Lua学习--4

结构控制

 

2.4.4 – Control Structures

The control structures if, while, and repeat have the usual meaning and familiar syntax:

	stat ::= while exp do block end
	stat ::= repeat block until exp
	stat ::= if exp then block {elseif exp then block} [else block] end

Lua also has a for statement, in two flavors (see §2.4.5).

The condition expression of a control structure can return any value. Both false and nil are considered false. All values different from nil and false are considered true (in particular, the number 0 and the empty string are also true).

In the repeat–until loop, the inner block does not end at the until keyword, but only after the condition. So, the condition can refer to local variables declared inside the loop block.

The return statement is used to return values from a function or a chunk (which is just a function). Functions and chunks can return more than one value, and so the syntax for the return statement is

	stat ::= return [explist]

The break statement is used to terminate the execution of a while, repeat, or for loop, skipping to the next statement after the loop:

	stat ::= break

A break ends the innermost enclosing loop.

The return and break statements can only be written as the last statement of a block. If it is really necessary to return or break in the middle of a block, then an explicit inner block can be used, as in the idioms do return end and do break end, because now return and break are the last statements in their (inner) blocks.

在结构控制中有循环 
repeat block until exp
do block while exp
if exp then block {elseif exp then block} {else block} end
for v=exp1,to_exp2{,step_exp3}  do block end

exp 表达式只有 false 和 nil 才会是false,其它值都是true（包括数值的0和字符串的空）
在循环block 中 可以通过 break  结束当前block最内层循环  或goto 跳转 到 label,格式如下：

:: Label ::

数值型for循环

For 语法

 do
       local var, limit, step = tonumber(e1), tonumber(e2), tonumber(e3)
       if not (var and limit and step) then error() end
       while (step > 0 and var <= limit) or (step <= 0 and var >= limit) do
         local v = var
         block
         var = var + step
       end
     end

Note the following:

• All three control expressions are evaluated only once, before the loop starts. They must all result in numbers.
• var, limit, and step are invisible variables. The names shown here are for explanatory purposes only.
• If the third expression (the step) is absent, then a step of 1 is used.
• You can use break to exit a for loop.
• The loop variable v is local to the loop; you cannot use its value after the for ends or is broken. If you need this value, assign it to another variable before breaking or exiting the loop.

 

针对上面的for 结构语法 做如下说明：
1、所有的表达式只在循环开始之前 计算一次，并且所有返回结果必须是数值型
2、如果步长表达式省略，默认为1
3、break 可以中止block的正常执行而退出最近的一级for循环
4、结构 中的 值 v 是for循环内部变量，循环体之外无效

泛型for循环

The generic for statement works over functions, called iterators. On each iteration, the iterator function is called to produce a new value, stopping when this new value is nil. The generic for loop has the following syntax:

	stat ::= for namelist in explist do block end
	namelist ::= Name {`,´ Name}

A for statement like

     for var_1, ···, var_n in explist do block end

is equivalent to the code:

     do
       local f, s, var = explist
       while true do
         local var_1, ···, var_n = f(s, var)
         var = var_1
         if var == nil then break end
         block
       end
     end

Note the following:

• explist is evaluated only once. Its results are an iterator function, a state, and an initial value for the first iterator variable.
• f, s, and var are invisible variables. The names are here for explanatory purposes only.
• You can use break to exit a for loop.
• The loop variables var_i are local to the loop; you cannot use their values after the for ends. If you need these values, then assign them to other variables before breaking or exiting the loop.

 

泛型循环，依赖于迭代器，每执行一次block 迭代器都会产生一个新的值，如果值为nil则循环结束

泛型 for 循环通过一个迭代器函数来遍历所有值，类似 java 中的 foreach 语句。

Lua 编程语言中泛型 for 循环语法格式:

--打印数组a的所有值  
a = {"one", "two", "three"}
for i, v in ipairs(a) do
    print(i, v)
end 
i是数组索引值，v是对应索引的数组元素值。ipairs是Lua提供的一个迭代器函数，用来迭代数组。

 

 

c1,d1=3
--print(c1,d1)
a,b=1,2
if( a>b ) then
    print('a='..a)
elseif(a ~= b ) then
    print('b='..b)
else
    print(' a equal b')
end

 

i=5
repeat 
    print(i) 
    i=i-1 
until i<=0