Unexpected multiplication result

Bugs that are actually features.
Post Reply
Pi-C
Filter Inserter
Filter Inserter
Posts: 575
Joined: Sun Oct 14, 2018 8:13 am
Contact:

Unexpected multiplication result

Post by Pi-C »

Code: Select all

local p = 0.025
local ret = {}

for x, name in ipairs({"A", "B", "C"}) do
    ret[x] = {
        item = name,
        probability = x * p,
    }
end
log("ret: " .. serpent.block(ret))
This results in:

Code: Select all

ret: {
  {
    item = "A",
    probability = 0.025
  },
  {
    item = "B",
    probability = 0.05
  },
  {
    item = "C",
    probability = 0.075000000000000009
  }
}
The last value for probability seems to be a bit off. This happened in 0.18.6, by the way … :-)
A good mod deserves a good changelog. Here's a tutorial (WIP) about Factorio's way too strict changelog syntax!

User avatar
Klonan
Factorio Staff
Factorio Staff
Posts: 4125
Joined: Sun Jan 11, 2015 2:09 pm
Contact:

Re: Unexpected multiplication result

Post by Klonan »

This is just how floating point numbers work

User avatar
darkfrei
Smart Inserter
Smart Inserter
Posts: 2479
Joined: Thu Nov 20, 2014 11:11 pm
Contact:

Re: Unexpected multiplication result

Post by darkfrei »

Klonan wrote:
Fri Feb 14, 2020 9:58 am
This is just how floating point numbers work
How to fix it?
x = math.floor (x * 10^4) / 10^4

User avatar
Klonan
Factorio Staff
Factorio Staff
Posts: 4125
Joined: Sun Jan 11, 2015 2:09 pm
Contact:

Re: Unexpected multiplication result

Post by Klonan »

darkfrei wrote:
Fri Feb 14, 2020 11:18 am
Klonan wrote:
Fri Feb 14, 2020 9:58 am
This is just how floating point numbers work
How to fix it?
x = math.floor (x * 10^4) / 10^4
Why do you need to fix it?

User avatar
darkfrei
Smart Inserter
Smart Inserter
Posts: 2479
Joined: Thu Nov 20, 2014 11:11 pm
Contact:

Re: Unexpected multiplication result

Post by darkfrei »

Klonan wrote:
Fri Feb 14, 2020 11:21 am
darkfrei wrote:
Fri Feb 14, 2020 11:18 am
Klonan wrote:
Fri Feb 14, 2020 9:58 am
This is just how floating point numbers work
How to fix it?
x = math.floor (x * 10^4) / 10^4
Why do you need to fix it?
I want to reed the
color = {r = 0.5, g = 0.25, b = 0.75}
but not the
color = {r = 0.5000000000012, g = 0.2500000000006, b = 0.75000000000018}

It's easier to read the short form.

Pi-C
Filter Inserter
Filter Inserter
Posts: 575
Joined: Sun Oct 14, 2018 8:13 am
Contact:

Re: Unexpected multiplication result

Post by Pi-C »

darkfrei wrote:
Fri Feb 14, 2020 11:25 am
Klonan wrote:
Fri Feb 14, 2020 11:21 am

Why do you need to fix it?
I want to reed the
color = {r = 0.5, g = 0.25, b = 0.75}
but not the
color = {r = 0.5000000000012, g = 0.2500000000006, b = 0.75000000000018}

It's easier to read the short form.
Correct. It doesn't really matter in the game if the calculated probability is a miniscule fraction bigger than expected, but it doesn't look nice. I've worked around it by using integers and dividing the result by 1000 -- basically the same thing darkfrei did.
A good mod deserves a good changelog. Here's a tutorial (WIP) about Factorio's way too strict changelog syntax!

movax20h
Fast Inserter
Fast Inserter
Posts: 109
Joined: Fri Mar 08, 2019 7:07 pm
Contact:

Re: Unexpected multiplication result

Post by movax20h »

The reason you are seeing these numbers is because "0.025" is not a representable floating point number. 0.25 is. But 0.025 isn't. Closest value is "0.025000000000000001387778780781".

Agreed with devs. Not a bug. If you want to format values to be "nicer" you need to do it manually when printing. Or by pre-multiplicating things by some reasonable constant (1000 for example) maybe.

Post Reply

Return to “Not a bug”

Who is online

Users browsing this forum: No registered users