Numeric help!

[Sheldon]

Hi Carl,

My days as a student is over for the most part. I am learning python on
my own and Numeric is not properly documented so I am learning by doing
and copying from others. I thought about the problem and a solution
another problem given to me earlier using "putmask" is the solution but
there is a bug that I cannot figure out:
************************
            index = 0
            for z in range_va:
                wk     = msgva # working arrary
                sattmp = sat_id # working array
                mask = where(equal(wk,z),1,0) # creating a mask of
valid data pixs
                putmask(sattmp,mask==0,-999)  # since zero is valid
data, -999 is used instead
                rdata =
compress(ravel(not_equal(sattmp,-999)),ravel(sattmp))
                if sum(sum(rdata)) == 0:
                    av = 0
                else:
                    av = average(rdata,axis=None)
                tmparray[0,index] = av
                tmparray[1,index] = z
                index += 1
            print tmparray
***********************************
But the tmparray is returning zeros as averages. When I try just one
value for z everything works. I can't see where I going wrong. I am not
using the original arrays, only the copies and when a new z is chosen
then these are recreated.

Care to help out with this?
/Sheldon

Carl Banks wrote:
> Sheldon wrote:
> > Hi,
> >
> > I have the following loop that I think can be written to run faster in
> > Numeric. I am currently using Numeric.
> > range_va = [60,61,62,63,64,65,66,67,68,69,70,71,72]
> > main.xsize= 600
> > main.ysize= 600
> > #msgva is an (600x600) Numeric array with mutiple occurrences of the
> > values in range_va
> > #sat_id is an (600x600) Numeric array with values ranging from -2 to 2
> > for z in range_va:
> >             count = 0
> >             mbias = 0
> >             for i in range(main.xsize):
> >                 for j in range(main.ysize):
> >                     if msgva[i,j] == z:
> >                         mbias += sat_id[i,j] # take the sum of the
> > values
> >                         count += 1 # count the occurrences
> >             tmp_array[0,index] = round(mbias/count,1) # store the mean
> >             tmp_array[1,index] = z
> >             index += 1
> >
> > Any help would be greatly appreciated!
>
> I'm not sufficiently sure this isn't a homework problem, so here's a
> partial answer.
>
> Your intuition is correct--there's almost always a faster way to do it
> when you're cycling through Numeric array indices in a Python for loop.
>  Numeric and successors exist mostly to get rid of them.
>
> Given z, you can calculate mbias in one line:
>
>     mbias = Numeric.sum(
>             Numeric.ravel(Numeric.where(msgva==z,sat_id,0)))
>
> Here, the Numeric.where function allows you to filter out entries: it
> returns an array with the elements of sat_id, but only  where the
> corresponding element of msvga==z; otherwise the entry is zero.
> Numeric.ravel flattens an multidimensional array into one dimension,
> and, of course, Numeric.sum adds up all the elements in the array.
>
> How to get count and to work this into your loop are left as an
> exercise.
> 
> 
> Carl Banks