[Lua-pil] Filling Some Gaps

读书笔记： Programming in Lua, 4th Edition.

do-end

do-end 就像是一个花括号，用来构成一个作用域。

> local x1
> do
>> local x1 = 1
>> print(x1)
>> end
1
> print(x1)
nil

if-then-elseif-else-end

if op == "+" then
    r = a + b
elseif op == "-" then
    r = a - b
elseif op == "*" then
    r = a * b
elseif op == "/" then
    r = a / b
else
    error("invalid operation")
end

while

local i = 1
while a[i] do
    print(a[i])
    i = i + 1
end

repeat-until

至少执行一次，就是C语言中的do-while

-- print the first non-empty input line
local line
repeat
    line = io.read()
until line ~= ""
print(line)

Differently from most other languages, in Lua the scope of a local variable declared inside the loop includes the condition:

-- computes the square root of 'x' using Newton-Raphson method
local sqr = x / 2
repeat
    sqr = (sqr + x/sqr) / 2
    local error = math.abs(sqr^2 - x)
until error < x/10000 -- local 'error' still visible here

for

The for statement has two variants: the numerical for and the generic for.

Numerical for

This loop will execute something for each value of var from exp1 to exp2, using exp3 as the step to increment var. This third expression is optional; when absent, Lua assumes one as the step value.

for var = exp1, exp2, exp3 do
    something
end

If we want a loop without an upper limit, we can use the constant math.huge:

for i = 1, math.huge do
    if (0.3*i^3 - 20*i^2 - 500 >= 0) then
        print(i)
        break
    end
end