Carl Banks wrote: > 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
Thanks for the tips about the array and how it is copied. I figured this out late yesterday and wrote a dirty solution to this problem but your is more elegant and concise. Yes, where statement is not needed and I removed it. The replacement is the following: average(compress(ravel(equal(wk,z)),ravel(sattmp)),axis=None) This is much more compact and elegant. Thanks for pointing this out. I don't know why average() returned a divide by zero error and to avoid this I inserted this if statement. Now it works much better ! Much obliged, Sheldon -- http://mail.python.org/mailman/listinfo/python-list
