[Numpy-discussion] please change mean to use dtype=float
Tim Hochberg
tim.hochberg at ieee.org
Thu Sep 21 17:56:47 CDT 2006
David M. Cooke wrote:
> On Thu, 21 Sep 2006 11:34:42 -0700
> Tim Hochberg <tim.hochberg at ieee.org> wrote:
>
>
>> Tim Hochberg wrote:
>>
>>> Robert Kern wrote:
>>>
>>>
>>>> David M. Cooke wrote:
>>>>
>>>>
>>>>
>>>>> On Wed, Sep 20, 2006 at 03:01:18AM -0500, Robert Kern wrote:
>>>>>
>>>>>
>>>>>
>>>>>> Let me offer a third path: the algorithms used for .mean() and .var()
>>>>>> are substandard. There are much better incremental algorithms that
>>>>>> entirely avoid the need to accumulate such large (and therefore
>>>>>> precision-losing) intermediate values. The algorithms look like the
>>>>>> following for 1D arrays in Python:
>>>>>>
>>>>>> def mean(a):
>>>>>> m = a[0]
>>>>>> for i in range(1, len(a)):
>>>>>> m += (a[i] - m) / (i + 1)
>>>>>> return m
>>>>>>
>>>>>>
>>>>>>
>>>>> This isn't really going to be any better than using a simple sum.
>>>>> It'll also be slower (a division per iteration).
>>>>>
>>>>>
>>>>>
>>>> With one exception, every test that I've thrown at it shows that it's
>>>> better for float32. That exception is uniformly spaced arrays, like
>>>> linspace().
>>>>
>>>> > You do avoid
>>>> > accumulating large sums, but then doing the division a[i]/len(a) and
>>>> > adding that will do the same.
>>>>
>>>> Okay, this is true.
>>>>
>>>>
>>>>
>>>>
>>>>> Now, if you want to avoid losing precision, you want to use a better
>>>>> summation technique, like compensated (or Kahan) summation:
>>>>>
>>>>> def mean(a):
>>>>> s = e = a.dtype.type(0)
>>>>> for i in range(0, len(a)):
>>>>> temp = s
>>>>> y = a[i] + e
>>>>> s = temp + y
>>>>> e = (temp - s) + y
>>>>> return s / len(a)
>>>>>
>>>>
>>>>
>>>>>> def var(a):
>>>>>> m = a[0]
>>>>>> t = a.dtype.type(0)
>>>>>> for i in range(1, len(a)):
>>>>>> q = a[i] - m
>>>>>> r = q / (i+1)
>>>>>> m += r
>>>>>> t += i * q * r
>>>>>> t /= len(a)
>>>>>> return t
>>>>>>
>>>>>> Alternatively, from Knuth:
>>>>>>
>>>>>> def var_knuth(a):
>>>>>> m = a.dtype.type(0)
>>>>>> variance = a.dtype.type(0)
>>>>>> for i in range(len(a)):
>>>>>> delta = a[i] - m
>>>>>> m += delta / (i+1)
>>>>>> variance += delta * (a[i] - m)
>>>>>> variance /= len(a)
>>>>>> return variance
>>>>>>
>> I'm going to go ahead and attach a module containing the versions of
>> mean, var, etc that I've been playing with in case someone wants to mess
>> with them. Some were stolen from traffic on this list, for others I
>> grabbed the algorithms from wikipedia or equivalent.
>>
>
> I looked into this a bit more. I checked float32 (single precision) and
> float64 (double precision), using long doubles (float96) for the "exact"
> results. This is based on your code. Results are compared using
> abs(exact_stat - computed_stat) / max(abs(values)), with 10000 values in the
> range of [-100, 900]
>
> First, the mean. In float32, the Kahan summation in single precision is
> better by about 2 orders of magnitude than simple summation. However,
> accumulating the sum in double precision is better by about 9 orders of
> magnitude than simple summation (7 orders more than Kahan).
>
> In float64, Kahan summation is the way to go, by 2 orders of magnitude.
>
> For the variance, in float32, Knuth's method is *no better* than the two-pass
> method. Tim's code does an implicit conversion of intermediate results to
> float64, which is why he saw a much better result. The two-pass method using
> Kahan summation (again, in single precision), is better by about 2 orders of
> magnitude. There is practically no difference when using a double-precision
> accumulator amongst the techniques: they're all about 9 orders of magnitude
> better than single-precision two-pass.
>
> In float64, Kahan summation is again better than the rest, by about 2 orders
> of magnitude.
>
> I've put my adaptation of Tim's code, and box-and-whisker plots of the
> results, at http://arbutus.mcmaster.ca/dmc/numpy/variance/
>
> Conclusions:
>
> - If you're going to calculate everything in single precision, use Kahan
> summation. Using it in double-precision also helps.
> - If you can use a double-precision accumulator, it's much better than any of
> the techniques in single-precision only.
>
> - for speed+precision in the variance, either use Kahan summation in single
> precision with the two-pass method, or use double precision with simple
> summation with the two-pass method. Knuth buys you nothing, except slower
> code :-)
>
> After 1.0 is out, we should look at doing one of the above.
>
One more little tidbit; it appears possible to "fix up" Knuth's
algorithm so that it's comparable in accuracy to the two pass Kahan
version by doing Kahan summation while accumulating the variance.
Testing on this was far from thorough, but in the tests I did it nearly
always produced identical results to kahan. Of course this is even
slower than the original Knuth version, but it's interesting anyway:
# This is probably messier than it need to be
# but I'm out of time for today...
def var_knuth2(values, dtype=default_prec):
"""var(values) -> variance of values computed using
Knuth's one pass algorithm"""
m = variance = mc = vc = dtype(0)
for i, x in enumerate(values):
delta = values[i] - m
y = delta / dtype(i+1) + mc
t = m + y
mc = y - (t - m)
m = t
y = delta * (x - m) + vc
t = variance + y
vc = y - (t - variance)
variance = t
assert type(variance) == dtype
variance /= dtype(len(values))
return variance
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