on floating-point numbers

[Roel Schroeven]

Op 2/09/2021 om 17:08 schreef Hope Rouselle:
> >>>> ls = [7.23, 8.41, 6.15, 2.31, 7.73, 7.77]
> >>>> sum(ls)
> > 39.599999999999994
> >
> >>>> ls = [8.41, 6.15, 2.31, 7.73, 7.77, 7.23]
> >>>> sum(ls)
> > 39.60000000000001
> >
> > All I did was to take the first number, 7.23, and move it to the last
> > position in the list.  (So we have a violation of the commutativity of
> > addition.)
>
> Suppose these numbers are prices in dollar, never going beyond cents.
> Would it be safe to multiply each one of them by 100 and therefore work
> with cents only?
For working with monetary values, or any value that needs to accurate 
correspondence to 10-based values, best use Python's Decimal; see the 
documentation: https://docs.python.org/3.8/library/decimal.html

Example:

from decimal import Decimal as D
ls1 = [D('7.23'), D('8.41'), D('6.15'), D('2.31'), D('7.73'), D('7.77')]
ls2 = [D('8.41'), D('6.15'), D('2.31'), D('7.73'), D('7.77'), D('7.23')]
print(sum(ls1), sum(ls2))

Output:
39.60 39.60

(Note that I initialized the values with strings instead of numbers, to 
allow Decimal access to the exact number without it already being 
converted to a float that doesn't necessarily exactly correspond to the 
decimal value)

-- 
"Your scientists were so preoccupied with whether they could, they didn't
stop to think if they should"
         -- Dr. Ian Malcolm