On Fri, Feb 13, 2015 at 2:22 PM, Grant Edwards <invalid@invalid.invalid> wrote: > On 2015-02-13, Dave Angel <da...@davea.name> wrote: >> On the other hand, the Decimal package has a way that the programmer >> can tell how many digits to use at each stage of the calculation. > > That's what surpised me. From TFM: > > https://docs.python.org/2/library/decimal.html: > > * The decimal module incorporates a notion of significant places so that > 1.30 + 1.20 is 2.50. The trailing zero is kept to indicate > significance. This is the customary presentation for monetary > applications. For multiplication, the “schoolbook” approach uses > all the figures in the multiplicands. For instance, 1.3 * 1.2 gives > 1.56 while 1.30 * 1.20 gives 1.5600.
Huh. That approach for multiplication is definitely not what I was taught in school. I was taught that the number of significant digits in the product is the lesser of the number of significant digits in either of the measured multiplicands. So 1.30 * 1.20 would be 1.56, while 1.3 * 1.2 would just be 1.6. Wikipedia appears to agree with me: http://en.wikipedia.org/wiki/Significance_arithmetic#Multiplication_and_division_using_significance_arithmetic Moreover: >>> D('1.304') * D('1.204') Decimal('1.570016') >>> D('1.295') * D('1.195') Decimal('1.547525') So 1.30 * 1.20 could be written approximately as 1.56 ± 0.01. Given that, I don't understand how the trailing zeros in 1.5600 could possibly be considered significant. -- https://mail.python.org/mailman/listinfo/python-list
