Re: Question about lua socket ftp example.

[Rena <hyperhacker@...>]

On Tue, Mar 12, 2013 at 11:09 PM, albert <albert_200200@126.com> wrote:
> Rena, you are so kind. The explaination is so good, it really unlock my
> mind. The question makes me confuse for a long time.
> I have another question then,
>
> if r then val = table.concat(t) end
> How could it work? table.concat(t) concat nothing to the the table t.
>
>
> On 2013/3/13 10:55, Rena wrote:
>>
>> On Tue, Mar 12, 2013 at 10:29 PM, albert <albert_200200@126.com> wrote:
>>>
>>> The example code is in the link:
>>>
>>> http://w3.impa.br/~diego/software/luasocket/old/luasocket-2.0-beta/ftp.html
>>>    1 require("ftp")
>>>    2 require("ltn12")
>>>    3 require("url")
>>>    4 function ls(u)
>>>    5     local t= {}
>>>    6     local p = socket.url.parse(u)
>>>    7     p.command = "nlst"
>>>    8     p.sink = ltn12.sink.table(t)
>>>    9     local r, e = socket.ftp.get(p)
>>>   10     return r and table.concat(t), e
>>>   11 end
>>>
>>>   I don't understand line 10, Does that mean "return (r and
>>> table.concat(t)),
>>> e"?
>>>   r is a string, Then what does "r and table.concat(t)" mean?
>>>
>>>
>> It's what you wrote there, (r and table.concat(t)), e. It's using
>> boolean short-circuiting. A longer way to write it would be:
>> local val
>> if r then val = table.concat(t) end
>> return val, e
>>
>> in both cases, if r is nil or false, the table.concat part of the
>> expression is ignored and the value returned is nil. socket.ftp.get
>> most likely returns (nil, error_message) on failure, so in that case
>> this function would also return (nil, e). It's comparable (but not
>> identical!) to C's ternary operator.
>>
>
>
>

See http://www.lua.org/manual/5.2/manual.html#pdf-table.concat
it concatenates the table's contents into a string.

-- 
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