[Home] [Downloads] [Search] [Help/forum]

Gammon Forum

See www.mushclient.com/spam for dealing with forum spam. Please read the MUSHclient FAQ!

[Folder]  Entire forum
-> [Folder]  MUSHclient
. -> [Folder]  Lua
. . -> [Subject]  if/elseif/else vs tables
Home  |  Users  |  Search  |  FAQ
Username:
Register forum user name
Password:
Forgotten password?

if/elseif/else vs tables

It is now over 60 days since the last post. This thread is closed.     [Refresh] Refresh page


Posted by Shaun Biggs   USA  (644 posts)  [Biography] bio
Date Thu 24 May 2007 06:55 PM (UTC)

Amended on Thu 24 May 2007 07:32 PM (UTC) by Shaun Biggs

Message
I got bored today, so I decided to test something out. I've wondered about the efficiency of tables and cascading if statements. Just which one is better in which setting. I ran the following:

function testif( i )
  if i == 1 then
    return 2
  elseif i == 2 then
    return 3
  elseif i == 3 then
    return 4
  elseif i == 4 then
    return 5
  elseif i == 5 then
    return 6
  elseif i == 6 then
    return 7
  elseif i == 7 then
    return 8
  elseif i == 8 then
    return 9
  elseif i == 9 then
    return 10
  elseif i == 10 then
    return 11
  else
    return 1
  end
end

function testtable1( i )
tab = { [1]=2, [2]=3, [3]=4, [4]=5, [5]=6, [6]=7, [7]=8 , [8]=9, [9]=10, [10]=11, [11]=1}
  return tab[i]
end

tab2={}
for i = 1,10 do
  tab2[i] = i+1
end
tab2[#tab2+1]=1
function testtable2( i )
  return tab2[i]
end

function speedtest( count )
  start = os.time()
  test = 1
  for i = 1,count do
    test = testif( test )
  end
  print( "if:"..os.difftime( os.time(), start ) )
  start = os.time()
  for i = 1,count do
    test = testtable1( test )
  end
  print( "table1:"..os.difftime( os.time(), start ) )
  start = os.time()
  for i = 1,count do
    test = testtable2( test )
  end
  print( "table2:"..os.difftime( os.time(), start ) )
end

And got this out of it:
/speedtest(30000000)
if:15
table1:107
table2:11

Yes, that's a loop of 30 million tries. I started off with 100,000, but if and table2 were both less than a second... not a terribly good comparison. What I've learned today: Predefine your tables whenever possible. Also, if you're just running things through for one shot, you won't get terribly much better performance with a table. It just looks neater. Granted, even with the slowest table version, that's over a quarter of a million function calls per second on my machine.

It is much easier to fight for one's ideals than to live up to them.
[Go to top] top

Posted by David Haley   USA  (3,881 posts)  [Biography] bio
Date Reply #1 on Thu 24 May 2007 07:31 PM (UTC)
Message
If you have a very long series of if statements, the table is likely to be faster. If you have a very short series of if statements, the ifs are likely to be faster. And if your ifs are ordered such that the most probable cases are first, that will make the ifs faster.

In brief the issues are:

- it is faster to check equalities (in an if statement) than it is to index into a hash table.
- however, if you have to make a large number of equality tests before you find the right one, it might have been cheaper to take the hashing cost from the beginning.


If you were to make the numbers go up to, say, 100, you would probably start seeing the table outperforming the if checks.


And yes, it is a much better idea to create the table *outside* of the execution of the testtable function, either as a global variable or using a closure.


And incidentally I would argue that the cleanliness of the table solution outweighs the if checks, even if it were to be slightly slower. It depends on what you care about. If you're not hitting the table millions of times per second, it doesn't really matter. One of the most common "errors" for novice (and not-so-novice) programmers is what's called premature optimization: trying to optimize things, at the expense of clarity, because understanding what really matters in the end of the day with respect to the program speed.

David Haley aka Ksilyan
Head Programmer,
Legends of the Darkstone

http://david.the-haleys.org
[Go to top] top

Posted by Shaun Biggs   USA  (644 posts)  [Biography] bio
Date Reply #2 on Thu 24 May 2007 07:49 PM (UTC)

Amended on Thu 24 May 2007 07:50 PM (UTC) by Shaun Biggs

Message
I agree with every point you made. I should have been a bit more specific. I was testing small sets of matching items. I did try it at first with only boolean values, and here are the results:
/speedtest( 30000000 )
if:11
table:45
table:15

Creating the table each time takes up a LOT of time. Having the pre-created tables just increases relative speed exponentially the more elements I put in. I stopped at 11, since I just got sick of cut/pasting it all, and didn't want to set up a script to deal with it. With two elements, the slower table comparison takes 3 times as long as the faster table comparison. With 11 elements, it takes 9 times as long.

I also noted at the end the speed of the slowest test, creating the table each time, as being quite fast. If your speed requirements are higher than a quarter of a million tests per second in a mud client, you are doing something seriously wrong. Or making a mapper, which would have an already created set of tables. Also mentioned it looking better, and I should have mentioned that it is much easier to follow.

It is much easier to fight for one's ideals than to live up to them.
[Go to top] top

The dates and times for posts above are shown in Universal Co-ordinated Time (UTC).

To show them in your local time you can join the forum, and then set the 'time correction' field in your profile to the number of hours difference between your location and UTC time.


7,318 views.

It is now over 60 days since the last post. This thread is closed.     [Refresh] Refresh page

Go to topic:           Search the forum


[Go to top] top

Quick links: MUSHclient. MUSHclient help. Forum shortcuts. Posting templates. Lua modules. Lua documentation.

Information and images on this site are licensed under the Creative Commons Attribution 3.0 Australia License unless stated otherwise.

[Home]


Written by Nick Gammon - 5K   profile for Nick Gammon on Stack Exchange, a network of free, community-driven Q&A sites   Marriage equality

Comments to: Gammon Software support
[RH click to get RSS URL] Forum RSS feed ( https://gammon.com.au/rss/forum.xml )

[Best viewed with any browser - 2K]    [Hosted at FutureQuest]