[Numpy-discussion] My identity

Charles R Harris charlesr.harris@gmail....
Mon Jul 20 11:03:31 CDT 2009


On Mon, Jul 20, 2009 at 9:39 AM, Keith Goodman <kwgoodman@gmail.com> wrote:

> Using a trick that Robert Kern recently posted to the list makes the
> identity function much faster.
>
> Current version:
>
> def identity(n, dtype=None):
>    a = array([1]+n*[0],dtype=dtype)
>    b = empty((n,n),dtype=dtype)
>    b.flat = a
>    return b
>
> Proposed version:
>
> def myidentity(n, dtype=None):
>    a = zeros((n,n), dtype=dtype)
>    a.flat[::n+1] = 1
>    return a
>
> >> timeit identity(1)
> 100000 loops, best of 3: 14.9 µs per loop
> >> timeit identity(10)
> 10000 loops, best of 3: 20 µs per loop
> >> timeit identity(100)
> 1000 loops, best of 3: 696 µs per loop
> >> timeit identity(1000)
> 10 loops, best of 3: 73.6 ms per loop
>
> >> timeit myidentity(1)
> 100000 loops, best of 3: 6.57 µs per loop
> >> timeit myidentity(10)
> 100000 loops, best of 3: 7.08 µs per loop
> >> timeit myidentity(100)
> 100000 loops, best of 3: 16.4 µs per loop
> >> timeit myidentity(1000)
> 100 loops, best of 3: 5.92 ms per loop
>
> It would also speed up the functions that use identity (for example
> np.linalg.inv). And it would make identity faster than eye.
>
> Are there any corner cases that the new version doesn't handle? Any
> other issues?


I believe r7130 made the change.

Chuck
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