort it by the vector maximums:
> l
[[1]]
[1] 7 8
[[2]]
[1] 5 6
[[3]]
[1] 3 4
[[4]]
[1] 2 3
[[5]]
[1] 1 2
Any idea how I could do this in a one liner? thx

One way is
l[order(sapply(l, max), decreasing=TRUE)]
#[[1]]
#[1] 7 8
#[[2]]
#[1] 5 6
#[[3]]
#[1] 3 4
#[[4]]
#[1] 2 3
#[[5]]
#[1] 1 2
You could replace sapply(l, max) with vapply(l, max, numeric(1L)) as well.
Or a compact form suggested by #DavidArenburg
l[order(-sapply(l, max))]

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