I just realized... where() belongs to Numeric, so I need sum([Numeric.where(a < 255, a, 0) for a in y]) duh. I did just compare Numeric vs. Masked arrays: =========================================================== # test.py from MA import masked_array, sum from RandomArray import * import time seed() y = randint(240,256, (480,640,16)) start = time.time() x=masked_array(y, y>=255) maskTime = time.time() - start sum_1 = sum(x,axis=2) maskedTime = time.time() - start print sum_1.shape print sum_1 print "mask make time: " + str(maskTime) print "time using MA: " + str(maskedTime) + "\n" z = Numeric.reshape(y, (16, 480, 640)) newStart = time.time() sum_2 = sum([Numeric.where(a < 255, a, 2) for a in z]) numTime = time.time() - newStart print sum_2.shape print sum_2 print "time using Numeric: " + str(numTime) + "\n" ====================================================== Result: C:\projects\Astro>python test.py (480, 640) array (480,640) , type = O, has 307200 elements mask make time: 1.07899999619 time using MA: 3.39100003242 (480, 640) array (480,640) , type = l, has 307200 elements time using Numeric: 2.39099979401 So, MA's sum() is slightly faster, but the penalty for making a mask first is large. Now I have to figure out why I had to reshape the array for the second computation. Thanks, Ray At 09:17 AM 1/26/2004 +0100, you wrote:

On 26.01.2004, at 07:14, RJS wrote:

...

The problem: I have a "stack" of 8, 640 x 480 integer image arrays from a FITS cube concatenated into a 3D array, and I want to sum each pixel such that the result ignores clipped values (255+); i.e., if two images have clipped pixels at (x,y) the result along z will be the sum of the other 6. Memory doesn't seem critical for such small arrays, so you can just do

sum([where(a < 255, a, 0) for a in images])

Hello Konrad, I just tried: from MA import masked_array, sum from RandomArray import * seed() y = randint(240,256, (480,640,2)) print sum([where(a < 255, a, 0) for a in y]) and it errors: Traceback (most recent call last): File "test.py", line 21, in ? print sum([where(a < 255, a, 0) for a in y]) NameError: name 'where' is not defined Could you enlighten me further? I have not found a good resource for compound Numeric statements yet. Thank you, Ray