[Numpy-discussion] A faster median (Wirth's method)

Citi, Luca lciti@essex.ac...
Wed Sep 2 03:50:03 CDT 2009


Hello Sturla,
I had a quick look at your code.
Looks fine.

A few notes...

In "select" you should replace numpy with np.

In "_median" how can you, if n==2, use s[] if s is not defined?
What if n==1?
Also, I think when returning an empty array, it should be of
the same type you would get in the other cases.

You could replace _median with the following.

Best,
Luca


def _median(x, inplace):
    assert(x.ndim == 1)
    n = x.shape[0]
    if n > 2:
        k = n >> 1
        s = select(x, k, inplace=inplace)
        if n & 1:
            return s[k]
        else:
            return 0.5*(s[k]+s[:k].max())      
    elif n == 0:
        return np.empty(0, dtype=x.dtype)
    elif n == 2:
        return 0.5*(x[0]+x[1])        
    else: # n == 1
        return x[0]



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