[Edu-sig] handling money and floats with beginners

[engelbert.gruber@ssg.co.at]

On Wed, 2 Oct 2002, Tim Wilson wrote:

> Hi everyone,
>
> I'm struggling a bit with explaining how important it is to handle
> floating point numbers correctly when doing calculations with money. My
> latest assignment (http://isd197.org/sibley/cs/icp/loancalc_html)
> involves calculating and printing a load payment schedule and I want my
> students to do things "the right way."
>
> My assignment instructions include a link to the Floating Point
> Arithmetic section of the python tutorial, but it's not quite enough.
>
> Does anyone have any code snippets that illustrate the problem clearly
> and are understandable by novices? My instructions say that the simplest
> approach is to do money-related calculations in pennies. Is that really
> the best approach for beginners? I didn't address this issue very well
> last year and I'd like to do a better job this time around.

1. since switching to euros some software must cope with with 1/100 cent.

2. ::
    >>> r=10e+15
    >>> "%20f" % r
    '10000000000000000.000000'
    >>> r+=1
    >>> "%20f" % r
    '10000000000000000.000000'
    # incrementing by one is not possible
    >>> r+=100
    >>> "%20f" % r
    '10000000000000100.000000'
    >>> r+=1
    >>> "%20f" % r
    '10000000000000100.000000'
    >>> r+=2
    >>> "%20f" % r
    '10000000000000102.000000'
    # by two works

3. use mpz or as in the old days bcd-coding

4. in my school days we had to do calculations on date::
   19871093 * 2 = 39742186
   39742186 * 3 = 119226558
  119226558 * 4 = 476906232
  476906232 / 2 = 238453116
  238453116 / 3 = 79484372
   79484372 / 4 = 19871093

  but going up to 10. now my son had the same kind of calcualtions
  but going up to 13, which exceeded the calcualtors precision.

cheers
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