lua table values to a string- performance issue

Question

I have a lua code where a certain table (lets say 'abc') has 50,000 key value pairs. Now i want to create a long string which will look like "(key1, value1) (key2,value2) (key3, value3) ... ".

This works for tables with smaller number of elements. for 50,000 it works but the pc becomes very very slow. Is there a way around this ?

I tried inserting in to another table and then concatenating since the string concatenation in lua is expensive but still the performance penalty is there

local pqr = {}
for key, value in pairs(app_revenue) do
  table.insert(pqr, "(")
  table.insert(pqr, key)
  table.insert(pqr, ",")
  table.insert(pqr, value)
  table.insert(pqr, ") ")
end
local x = table.concat(pqr)

Any insight on this issue is much appreciated. Thank you

And BTW I found this during my search. May be will help someone in future. (lua.org/pil/11.6.html) — user4700203
What do the key value pairs look like for app_revenue? I did a test with this and it doesn't seem that slow. On my machine it finished in 0.381 seconds. On repl.it it took less than 2 seconds to finish which isn't too bad considering it's interpreted inside a javascript vm. — greatwolf
the key values look like ('application|cricket2015|totalscore|match1', 623 ).. I think there is an issue with the scenario i am working on. (machine and the frameworks i am using) — user4700203

Dmitry Ledentsov Dmitry Ledentsov · Accepted Answer · 2015-03-26T09:49:28

You can use a temporary table and string formatting. Look, pretty fast:

local t={}

for i=1,50000 do 
    t[i] = ''..i
end

local temp = {}

for k,v in ipairs(t) do
    temp[#temp+1] = string.format('(%d, %s)', k, v)
end

local x = table.concat(temp,' ')

print(string.sub(x,1,99), #x)

less than a second: http://ideone.com/tSLeBH

1.3 seconds for 500000 entries on my machine

lua table values to a string- performance issue

2 Answers