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pythonfloating-pointsubtraction
NCall
NCall

Reputation: 133

Basic floating subtraction in python

I have two float values, I would like to subtract them and compare the result with another value in an if statement. Problem, I didn't manage to have the expected result. I tried to use Decimal module.

from decimal import Decimal, getcontext    
getcontext().prec = 3
value_1 = 60.32
value_2 = 59.72
condition = 0.6

sub = Decimal(value_1) - Decimal(value_2)

But,

sub <= condition
FALSE

sub <= Decimal(condition)
FALSE

Upvotes: 0

Views: 1559

Answers (1)

Patrick Artner
Patrick Artner

Reputation: 51643

This answer explains why this question is a dupe of Is floating point math broken?.

If you use

 k = Decimal(0.6)

and inspect k you get:

is floating math broken

This is a case of "Is floating math broken":

  • the number 0.6 is first converted to a float (wich gives the nearest float-approximation)
  • the float is passed on to the Decimal constructor wich dutifully constructs a Decimal that is just a little bit smaller then the 0.6 originally put in your source code.

You can avoid this by providing a string of your number to the decimal constructor instead to bypass the "float"-conversion.

Inspecting

value_1 = 60.32
value_2 = 59.72
sub = Decimal(value_1) - Decimal(value_2)

leads to :

exactly 0.6

which is exactly 0.6. As k is now slightly less then 0.6 your test of sub <= k returns False.

You can use k = Decimal("0.6") to get a decimal value that is exactly 0.6.

Upvotes: 1

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