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DrifterNL

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Joined: 27/09/2018
Location: Netherlands
Posts: 58

Posted: 09:54am 21 Apr 2023

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Hi all,
I've been trying to wrap my head around this.
Is this one of those floating point quirks, is my thinking wrong or am I getting something mixed up?:
* PicoMite Version 5.0706

Dim a = 100
Dim b = 0
Dim c = 0.1

start:
Print b
If b = a Then GoTo equ
If b > a Then GoTo gre
If b < a Then Inc b,c
GoTo start

equ:
Print "equal"
Print "a= " a
Print "b= " b
End

gre:
Print "greater than"
Print "a= " a
Print "b= " b
End

If I run the program as is I get:

greater than
a= 100
b= 100.1

b = a isn't recognized?
------------------------------------------
When I change c to 0.2 I get:

greater than
a= 100
b= 100

b is greater then a?
------------------------------------------
When I change c to 0.25 I get:

equal
a= 100
b= 100

b = a is recognized, seems to work
Edited 2023-04-21 20:00 by DrifterNL

Floating Point Keeps Sinking Me!
Back To Integer So I Don't Get Injured.

Volhout
Guru

Joined: 05/03/2018
Location: Netherlands
Posts: 5089

Posted: 10:06am 21 Apr 2023

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Option default float ?

Yes, this is strange. As if behind the comma, there is a difference that is not printed on screen.

Volhout
Edited 2023-04-21 20:13 by Volhout

PicomiteVGA PETSCII ROBOTS

matherp
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Joined: 11/12/2012
Location: United Kingdom
Posts: 10310

Posted: 10:12am 21 Apr 2023

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This is standard floating point behaviour 0.1 cannot be exactly expressed as a float therefore 1000*0.1 will not equal 100. In general you should never test for equality of floats after any arithmetic but must allow a small delta e.g.

if abs(a-b)<0.000000001

DrifterNL

Regular Member

Joined: 27/09/2018
Location: Netherlands
Posts: 58

Posted: 10:28am 21 Apr 2023

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Thanks for the info!
Darn, floating point got me again.

Floating Point Keeps Sinking Me!
Back To Integer So I Don't Get Injured.

CaptainBoing

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Joined: 07/09/2016
Location: United Kingdom
Posts: 2170

Posted: 10:46am 21 Apr 2023

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Think I might have mentioned this before, Years ago HiSoft BASIC had the == operator which meant "roughly equal to" precisely because of FP wrinkles like this

so

IF 1000*0.1 == 100

would be true

no idea how the thresholds were set up

As a callow youth taking my first steps in this stuff professionally, It was always drummed into us to "get things into integer ASAP"

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