[Python-bugs-list] [ python-Bugs-473456 ] float round up on decimal numbers (original) (raw)

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Sun, 21 Oct 2001 13:01:48 -0700

Bugs item #473456, was opened at 2001-10-21 11:30 You can respond by visiting: http://sourceforge.net/tracker/?func=detail&atid=105470&aid=473456&group_id=5470

Category: Python Interpreter Core Group: Not a Bug Status: Closed Resolution: Invalid Priority: 5 Submitted By: Frederic Giacometti (giacometti) Assigned to: Tim Peters (tim_one) Summary: float round up on decimal numbers

Initial Comment:

We used to get this right on python 1.5.2; and this breaks since Python 2.0:

for i in range( 10): x = 0.1 * i; print x, str( x), repr( x) ... 0.0 0.0 0.0 0.1 0.1 0.10000000000000001 0.2 0.2 0.20000000000000001 0.3 0.3 0.30000000000000004 0.4 0.4 0.40000000000000002 0.5 0.5 0.5 0.6 0.6 0.60000000000000009 0.7 0.7 0.70000000000000007 0.8 0.8 0.80000000000000004 0.9 0.9 0.90000000000000002 print 0.6 == 0.60000000000000009 0

0.3 0.29999999999999999 0.1 0.10000000000000001 1.0 1.0 1.01 1.01 0.8 0.80000000000000004 0.5 0.5 1.3 1.3

We get the same problem on Unix and Windows; and python1.5.2 gives the exact decimal values everywhere.

Comment By: Tim Peters (timone) Date: 2001-10-21 13:01

Message: Logged In: YES user_id=31435

BTW, that was the canned "float precision" response (this is a FAQ). Your last example illustrates why Python made this change: the best possible 754 double approximation to 0.6 is indeed not equal to the best possible 754 double approximation to the product of 6 with the best possible 754 double approximation to 0.1. So if you expected 0.6 to equal 0.1 * 6 exactly, you probably shouldn't be using floating-point at all, in Python or any other language .

Comment By: Tim Peters (tim_one) Date: 2001-10-21 12:50

Message: Logged In: YES user_id=31435

This is not a bug.

Binary floating point cannot represent decimal fractions exactly, so some rounding always occurs (even in Python 1.5.2).

What changed is that Python 2.0 shows more precision than before in certain circumstances (repr() and the interactive prompt).

You can use str() or print to get the old, rounded output:

print 0.1+0.1 0.2

Follow the link for more information:

http://python.sourceforge.net/devel-docs/tut/node14.html

You can respond by visiting: http://sourceforge.net/tracker/?func=detail&atid=105470&aid=473456&group_id=5470