lua程序设计（第四版）练习答案自做（第二十章）

文章目录

• • • 20.1
    • 20.2
    • 20.3
    • 20.4
    • 20.5

仓库

20.1

#!/usr/bin/lua
local Set={}
local mt={}
function Set.new(l)
	local set={}
	setmetatable(set,mt)
	for _,v in pairs(l) do
		set[v]=true
	end
	return set
end
function Set.union(a,b)
	if getmetatable(a)~=mt or getmetatable(b)~=mt then
		error("attempt to 'add' a set with a non-set value",2)
	end
	local res=Set.new{}
	for k in pairs(a) do
		res[k]=true
	end
	for k in pairs(b) do
		res[k]=true
	end
	return res
end
function Set.intersection(a,b)
	if getmetatable(a)~=mt or getmetatable(b)~=mt then
		error("attempt to 'multiply' a set with a non-set value",2)
	end
	local res=Set.new{}
	for k in pairs(a) do
		res[k]=b[k]
	end
	return res
end
function Set.subtraction(a,b)
	if getmetatable(a)~=mt or getmetatable(b)~=mt then
		error("attempt to 'subtract' a set with a non-set value",2)
	end
	local res=Set.new{}
	for k in pairs(a) do
		res[k]=true
	end
	for k in pairs(b) do
		res[k]=nil
	end
	return res
end
function Set.tostring(set)
	local l={}
	for e in pairs(set) do
		l[#l+1]=e
	end
	return "{"..table.concat(l,",").."}"
end
function Set.length(set)
	local count=0
	for i in pairs(set) do
		count=count+1
	end
	return count
end
mt.__add=Set.union
mt.__mul=Set.intersection
mt.__sub=Set.subtraction
mt.__len=Set.length
mt.__tostring=Set.tostring
mt.__le=function (a,b)
	for k in pairs(a) do
		if not b[k] then
			return false
		end
	end
		return true
end
mt.__lt=function (a,b)
	return a<=b and not(b<=a)
end
mt.__eq=function (a,b)
	return a<=b and b<=a
end
return Set

for test

#!/usr/bin/lua
t=require("20-1and20-2")
a=t.new({1,2,3,4})
b=t.new{3,5}
print(a-b)
print(a+b)
print(a*b)
print("#a="..#a)

20.2

同上

20.3

#!/usr/bin/lua
local proxy={}
local mt={}
function readOnly(t)
	mt.__index=function (_,k) return rawget(t,k) end
	mt.__newindex=function (t,k,v) error("attempt to update a read-only table",2) end
	setmetatable(proxy,mt)
	return proxy
end
days={"Sunday","Monday","Tuesday","Wednesday","Tursday","Friday","Saturday"}
days=readOnly(days)
print(days[1])
days[2]="Noday"

20.4

#!/usr/bin/lua
function fileAsArray(filename)
	local proxy={}

	local filearray={}
	local file=assert(io.open(filename,"r"))
	for block in file:lines(1) do
		filearray[#filearray+1]=block
	end
	file:close()
	
	local mt={
		__index=function (_,k) return filearray[k] end,
		__newindex=function (_,k,v)
			filearray[k]=v
			local file=assert(io.open(filename,"w"))
			for _,w in ipairs(filearray) do
				file:write(w)
			end
			file:close()
		end,
		__pairs=function ()
			return function (_,k)
				local nextkey,nextvalue=next(filearray,k)
				return nextkey,nextvalue
			end
		end,
		__len=function ()
			return #filearray
		end
	}


	setmetatable(proxy,mt)
	return proxy
end


--the content under the line are for test 
f=fileAsArray("text")
print(f[2])
f[2]="i"
for x,y in pairs(f) do
	print(x,y)
end
print(#f)

20.5

同上

标签：function,Set,end,自做,mt,lua,第四版,return,local
来源： https://blog.csdn.net/fuluoyide312/article/details/114219240