On Monday, October 30, 2006, at 08:30AM, "Arthur" <[EMAIL PROTECTED]> wrote: > >Looking for a little education on edu-sig. > >On one hand I am feeling like a big boy, having announced today on the >vpython list that I think I have adequately accomplished the necessary >fixes to the vpython C++ code to accomplish compatibility with the newly >released numpy 1.0, and in a way that I think in the end (it was a >round-about process for me) is straightforward and unlikely to >introduce bugs into the existing stable codebase. > >OTOH, I am learning there is some very basic Python I do not understand. > >Now having a vpython that runs against numpy, I am learning about some >of the changes in the numpy Python API versus that of Numeric and >numarray. One basic incompatibility is in that the older libraries did >something to give a result in comparing 2 arrays that numpy backs away >from - complaining about ambiguity. > >For example, in the vpython faces_heightfield.py demo there is a line: > > >>>normal[i] = vertex_map[tp] and norm( vertex_map[ tp ] ) > >which ran fine under Numeric and numarray, and now fails with a message: > >""" >The truth value of an array with more than one element is ambiguous. >Use a.any() or a.all() >""" > >For the purposes of the exercise assume that both "vertex_map[tp]" and >"norm( vertex_map[ tp ]" >each return one dimensional, 3 element numpy arrays. Not exactly true, >but I think the result would be the same if it were. > >In the original code, what is the "and" doing, and in what way might the >"any" or "all" built-ins be used to get the result intended by it?
I've not used .any or .all, but having just taught my CS1 students about boolean operators, I was reminded that Python works as the following example describes: x = a and b # if both a and b are true, x is assigned b, otherwise x is assigned a x = 2 and 3 # x is assigned 3 x = 0 and 2 # x is assigned 0 x = a or b # if a is true, x is assigned a, if a is true, x is assigned a, if a is false and b is true, x is assigned b, if both a and b are false, x is assigned False You need to determine what should make vertex_map[tp] be true (maybe it is all the values being non-zero or maybe it is just the array being non-empty) and assuming norm is then a single floating point value, if it's non-zero, it is true. And then assign normal[i] the appropriate one. HTH, Dave _______________________________________________ Edu-sig mailing list Edu-sig@python.org http://mail.python.org/mailman/listinfo/edu-sig
