Numeric help!

[Carl Banks]

Sheldon wrote:
>Carl Banks wrote:
>> I'm not sufficiently sure this isn't a homework problem, so here's a
>> partial answer.
[snip]
>
> 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.

Are you aware of this guide?

http://numeric.scipy.org/numpydoc/numdoc.htm

I've seen better documents but it's fairly complete--I wouldn't call it
improper.


> 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

This appears to be doing something you don't expect.  In Python all
names are references; assignments don't create new objects but rather
new names for the same object.  In the above, wk and msvga are the same
array: any changes in wk also appear in msvga.  So wk[1,1] = 4 also
causes msgva[1,1] to be 4.

Numeric arrays even share data when slicing.  If you were to take a
slice of wk, for example, a = wk[0:10,0:10], then modifying a would
also show up in wk (and msvga).  (However, slicing most other objects
copies rather than shares the data.)

It's completely different from Matlab, which always copies data.

Here's what you should do:

    wk = array(msgva) # this creates a new array for wk
    sattmp = array(sat_id)

>                 mask = where(equal(wk,z),1,0) # creating a mask of
> valid data pixs

Note that the "where" is unnecessary here.  equal creates an array of
ones and zeros.

>                 putmask(sattmp,mask==0,-999)  # since zero is valid
> data, -999 is used instead

This would overwrite sat_id unless you copy the array as I've shown
above.

>                 rdata =
> compress(ravel(not_equal(sattmp,-999)),ravel(sattmp))

I believe you could avoid the above putmask above step and just use
ravel(mask) as the first argument of compress here, or even just
equal(wk,z).

"equal(wx,z)", "mask", and "not_equal(sattmp,-999)" are all equal
arrays.

>                 if sum(sum(rdata)) == 0:
>                     av = 0

rdata is one-dimensional, so you only need one sum call; but why do
this?  average will still work if the sum is zero.

>                 else:
>                     av = average(rdata,axis=None)

axis argument isn't necessary here since rdata is one-dimesional.

>                 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

Other than the array sharing mistake, it looks like it should work.

Also, note that this group disdains top-posting; replies should go
below the quoted text because in these sorts of discussions it's best
to keep things in conversational order, especially for the sake of
other interested readers.

Good luck.


Carl Banks