(Don't read the title, which belongs to the categories how to read a book and remember everything / how to learn a programming language in three days / how to loose weight by doing this five minutes per day... What follows is a seed. It needs you and your effort to grow. With diligence and luck, you might thank me for the flower later...)

Here is a glimpse into functional programming in R for the future me.

Filter

Filter selects a subset of a sequence based on a predicate.

``````ll <- list(1, 2, "a", "b")
Filter(is.numeric, ll)
# [[1]]
# [1] 1
#
# [[2]]
# [1] 2``````

The equivalent function in the purrr package is keep:

``````purrr::keep(.x = ll, .p = is.numeric)
# [[1]]
# [1] 1
#
# [[2]]
# [1] 2``````

Here two examples from the book Functional Programming in R by Thomas Mailund:

``````is_even <- function(x){
x %% 2 == 0
}
unlist(Filter(is_even, 1:10))
#[1]  2  4  6  8 10``````

Filtering is more powerful with closures:

``````larger_than <- function(x){
function(y){
y > x
}
}
larger_than(1:10)(3)
#TRUE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE

`%>%` <- magrittr::`%>%`
Filter(larger_than(3), 1:10) %>%
unlist(.)
#[1]  4  5  6  7  8  9 10``````

Negate

Negate negates a predicate function.

``````isTRUE(TRUE)
# TRUE

Negate(isTRUE)(TRUE)
# FALSE

ll <- list(1, 2, "a", "b")
Filter(f = Negate(is.numeric), x = ll)
# [[1]]
# [1] "a"
#
# [[2]]
# [1] "b"

larger_than <- function(x){
function(y){
y > x
}
}
larger_than(1:10)(3)
# TRUE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE

Negate(larger_than(1:10))(3)
# FALSE FALSE  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE

`%>%` <- magrittr::`%>%`
Filter(larger_than(3), 1:10) %>%
unlist(.)
#[1]  4  5  6  7  8  9 10
Filter(Negate(larger_than(3)), 1:10) %>%
unlist(.)
# [1] 1 2 3``````

The equivalent function in the purrr package is discard:

``````ll <- list(1, 2, "a", "b")
purrr::discard(.x = ll, .p = is.numeric)
# [[1]]
# [1] "a"
#
# [[2]]
# [1] "b"``````

Map

The Map function evaluates a function for each element in a vector or list and returns a vector or list with the results:

``````is_even <- function(x){
x %% 2 == 0
}
`%>%` <- magrittr::`%>%`
Map(is_even, 1:5) %>%
unlist(.)
# [1] FALSE  TRUE FALSE  TRUE FALSE  TRUE FALSE  TRUE FALSE  TRUE

Map(isTRUE, list(FALSE, TRUE, NULL, NA, "", 1)) %>%
unlist(.)
# FALSE  TRUE FALSE FALSE FALSE FALSE

purrr::map(.x = 1:5, .f = is_even) %>%
unlist(.)
#[1] FALSE  TRUE FALSE  TRUE FALSE``````

Similar to the Filter function, Map can be combined with closures:

``````add <- function(x) {
function(y) {
x + y
}
}
unlist(.)
# 4  5  6  7  8  9 10 11 12 13

purrr::map(.x = 1:10, .f = add, y = 3) %>%
unlist(.)
[1]  4  5  6  7  8  9 10 11 12 13``````

One can pass named arguments to a Map call via a list provided to the MoreArgs parameter:

``````add <- function(x, y) {
x + y
}
Map(f = add, 1:10, MoreArgs = list(y = 3)) %>%
unlist(.)
# [1]  4  5  6  7  8  9 10 11 12 13``````

If you want to loop over multiple inputs, use purrr::map2:

``````purrr::map2(.x = 1:10, .y = 3, .f = `+`) %>%
unlist(.)
[1]  4  5  6  7  8  9 10 11 12 13``````

Applications with data.table

``````dt <- data.table::data.table(
market = letters[1:3],
symbol = LETTERS[1:3],
price = 10:12,
ts = bit64::as.integer64(1:3)
)
dt
#    market symbol price ts
# 1:      a      A    10  1
# 2:      b      B    11  2
# 3:      c      C    12  3

Filter(is.numeric, dt)
#    price ts
# 1:    10  1
# 2:    11  2
# 3:    12  3
purrr::keep(.x = dt, .p = is.numeric)
#    price ts
# 1:    10  1
# 2:    11  2
# 3:    12  3

Filter(Negate(is.numeric), dt)
#    market symbol
# 1:      a      A
# 2:      b      B
# 3:      c      C
purrr::discard(.x = dt, .p = is.numeric)
#   market symbol
# 1:      a      A
# 2:      b      B
# 3:      c      C``````

Convert columns of type integer64 to numeric:

``````str(dt)
# Classes ‘data.table’ and 'data.frame': 3 obs. of  4 variables:
# \$ market: chr  "a" "b" "c"
# \$ symbol: chr  "A" "B" "C"
# \$ price : int  10 11 12
# \$ ts    :integer64 1 2 3

cols_to_convert <- names(Filter(bit64::is.integer64, dt))
dt[, (cols_to_convert) := lapply(.SD, as.numeric), .SDcols = cols_to_convert]
str(dt)
# Classes ‘data.table’ and 'data.frame': 3 obs. of  4 variables:
# \$ market: chr  "a" "b" "c"
# \$ symbol: chr  "A" "B" "C"
# \$ price : int  10 11 12
# \$ ts    : num  1 2 3``````

Create a column based on other columns:

``````dt[, id := Reduce(x = .SD, f = paste), .SDcols = c("market", "symbol")]
str(dt)
# Classes ‘data.table’ and 'data.frame': 3 obs. of  5 variables:
# \$ market: chr  "a" "b" "c"
# \$ symbol: chr  "A" "B" "C"
# \$ price : int  10 11 12
# \$ ts    : num  1 2 3
# \$ id    : chr  "a A" "b B" "c C"

my_paste <- function(...){
paste(..., sep = "@")
}
dt[, id2 := Reduce(x = .SD, f = my_paste), .SDcols = c("market", "symbol")]
dt
#    market symbol price ts  id id2
# 1:      a      A    10  1 a A a@A
# 2:      b      B    11  2 b B b@B
# 3:      c      C    12  3 c C c@C

dt[
, id3 := purrr::reduce(.x = .SD, .f = paste, sep = "@"),
.SDcols = c("market", "symbol")
]
dt
#    market symbol price ts  id id2 id3
# 1:      a      A    10  1 a A a@A a@A
# 2:      b      B    11  2 b B b@B b@B
# 3:      c      C    12  3 c C c@C c@C``````

What if you want to use only base function and not the purrr package, but you still want to be able to freely select the "sep" argument? The base Reduce function does not offer the possibility to give any arguments to the argument ".f".

However, we can use closures:

``````# this does not work:
my_paste_2 <- function(..., sep) {
paste(..., sep = sep)
}

# but this does:
my_paste_3 <- function(..., sep){
function(...){
paste(..., sep = sep)
}
}
dt[
, id4 := Reduce(x = .SD, f = my_paste_3(sep = "|")),
.SDcols = c("market", "symbol")
]
dt
#    market symbol price ts  id id2 id3 id4
# 1:      a      A    10  1 a A a@A a@A a|A
# 2:      b      B    11  2 b B b@B b@B b|B
# 3:      c      C    12  3 c C c@C c@C c|C``````

Too much?