Timothy Hochberg wrote: > > > On 9/11/07, *Robert Kern* <[EMAIL PROTECTED] > <mailto:[EMAIL PROTECTED]>> wrote: > > Mike Ressler wrote: > > The following seems to be a wart: is it expected? > > > > Set up a 10x10 array and some indexing arrays: > > > > a=arange(100) > > a.shape=(10,10) > > q=array([0,2,4,6,8]) > > r=array([0,5]) > > > > Suppose I want to extract only the "even" numbered rows from a - > then > > > > print a[q,:] > > > > <works - output deleted> > > > > Every fifth column: > > > > print a[:,r] > > > > <works - output deleted> > > > > Only the even rows of every fifth column: > > > > print a[q,r] > > > > > > --------------------------------------------------------------------------- > > > <type 'exceptions.ValueError'> Traceback (most recent > call last) > > > > /.../.../.../<ipython console> in <module>() > > > > <type 'exceptions.ValueError '>: shape mismatch: objects cannot be > > broadcast to a single shape > > > > But, this works: > > > > print a[q,:][:,r] > > > > [[ 0 5] > > [20 25] > > [40 45] > > [60 65] > > [80 85]] > > > > So why does the a[q,r] form have problems? Thanks for your insights. > > It is intended that the form a[q,r] be the general case: q and r > are broadcasted > against each other to a single shape. The result of the indexing > is an array of > that broadcasted shape with elements found by using each pair of > elements in the > broadcasted q and r arrays as indices. > > There are operations you can express with this form that you > couldn't if the > behavior that you expected were the case whereas you can get the > result you want > relatively straightforwardly. > > In [6]: a[q[:,newaxis], r] > Out[6]: > array([[ 0, 5], > [20, 25], > [40, 45], > [60, 65], > [80, 85]]) > > > > At the risk of making Robert grumpy: while it is true the form we > ended up with is more general I've come to the conclusion that it was > a bit of a mistake. In the spirit of making simple things simple and > complex things possible, I suspect that having fancy-indexing do the > obvious thing here[1] and delegating the more powerful but also more > difficult to understand case to a function or method would have been > overall more useful. Cases where the multidimensional features of > fancy-indexing get used are messy enough that they don't benefit much > from the conciseness of the indexing notation, at least in my experience. This is a reasonable argument. It is reasonable enough that I intentionally made an ix_ function to do what you want.
a[ix_(q,r)] does as originally expected if a bit more line-noise. -Travis _______________________________________________ Numpy-discussion mailing list [email protected] http://projects.scipy.org/mailman/listinfo/numpy-discussion
