julia_functions.jl

# functions

# 基本形
function f(x, y)
    x^2 + y^2
end

f(3, 4)

# 勿論unicodeも使える
function µ(x)
    y = sum(x) / length(x)
    return x # returnで指定した文字列を強制的に吐き出す
    x = sum(x) # それ以降で値を変更しても無駄
end

µ([1, 2, 3, 4, 5, 6])
# more terse syntax
g(x) = sum(x) / length(x)
g([1, 2, 3, 4, 5, 6])
# specify return type
h(x)::Int8 = sum(x) / length(x) # function~の書き出しでも使用できる。
h([1, 2, 3])

# anonymus function + map()
map(x -> sqrt(x), [1, 2, 3, 4, 5, 6, 7, 8, 9])
map(x -> sqrt(x), [1, 2, 3, 4, 5, 6, 7, 8, 0])
# Use do function, is equivalent above example
map([1:1:9;]) do x
    sqrt(x)
end

# tuples: a built-in data structure that can hold any value, but cannot be modified.
# It is constructer with commas and parentheses, and can be accessed by indexing.
x = ("1", 1, "abc", [1, 2, 3, 4])
x[4]
x[[1, 3]]
(1,) # this tuples length is 1
() # length-0 tuples
# named tuples
y = (a = 1, b = "1", c = f(x) = x^2)
y.c(4) #tuplesから呼び出し。なんと関数もはいっちゃう。
1, 2 # tuples without parentheses
x[1] = 1 # immutable

# Varags Function: variable length function
bar(a, b, x...) = (a, b, x)
bar(1, 2, 3, 4, 5)
bar(1, 2, [1, 2, 3, 4, 5]...)
bar(1, 2, [1, 2, 3, 4, 5]) # not same
x = [1, 2]
bar(1, 2, x...)
# Optional Arguments
#using Dates
#function Date(y::Int64, m::Int64=1, d::Int64=1)
    # err = Dates.validargs(Date, y, m, d)
    #Dates パッケージの提供する関数のようだが，使い方は不明
    # err === nothing || throw(err)
    #return Date(UTD(totaldays(y, m, d)))
#end
#Date(2000, 12, 12)

# Do-Block Syntax
map(x -> begin
    if x < 0 && iseven(x) # 負，かつ偶数ならば
        return 0
    elseif x == 0
        return 1
    else
        return x
    end
end, [-2, -1, 0, 1, 2])
# 同じことがdoをもちいても書ける
map([-2, -1, 0, 1, 2]) do x
    if x < 0 && iseven(x)
        return 0
    elseif x == 0
        return 1
    else
        return x
    end
end

# Dot Syntax
# f.() is elementwise
A = [1, 2, 3, 4, 5]
sin(A)
sin.(A)
map(sin, A) # これと同じ意味
# effective syntax
Y = [1.0, 2.0, 3.0, 4.0]; # ちなみに;を最後につければ，結果がよこに表示されないようだ。
# pre-allocate output array
X = similar(Y); # similar(): create an uninitialized mutable array
X = sin.(cos.(Y))
@. X = sin(cos(Y)) # いちいち.を書かなくてもOK
broadcast(*, 2.0, (2.0, 3.0))# これでもOK