opPow, opDollar

[Andrei Alexandrescu]

Robert Jacques wrote:
> On Sat, 07 Nov 2009 12:56:35 -0500, Andrei Alexandrescu 
> <SeeWebsiteForEmail at erdani.org> wrote:
> 
>> Robert Jacques wrote:
>>> I'd recommend rolling that into a basic statistics struct containing 
>>> common single pass metrics: i.e. sum, mean, variance, min, max, etc.
>>
>> Well the problem is that if you want to compute several one-pass 
>> statistics in one pass, you'd have to invent means to combine these 
>> functions. That ability is already present in reduce, e.g. reduce(min, 
>> max)(range) yields a pair containing the min and the max element after 
>> exactly one pass through range.
>>
>> Andrei
> 
> Yes, but reduce(mean, std)(range) doesn't work.

 From std.algorithm's doc:

// Compute sum and sum of squares in one pass
r = reduce!("a + b", "a + b * b")(tuple(0.0, 0.0), a);
// Compute average and standard deviation from the above
auto avg = r.field[0] / a.length;
auto stdev = sqrt(r.field[1] / a.length - avg * avg);

I'm not saying there's no need for a more specialized library, just that 
I purposely designed reduce to be no slouch either.

> Even reduce(count) would 
> require the range to be mapped.

(This I don't get.)

> Besides, in my use case I need lazy 
> evaluation, and I'd much rather add elements to a statistics struct, 
> than write a range wrapper.

Well if you go for surgery on an existing struct then the opportunity 
for reuse is diminished.


Andrei