Head 和 Tail
Head(头) 代表List的第一个值，Tail(尾巴) 代表了List除第一个值以外构成的子列表。例如：
iex(1)> [ head | tail ] = [1, 2, 3]
#[1, 2, 3]
iex(2)> head
#1
iex(3)> tail
#[2, 3]
示例：利用递归计算list的length
defmodule MyList do
def len([]), do: 0
def len([_|tail]), do: 1 + len(tail)
end
#-----------------------------------------
$iex mylist.exs
iex(1)> MyList.len([])
#0
iex(2)> MyList.len([2,5,6])
#3
示例2：以递归的方式实现map,比如：返回当前list元素的平方
defmodule MyList do
def square([]), do: []
def square([h|t]), do: [h * h | square(t)]
end
构建Map函数
自定义map 函数
defmodule MyList do
def map([],_func), do: []
def map([h|t], func), do: [func.(h)|map(t, func)]
end
#----------------------
iex(1)> MyList.map([1,2,3],&(&1 * 3))
#[3, 6, 9]
Reduce 实现原理
List 求和函数第一种实现：
defmodule MyList do
def sum(list), do: _sum(list, 0)
defp _sum([], total), do: total
defp _sum([h|t],total), do: _sum(t,total+h)
end
iex(1)> MyList.sum([1,2,3])
#6
List 求和函数第二种实现：
defmodule MyList do
def sum([]), do: 0
def sum([h|t]), do: h + sum(t)
end
#------------------------------------------------
iex(1)> MyList.sum([1,2,3])
#6
自定义reduce函数
defmodule MyList do
def reduce([], value, _), do: value
def reduce([h|t], value, func), do: reduce(t, func.(h, value), func)
end
#------------------------------------------------
iex(1)> MyList.reduce([1,2,3,4,5], 1, &(&1 * &2))
#120
练习1：mapsum 函数，先做map再求和
defmodule MyList do
def mapsum([], _func), do: 0
def mapsum([h|t], func), do: func.(h) + mapsum(t, func)
end
#------------------------------------------------
iex(1)> MyList.mapsum [1, 2, 3], &(&1 * &1)
14
练习2：max函数求list中的最大值
defmodule MyList do
def max([]), do: "Empty List"
def max([a]), do: a
def max([h|t]) do
if h > max(t) do
h
else
max(t)
end
end
end
#------------------------------------------------
iex(1)> MyList.max([12,1,3,4,13,1,12,2])
#13
iex(2)> MyList.max([])
#"Empty List"
iex(3)> MyList.max([1])
#1
练习3： 过滤数组的数组
defmodule WeatherHistory do
def for_location_27([]), do: []
def for_location_27([ h = [_, 27, _, _ ] | tail]) do
[ h | for_location_27(tail) ]
end
def for_location_27([ _ | tail]), do: for_location_27(tail)
end
#------------------------------------------------
iex(1)> MyList.for_location_27([[1366225622, 26, 15, 0.125],[1366225622, 27, 15, 0.45],[1366225622, 28, 21, 0.25],[1366229222, 26, 19, 0.081],[1366229222, 27, 17, 0.468]])
#[[1366225622, 27, 15, 0.45], [1366229222, 27, 17, 0.468]]
练习4：给定一个范围，返回一个列表
MyList.span(1,5)
#[1,2,3,4,5]
defmodule MyList do
def span(from,from), do: [from]
def span(from,to), do: [from|span(from+1,to)]
end
List Module 官方文档 https://hexdocs.pm/elixir/List.html
