[Numpy-discussion] Inverting argsort(a, axis=0) to obtain column-wise ranks

[Robert Kern]

On Thu, Sep 9, 2010 at 15:59, Alexander Michael <lxander.m at gmail.com> wrote:
> Clever and concise (and expect that it works), but isn't this less
> efficient? Sorting is O(n*log(n)), while the code I gave is O(n).
> Using argsort has the potential to use less memory, though.

No, the code you gave is also O(n*log(n)) because it uses an
argsort(). Using two argsort()s won't change the O() complexity. O()
often tells you very little. Time it with typical values and find out.
Memory use is often the bottleneck. And sometimes, the real
performance differences just don't matter.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco