[Numpy-discussion] Efficient array equivalent to cmp(x,y)

Kim Hansen slaunger@gmail....
Thu Sep 17 15:13:58 CDT 2009


Hi,

Is there an array-like function equivalent with the builtin method for the
Python single-valued comparison cmp(x,y)?

What I would like is a cmp(a, lim), where a is an ndarray and lim is a
single value, and then I need an array back of a's shape giving the
elementwise comparison
array([cmp(a[0], lim), cmp(a[1], lim), ...])

I can do it somewhat ackwardly doing this:

In [1]: a = randint(5, size=10); print a
[0 2 4 1 3 0 3 4 0 1]
In [2]: lim = 2
In [3]: acmp = empty(a.shape, dtype='i1')
In [4]: acmp[a < lim] = -1
In [5]: acmp[a == lim] = 0
In [6]: acmp[a > lim] = 1
In [7]: acmp
Out[7]: array([-1,  0,  1, -1,  1, -1,  1,  1, -1, -1], dtype=int8)

But that is not very elegant and since this is a computational bottleneck I
would rather like to avoid all the intermediate creations of three mask
arrays for fancy indexing in this example.

Is there a simpler/more elegant way to acheive the same result?

Thanks in advance,

Kim
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