Package provides Python-style list comprehensions for R. List comprehension expressions use usual loops (for, while and repeat) and usual if as list producers. Syntax is very similar to Python. The difference is that returned value should be at the end of the loop body.
Rather unpractical example - squares of even numbers:
Pythagorean triples:
to_list(for (x in 1:20) for (y in x:20) for (z in y:20) if (x^2 + y^2 == z^2) c(x, y, z))
#> [[1]]
#> [1] 3 4 5
#>
#> [[2]]
#> [1] 5 12 13
#>
#> [[3]]
#> [1] 6 8 10
#>
#> [[4]]
#> [1] 8 15 17
#>
#> [[5]]
#> [1] 9 12 15
#>
#> [[6]]
#> [1] 12 16 20More examples:
colours = c("red", "green", "yellow", "blue")
things = c("house", "car", "tree")
to_vec(for(x in colours) for(y in things) paste(x, y))
#> [1] "red house" "red car" "red tree" "green house"
#> [5] "green car" "green tree" "yellow house" "yellow car"
#> [9] "yellow tree" "blue house" "blue car" "blue tree"
# prime numbers
noprimes = to_vec(for (i in 2:7) for (j in seq(i*2, 99, i)) j)
primes = to_vec(for (x in 2:99) if(!x %in% noprimes) x)
primes
#> [1] 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83
#> [24] 89 97You can iterate over multiple lists if you provide several loop variables in backticks:
to_vec(for(`i, j` in numerate(letters)) if(i %% 2==0) paste(i, j))
#> [1] "2 b" "4 d" "6 f" "8 h" "10 j" "12 l" "14 n" "16 p" "18 r" "20 t"
#> [11] "22 v" "24 x" "26 z"
set.seed(123)
rand_sequence = runif(20)
# gives only locally increasing values
to_vec(for(`i, j` in lag_list(rand_sequence)) if(j>i) j)
#> [1] 0.7883051 0.8830174 0.9404673 0.5281055 0.8924190 0.9568333 0.6775706
#> [8] 0.8998250 0.3279207 0.9545036