高阶函数 #

map(x -> x^2, [1, 2, 3, 4, 5])
map(uppercase, ["hello", "world"])

map((x, y) -> x + y, [1, 2, 3], [10, 20, 30])
map(*, [1, 2, 3], [4, 5, 6])

arr = [1, 2, 3, 4, 5]
map(x -> x^2, arr)
arr .^ 2

map(sqrt, arr)
sqrt.(arr)

arr = [1, 2, 3, 4, 5]
map!(x -> x^2, arr, arr)
arr

filter(x -> x > 2, [1, 2, 3, 4, 5])
filter(iseven, 1:20)
filter(x -> length(x) > 3, ["a", "ab", "abc", "abcd"])

d = Dict("a" => 1, "b" => 2, "c" => 3, "d" => 4)
filter(p -> p.second > 2, d)
filter(p -> p.first in ["a", "b"], d)

arr = [1, 2, 3, 4, 5]
filter!(x -> x > 2, arr)
arr

reduce(+, [1, 2, 3, 4, 5])
reduce(*, [1, 2, 3, 4, 5])
reduce(max, [3, 1, 4, 1, 5, 9, 2, 6])

reduce(+, [1, 2, 3], init=100)
reduce(*, [], init=1)

reduce((acc, x) -> acc * x + 1, [1, 2, 3, 4])
reduce((acc, x) -> push!(acc, x^2), [1, 2, 3, 4], init=Int[])

foldl(-, [1, 2, 3])
foldl((a, b) -> a - b, [1, 2, 3])

foldr(-, [1, 2, 3])
foldr((a, b) -> a - b, [1, 2, 3])

foldl(/, [1, 2, 3, 4])
foldr(/, [1, 2, 3, 4])

cumsum([1, 2, 3, 4, 5])

cumprod([1, 2, 3, 4, 5])

accumulate(+, [1, 2, 3, 4, 5])
accumulate(*, [1, 2, 3, 4, 5])
accumulate(max, [3, 1, 4, 1, 5, 9, 2, 6])

any(x -> x > 3, [1, 2, 3, 4, 5])
all(x -> x > 0, [1, 2, 3, 4, 5])

findfirst(x -> x > 2, [1, 2, 3, 4, 5])
findall(x -> x % 2 == 0, [1, 2, 3, 4, 5, 6])

count(x -> x > 2, [1, 2, 3, 4, 5])
count(iseven, 1:20)

foreach(println, [1, 2, 3, 4, 5])
foreach(x -> println(x^2), [1, 2, 3, 4, 5])

double(x) = 2x
increment(x) = x + 1

double_then_increment = increment ∘ double
double_then_increment(5)

[1, 2, 3, 4, 5] |> (x -> filter(iseven, x)) |> (x -> map(y -> y^2, x))

using Pipe

@pipe [1, 2, 3, 4, 5] |> filter(iseven, _) |> map(x -> x^2, _)

function process_data(data)
    data |>
    (x -> filter(y -> y > 0, x)) |>
    (x -> map(y -> y^2, x)) |>
    (x -> reduce(+, x))
end

process_data([1, -2, 3, -4, 5])

function word_stats(text)
    words = split(lowercase(text))
    (
        total = length(words),
        unique = length(unique(words)),
        avg_length = reduce(+, length.(words)) / length(words)
    )
end

word_stats("Hello world hello Julia")

function matrix_stats(mat)
    rows = mapslices(sum, mat, dims=2)
    cols = mapslices(sum, mat, dims=1)
    (row_sums=vec(rows), col_sums=vec(cols), total=sum(mat))
end

mat = [1 2 3; 4 5 6; 7 8 9]
matrix_stats(mat)

function group_reduce(arr, key_func, value_func, reduce_func)
    groups = Dict()
    for x in arr
        key = key_func(x)
        value = value_func(x)
        if !haskey(groups, key)
            groups[key] = []
        end
        push!(groups[key], value)
    end
    Dict(k => reduce_func(v) for (k, v) in groups)
end

data = [(type="A", value=1), (type="A", value=2), (type="B", value=3)]
group_reduce(data, x -> x.type, x -> x.value, sum)