[Numpy-discussion] simple array multiplication question
per freem
perfreem@gmail....
Mon Oct 12 22:53:05 CDT 2009
hi all,
i am trying to write a simple product of 3 arrays (as vectorized code)
but am having some difficulty. i have three arrays, one is a list
containing several lists:
p = array([[ 0.2, 0.8], [ 0.5, 0.5], [ 0.3, 0.7]])
each list in the array 'p' is of size N -- in this case N = 2. i have
a second array containing a set of numbers, each between 0 and N-1:
l = array([0, 0, 1])
and finally an array of the same size as l:
s = array([10, 20, 30])
what i want to do is pick the columns l of p, and multiply each one by
the numbers in s. the first step, picking columns l of p, is simply:
cols = p[arange(3), l]
then i want to multiply each one by the numbers in s, and i do it like this:
cols * s.reshape(3,1)
this seems to work, but i am concerned that it might be inefficient.
is there a cleaner way of doing this? is 'arange' operation necessary
to reference all the 'l' columns of p? also, is the reshape operation
expensive?
thanks very much.
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