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| -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): |