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Re: Another one of those times where Lua's floats make me do the HERP-A-DERP

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NecroBumpist is not online. NecroBumpist
Joined: 12 Sep 2010
Total Posts: 4198
12 Jul 2011 05:43 PM
So, I was strolling through the wiki when I came accross the math library page, and decided to take a looksies.

One of the first things I noticed was the derpy usage of numbers like 1.5707963267948966192313216916398, and 2.71828182845904523536.

So I decided to do a little research.

print(1.5707963267948966192313216916398)

results in 1.5707963267949
That's because Lua automagically rounds number constants/literals at compilation so that they fit within an IEEE 754 64bit double.

Now this made no difference in the example:
print(math.deg (1.5707963267948966192313216916398)) --> 90
print(math.deg (1.5707963267949)) --> 90

Now that's all fine and dandy, but it was the second, smaller number and its example that really made me herp my derp.

print(math.log (2.71828182845904523536)) --> 1

So that time it prints one, but what happens when we try shortening the number ?

print(2.71828182845904523536) --> 2.718281828459
print(math.log (2.71828182845904523536)) --> 1
print(math.log (2.718281828459)) --> 0.99999999999998

So this made me derp quite a bit.
So what causes these two different answers ?

One would think that "if the big number rounds down to the little one why doesn't it give the same answer ?"

The only speculation that I can offer you is that Lua has some derpy Float-To-ASCII conversion routines and vice versa. (aka; when you print the big number it leaves out just a tiny bit of accuracy that can be influenced by entering the larger number in full)

I'm going to assume my conjecture is correct because after a quick disassembly, it turns out that Lua uses two different constants for the larger number and the smaller number, leading me to believe they are encoded slightly different (though they both resolve to "2.718281828459" according to a function supplied by XlegoX), and I'm too lazy to compare the bytecode by hand.




I hope you guys have enjoyed this little adventure.



On a special note, this isn't the first time I've encountered problems with Lua's derpy handling of floats, problems can also arise from functions like math.modf()

local a, b = math.modf(3.16)
b = b * 100

print(b) --> 16
print(16 == b) --> false
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bunnybunn264 is not online. bunnybunn264
Joined: 04 Oct 2010
Total Posts: 5409
12 Jul 2011 05:45 PM
Why do floats in any programming language ever have to be so limited? :L
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LocalChum is not online. LocalChum
Joined: 04 Mar 2011
Total Posts: 6906
12 Jul 2011 05:53 PM
MahBrain.Status == "Exploded";

TL;DR
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JulienDethurens is not online. JulienDethurens
Joined: 11 Jun 2009
Total Posts: 11046
12 Jul 2011 05:58 PM
"Why do floats in any programming language ever have to be so limited? :L"

Limited? How often do you have to use a number like 1.5707963267948966192313216916398?

"MahBrain.Status == "Exploded";"

This thread isn't that hard to understand..
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LocalChum is not online. LocalChum
Joined: 04 Mar 2011
Total Posts: 6906
12 Jul 2011 06:00 PM
TL;DR == BLAARGH TOO MUCH TEXT
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blocco is not online. blocco
Joined: 14 Aug 2008
Total Posts: 29474
12 Jul 2011 09:52 PM
they truncate the float to convert it to a string
duh
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burgly is not online. burgly
Top 100 Poster
Joined: 20 Aug 2006
Total Posts: 2843
12 Jul 2011 10:10 PM
Of course it's limited. I was once stupid enough to try to use Lua to do competition programming: It was Slow, and couldn't support huge numbers.
Lua took 2 and a half hours to do what C++ did in about 45 seconds.
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blocco is not online. blocco
Joined: 14 Aug 2008
Total Posts: 29474
12 Jul 2011 10:11 PM
Coding a huge C++ program in 45 program takes like, hyper fingers and freaking skillz.
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blocco is not online. blocco
Joined: 14 Aug 2008
Total Posts: 29474
12 Jul 2011 10:12 PM
program (2) => seconds
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JulienDethurens is not online. JulienDethurens
Joined: 11 Jun 2009
Total Posts: 11046
12 Jul 2011 10:51 PM
"they truncate the float to convert it to a string"

Exactly. The print method calls tostring() on the string. io.write does it too, but unlike print, it doesn't print memory adresses.

print({})

--> table: insert memory adress in hex here

io.write({})

--> UMG DER'S AN ERROR UMG UMG
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burgly is not online. burgly
Top 100 Poster
Joined: 20 Aug 2006
Total Posts: 2843
12 Jul 2011 11:03 PM
@blocco, I was talking about run time.
C++ in 45 seconds? You must be crazy, XD
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Merlin11188 is not online. Merlin11188
Joined: 20 Dec 2007
Total Posts: 4158
12 Jul 2011 11:10 PM
That's an unfair measurement.
C:
Compiles in an hour.
Runs in 45 seconds.

Lua:
Is interpreted, compiles, and runs in 30 minutes.

Hmmm... I think I see the problem...
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pokelover980 is not online. pokelover980
Joined: 12 Jun 2009
Total Posts: 4141
12 Jul 2011 11:21 PM
"Lua took 2 and a half hours to do what C++ did in about 45 seconds."

I'm going to give you the same answer I give everyone else who claims such incredibly large differences: You're doing it wrong.
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NecroBumpist is not online. NecroBumpist
Joined: 12 Sep 2010
Total Posts: 4198
12 Jul 2011 11:41 PM
pokelover980, he probably didn't use LuaJIT
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pokelover980 is not online. pokelover980
Joined: 12 Jun 2009
Total Posts: 4141
12 Jul 2011 11:42 PM
LuaJIT master race! I <3 LuaJIT
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burgly is not online. burgly
Top 100 Poster
Joined: 20 Aug 2006
Total Posts: 2843
12 Jul 2011 11:50 PM
I was using SciTE at the time. How good is LuaJIT?
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NecroBumpist is not online. NecroBumpist
Joined: 12 Sep 2010
Total Posts: 4198
12 Jul 2011 11:51 PM
LuaJIT is just Lua on steroids.
SciTE is an editor, LuaJIT is an entirely different implementation of Lua.
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pokelover980 is not online. pokelover980
Joined: 12 Jun 2009
Total Posts: 4141
12 Jul 2011 11:59 PM
"LuaJIT is just Lua on steroids."

Nah, it's more like if you took soma (lol Brave New World reference. If you Google it, make sure you add in Brave New World as well or you'll get the wrong thing), mixed it with caffeine, and then finally added steroids.
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bunnybunn264 is not online. bunnybunn264
Joined: 04 Oct 2010
Total Posts: 5409
13 Jul 2011 09:07 AM
Then LuaJIT isn't vanilla Lua and it isn't really Lua we're testing here, is it?

btw, what's a huge calculation I can do that will take a long time suitable for a program race? I'll probably try C/C++ vs. Java/Lua/JavaScript/Python. I'm not a mathematician, so I have no idea what calculations I could do..
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booing is not online. booing
Joined: 04 May 2009
Total Posts: 6594
13 Jul 2011 09:17 AM
trolololool
int a = 1
double b = 12495071342895127
triple c = nu triplez? :(
Again, assembly
__asm {
MOV AX,9999
MOV BX,9999
MOV CX,9999
MOV DX,9999
}
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TheCapacitor is not online. TheCapacitor
Joined: 19 Jan 2011
Total Posts: 7045
13 Jul 2011 09:20 AM
octo d=nu octo? :(

(For dat we gotta go to Japan)
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