Gael Varoquaux wrote: > On Fri, Apr 25, 2008 at 01:40:29PM -0400, Alan G Isaac wrote: >> In contrast, there *is* universal agreement that >> x[0][0]==x[0,0] is desirable. Or so I've understood the >> discussion.
> I don't know why people are indexing matrices with A[x][y], but they > shouldn't. I think there has been a misunderstanding here. I don't think anyone is suggesting that if a coder wants an element of a matrix, that s/he should write that: element = M[i,j] Rather, that one might want to extract a row from a Matrix, and then index THAT object with a scalar: row = M[i] now do something with each element in that row: something = row[j] This is a rational, normal thing to do, and I think the desire for this is the core of this entire discussion, coming from the fact that in the current version of matrix both of M[i] and M[i,:] yield a matrix, which is 2-d, and cannot be indexed with a scalar. This is odd, as it is pretty natural to expect a single row (or column) to behave like a 1-d object. Alan G Isaac wrote: > I believe that this conflicts with submatrix extraction. > If we want to keep submatrix extraction (as we should), why? with 2-d arrays, you get a subarray with: A[i:j,:] and a 1-d array with A[i,:] -- why does that need to be different for Matrices? > 3. If ``v`` is a "RowVector" what behavior do we get? > Suppose the idea is that this will rescue > ``x[0][0]==x[0,0]`` **and** ``x[0]=x[0,:]``. > But then we must give up that ``x[0,:]`` is a submatrix. Correct, but why should it be -- we can get a submatrix with slicing, as above. Indeed, I was about to post this comment the other day, as someone was concerned that there needs to be a distinction between a ColumnVector and a Matrix that happens to have a second dimension of one. I think that the Vector proposal satisfies that: if you have code like: SubMatrix = M[i:j, k] Then you will always get a SubMatrix, even if j == i+1. If you index like so: Vector = M[i,k] you will always get a vector (a 1-d object) > Must ``v`` deviate from submatrix behavior in an important way? > Yes: ``v[0][0]`` is an IndexError. Correct, but if you're writing the code that generated that vector, you'd know it was a 1-d object. > Since submatrix extraction is fundamental, I think it is > a *very* bad idea to give it up. I agree -- but we're not giving it up, what we are doing is making a distinction between extracting a single row or column, and extracting a submatrix (that may or may not be a single row or column) -- just like that distinction is make for regular old ndarrays. > RowVector proposal we must give up ``x[0]==x[0,:]``. > But this is just what we give up with > the much simpler proposal that ``v`` be a 1d array. no, we also give up that v acts like a row or column (particularly column) vector in computation (*, **) We still need real linear algebra computation examples.... Alan G Isaac wrote: > I weight the future more heavily. We are approaching a last > chance to do things better, and we should seize it. Yes, but it seems that while a consensus will not be reached in time for 1.1, there is one that a change will probably occur in the next version, so there is a lot to be said for waiting until a proposal is settled on, then make the whole change at once. > The right questions looking forward: > > - what behavior allows the most generic code? > - what behavior breaks fewest expectations? > - what behavior is most useful? Yes, but I'm going to try to put down what I think are the key, very simple, questions: 1) Do we want to be able to extract a single row or column from a Matrix, and have it be indexable like a 1-d object? 2) Do we want to be able to do that without having to explicitly call the Array attribute: M.A[i]? 3) Do we want to have those 1-d objects act like Row or Column vectors in linear algebra operations (and broadcasting)? 4) Do we want to require slice notation to get a submatrix that happens to have a single row or column? If we want (1) and (2), then we need to make a change. If we want (3) and (4), then something like the Row/ColumnVector proposal is needed. I really think it's that simple. -Chris -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception [EMAIL PROTECTED] _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion
