Robert Kern wrote: > Sheldon wrote: > > Hi, > > > > I am trying to build a large array using concatenate function in > > python. > > So as I loop over the number of arrays, of which there are 12 (4 down > > and 3 across), I create 3 long arrays by concatenating them at the > > bottom and then concatenating them side by side: > > [snip] > > > print 'Max lat', max(max(ppslat_all)), '\t','Min lat', > > min(min(ppslat_all)) > > print 'Max lon', max(max(ppslon_all)), '\t','Min lon', > > min(min(ppslon_all)) > > > > ***************** > > Now this works, the array size is correct but the longitude values > > given for max and min are wrong. What is even stranger to me is that > > when I write the array in binary format to a file and read it with > > Matlab, the max and min are correct but when I read it back with python > > the max and min are again incorrect for only the longitude data. I > > saved the max and min for the longitude for each array and then check > > it in the python program and they are correct at the end but the > > max(max(ppslon)) values is incorrect. Does anyone knows why this is > > so? > > If I was doing something wrong then Matlab would not have returned > > correct values. > > Don't use min() and max() on multidimensional arrays. They won't give sensible > answers. > > > In [11]: a = RA.random([3,5]) > > In [12]: a > Out[12]: > array([[ 0.01721657, 0.64291363, 0.33210659, 0.89887972, 0.24437849], > [ 0.88205348, 0.00839329, 0.35999039, 0.9966411 , 0.54957126], > [ 0.59983864, 0.18983323, 0.13727718, 0.8987289 , 0.05425076]]) > > In [13]: min(a) > Out[13]: array([ 0.59983864, 0.18983323, 0.13727718, 0.8987289 , > 0.05425076]) > > > The builtin min() and max() compare the values in the sequence. In this case, > those values are the rows of the arrays. Numeric uses rich comparisons, so the > result of a comparison is a boolean array. Numeric also has the convention > that > if any of the elements of an array are considered to be True, then the whole > array is. > > > In [16]: a[1] < a[2] > Out[16]: array([0, 1, 0, 0, 0]) > > In [17]: bool(_) > Out[17]: True > > In [18]: a[2] < a[1] > Out[18]: array([1, 0, 1, 1, 1]) > > In [19]: bool(_) > Out[19]: True > > > This makes min(a) incorrect when len(a.shape) > 1. Instead, use the minimum > and > maximum ufuncs provided with Numeric: > > > In [21]: N.minimum.reduce(a.flat) > Out[21]: 0.0083932917161983426 > > In [22]: N.maximum.reduce(a.flat) > Out[22]: 0.99664110397663608 > > > -- > Robert Kern > > "I have come to believe that the whole world is an enigma, a harmless enigma > that is made terrible by our own mad attempt to interpret it as though it > had > an underlying truth." > -- Umberto Eco
Hi Robert, Thanks again for showing me this. I have been trying to read up on reduce() as I have never used it before. I would like to know what it does. So far my search has found nothing that I can grasp. The reference library notes are estoteric at best. Can you enlighten me on this matter?' /Sheldon -- http://mail.python.org/mailman/listinfo/python-list
