Find the maximum values of a variable in a list - r

Question

I have a list of the following structure & I intend to find the maximum value of X2 in the second variable (b) in the list
sample data
[[1]]
[[1]]$a
[1] 2
[[1]]$b
X1 X2
1 58 1686729
2 106 1682303
[[2]]
[[2]]$a
[1] 3
[[2]]$b
X1 X2
1 24 1642468
2 89 1695581
3 156 1634019
I looked into multiple filters that can be applied to the list like:
library(rlist)
list.filter(result, max(b$area))
and also tried lapply but with no success
lapply(result, function(x) x[which.max(x$b)])
I need the following output:
a x1 x2
2 58 1686729
3 89 1695581

Answer 1

With lapply() you can find the max of X2 in $b in each list, then cbind() with the a element.
l_max <- lapply(l, function(x) {
b <- x$b
cbind(a=x$a, b[which.max(b$X2),])
})
Use bind_rows() from dplyr for binding together.
l_max %>%
dplyr::bind_rows()
# a X1 X2
# 1 2 58 1686729
# 2 3 89 1695581
Example data:
l <- list(
list(a = 2,
b = data.frame(X1 = c(58, 106), X2 = c(1686729, 1682303))),
list(a = 3,
b = data.frame(X1 = c(24, 89,156), X2 = c(1642468, 1695581,1634019)))
)
With your example:
l_max <- lapply(l, function(x) {
b <- x$b
cbind(a = x$a, b[which.max(b[,2]),]) # NOTICE I used [,2] to refer to the second column
#b$area works too if all df share the col name
})
l_max %>%
dplyr::bind_rows()
# a rt area
# 1 2 58 1686729
# 2 3 89 1695581
# 3 4 101 1679889
# 4 5 88 1695983
# 5 6 105 1706445
# 6 7 121 1702019
Another solution with purrr::map_df() avoids the use of bind_rows():
purrr::map_df(l, function(x) {
b <- x$b
cbind(a = x$a, b[which.max(b[,2]),])
})
All base R using mapply():
t(mapply(function(x) {
b <- x$b
cbind(a = x$a, b[which.max(b[,2]),])
}, l))
Or with Map():
do.call("rbind", Map(function(x) {
b <- x$b
cbind(a = x$a, b[which.max(b[,2]),])
}, l))

Answer 2

You can also use sapply():
t(sapply(list, function(elem){
c(a = elem$a, elem$b[which.max(elem$b$area), ])
}))