A mathematical bug

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Post » Mon Oct 06, 2014 12:57 am

A mathematical bug
Sometimes the software calculates decimal numbers incorrectly.
For example, instead of 0.2 it calculates 0.1999999, and sometimes 0.200000001.
I do not know why this happens, but it is repetitive.

Attach a Capx
https://googledrive.com/host/0B5et6AUuw ... 20Bug.capx

Description of Capx
It is a very short CAPX, so here's a picture:
Image

Steps to Reproduce Bug
Just run the program with the debugger running, and see the bug.

Observed Result
Variable2 gets incorrect value.

Expected Result
Variable2 should be equal to 0.2

Affected Browsers
Tested on Chrome, Firefox, and Node-Webkit

Operating System and Service Pack
Win7

Construct 2 Version ID
I'm currently using r183, but I know that the bug was also in previous versions.
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Post » Mon Oct 06, 2014 1:01 am

https://www.scirra.com/blog/141/common- ... nd-gotchas

scroll a little past halfway down to "Expecting math calculations to be exact"
or google floating point numbers and computers...

(I once posted this as a bug, too. heh..)
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Post » Mon Oct 06, 2014 2:21 am

Hey spacedoubt,
Thanks for the comment.

I know about the floating point issue, but I do not think it is valid here.

After all, if I do: Set Variable2 to 1.2 - 1, Variable 2 will equal 0.2.
And the example I gave is not so different.
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Post » Mon Oct 06, 2014 7:55 am

1.2+var1=2.2
but....
1.2-var1=0.199999999999996

Strange indeed, no problem on addition. I understand if this is involve multiplication/division problem, but substraction?
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Post » Mon Oct 06, 2014 8:45 am

It is rounding error. Just don't expect exact results with real (decimal) numbers.

Try adding 0.1 to variable2 twelve times instead of 1.2 one time. You will get 2.200000000000001
But it will display in a text field as 2.2 :)
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Post » Mon Oct 06, 2014 9:09 am

I wonder if this is a normal thing that everybody should expect?
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Post » Mon Oct 06, 2014 9:24 am

DuckfaceNinja wrote:Strange indeed, no problem on addition. I understand if this is involve multiplication/division problem, but substraction?


That's what I thought.

codah wrote:It is rounding error. Just don't expect exact results with real (decimal) numbers.

Try adding 0.1 to variable2 twelve times instead of 1.2 one time. You will get 2.200000000000001
But it will display in a text field as 2.2 :)


I am building a software that displays advanced graphs for external sensors system, the entire software is one big math problem.
Of course, once I start division, multiplication, and other mathematical manipulations, I am aware of the floating point Inaccuracy.

In case of 1.2 - 1 however, I do not expect it.
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Post » Mon Oct 06, 2014 9:35 am

SES wrote:I am building a software that displays advanced graphs for external sensors system, the entire software is one big math problem.


In C2 ? :D Seriously though, which language are you using for that? I just tried the 1.2 + 1.0 in C++ with similar results.
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Post » Mon Oct 06, 2014 9:57 am

codah wrote:In C2 ? :D Seriously though, which language are you using for that? I just tried the 1.2 + 1.0 in C++ with similar results.


Yes, in C2, it looks something like this:

Image

I use a Python server to connect to the sensors and MatLab for the functions, but all of the graphics calculations are done within the C2.
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Post » Mon Oct 06, 2014 10:12 am

SES wrote:
codah wrote:In C2 ? :D Seriously though, which language are you using for that? I just tried the 1.2 + 1.0 in C++ with similar results.


Yes, in C2, it looks something like this:

Image

I use a Python server to connect to the sensors and MatLab for the functions, but all of the graphics calculations are done within the C2.


ok man that's cool! sorry for the smiley face :) It looks great.
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