Concatenation of string in an array into a long string in Lua

Question

I have this so far

mk= {}
mk = conn:query({ live=true, sql="select * from OrderReports where OrderId= '"..T.OrderId.."'"})
 for a=1, # mk do

 end

I want strings at different indexes in mk to be concatenated and stored in a variable let's say lk, what should be in the body ?

Be wary of concatenating strings to get data into SQL queries; it opens the door to SQL injection attacks. While this example is probably not too insecure, it's better to use prepared statements and binding if the the engine supports it (and all competent ones should). — Colonel Thirty Two
– Colonel Thirty Two, Commented Mar 24, 2014 at 14:04

Yu Hao · Accepted Answer · 2014-03-22 17:44:32Z

5

The direct way is to concatenate the strings in the for body like this:

local lk = ""
for a = 1, # mk do
    lk = lk .. mk[a]
end

But this has a poor performance when the table is large because strings are immutable in Lua. A new string is created and the contents are copied in each loop.

Instead, you should use the built-in function table.concat() for this job:

local lk = table.concat(mk)

edited Mar 22, 2014 at 17:44

answered Mar 22, 2014 at 17:26

Yu Hao

123k50 gold badges252 silver badges305 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

systemdebt Over a year ago

The first one gives error that says nodes with children can not be used in concatenaton.

systemdebt Over a year ago

second one gives error saying" bad argument #1 to '?' (table expected, got userdata"

Yu Hao Over a year ago

@Simrankaur You said mk is an array, in Lua, that means it's a table with index from 1 to some integral value only. But from your error message, it's not, its type is userdata.

systemdebt Over a year ago

Thank you Yu hao, it worked :), that was error because of something else, My mistake .thanks :)

Collectives™ on Stack Overflow

Concatenation of string in an array into a long string in Lua

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related