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author | Raymond Hettinger <python@rcn.com> | 2009-04-26 20:10:50 (GMT) |
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committer | Raymond Hettinger <python@rcn.com> | 2009-04-26 20:10:50 (GMT) |
commit | f0320c7ee6863eb359bbfce7f69420dedefb8538 (patch) | |
tree | bd1b0feceba9d7453caf7f0d6ec4a9bf3191cec4 /Doc/tutorial/floatingpoint.rst | |
parent | ce95e5646bdd1b74b6f02386b2dfac87b666fc23 (diff) | |
download | cpython-f0320c7ee6863eb359bbfce7f69420dedefb8538.zip cpython-f0320c7ee6863eb359bbfce7f69420dedefb8538.tar.gz cpython-f0320c7ee6863eb359bbfce7f69420dedefb8538.tar.bz2 |
Remove the round() discussion which is now out-of-date.
Diffstat (limited to 'Doc/tutorial/floatingpoint.rst')
-rw-r--r-- | Doc/tutorial/floatingpoint.rst | 19 |
1 files changed, 2 insertions, 17 deletions
diff --git a/Doc/tutorial/floatingpoint.rst b/Doc/tutorial/floatingpoint.rst index f060e69..78dc1d6 100644 --- a/Doc/tutorial/floatingpoint.rst +++ b/Doc/tutorial/floatingpoint.rst @@ -108,23 +108,8 @@ reproduce *x*, but the output may be more pleasant to look at:: It's important to realize that this is, in a real sense, an illusion: you're simply rounding the *display* of the true machine value. -Other surprises follow from this one. For example, after seeing :: - - >>> format(0.1, '.17g') - '0.10000000000000001' - -you may be tempted to use the :func:`round` function to chop it back to the -single digit you expect. But that makes no difference:: - - >>> format(round(0.1, 1), '.17g') - '0.10000000000000001' - -The problem is that the binary floating-point value stored for "0.1" was already -the best possible binary approximation to 1/10, so trying to round it again -can't make it better: it was already as good as it gets. - -Another consequence is that since 0.1 is not exactly 1/10, summing ten values of -0.1 may not yield exactly 1.0, either:: +One illusion may beget another. For example, since 0.1 is not exactly 1/10, +summing ten values of 0.1 may not yield exactly 1.0, either:: >>> sum = 0.0 >>> for i in range(10): |