# HG changeset patch # User David Bateman # Date 1225375730 -3600 # Node ID 242756f065f08bc846ae1399b0163721c1acf76b # Parent 53f35799b2355409f6b3883069cfab2136127765 Minor NEWS/FAQ update diff -r 53f35799b235 -r 242756f065f0 ChangeLog --- a/ChangeLog Thu Oct 30 14:15:12 2008 +0100 +++ b/ChangeLog Thu Oct 30 15:08:50 2008 +0100 @@ -1,3 +1,7 @@ +2008-10-30 David Bateman + + * NEWS: Minor update to document improved indexing code. + 2008-10-23 Jason Riedy * configure.in: Take care to use FT2_CFLAGS when checking diff -r 53f35799b235 -r 242756f065f0 NEWS --- a/NEWS Thu Oct 30 14:15:12 2008 +0100 +++ b/NEWS Thu Oct 30 15:08:50 2008 +0100 @@ -105,4 +105,8 @@ so users that make use of data generated by Octave with R or visa-versa are warned that compatibility might not be assured. + ** Improved array indexing + The underlying code used for indexing of arrays has been completely + rewritten and so the indexing of arrays is now significantly faster. + See NEWS.3 for old news. diff -r 53f35799b235 -r 242756f065f0 doc/ChangeLog --- a/doc/ChangeLog Thu Oct 30 14:15:12 2008 +0100 +++ b/doc/ChangeLog Thu Oct 30 15:08:50 2008 +0100 @@ -1,5 +1,9 @@ 2008-10-30 David Bateman + * faq/Octave-FAQ.texi: Document improved indexing and add an faq for + the compatibility of mldivide/mrdivide for singualr, under- and + over-determined matrices. + * interpreter/plot.txi: Document contour groups. 2008-10-29 Thorsten Meyer diff -r 53f35799b235 -r 242756f065f0 doc/faq/Octave-FAQ.texi --- a/doc/faq/Octave-FAQ.texi Thu Oct 30 14:15:12 2008 +0100 +++ b/doc/faq/Octave-FAQ.texi Thu Oct 30 15:08:50 2008 +0100 @@ -251,6 +251,10 @@ underlying LAPACK code. @item Single precision type + +@item Improved array indexing +The underlying code used for indexing of arrays has been completely +rewritten and so the indexing of arrays is now significantly faster. @end itemize @@ -861,7 +865,7 @@ @item Block comments Block comments denoted by "%@{" and "%@}" markers are supported by Octave with some limitations. The major limitation is that block -comments are not supported with [] or @{@}. +comments are not supported within [] or @{@}. @item Mat-File format There are some differences in the mat v5 file format accepted by @@ -981,6 +985,80 @@ any elements are nonzero. Octave however duplicates this behavior for if statements containing empty matrices. +@item Solvers for singular, under- and over-determined matrices + +Matlab's solvers as used by the operators mldivide (\) and mrdivide (/), +use a different approach than Octave's in the case of singular, under-, +or over-determined matrices. In the case of a singular matrix, Matlab +returns the result given by the LU decomposition, even though the underlying +solver has flagged the result as erroneous. Octave has made the choice +of falling back to a minimum norm solution of matrices that have been +flagged as singular which arguably is a better result for these cases. + +In the case of under- or over-determined matrices, Octave continues to +use a minimum norm solution, whereas Matlab uses an approach that is +equivalent to + +@example +@group +function x = mldivide (A, b) + [Q, R, E] = qr(A); + x = [A \ b, E(:, 1:m) * (R(:, 1:m) \ (Q' * b))] +end +@end group +@end example + +@noindent +While this approach is certainly faster and uses less memory than +Octave's minimum norm approach, this approach seems to be inferior in +other ways. + +A numerical question arises: how big can the null space component become, +relative to the minimum-norm solution? Can it be nicely bounded, or can it be +arbitrarily big? Consider this example: + +@example +@group +m = 10; +n = 10000; +A = ones(m, n) + 1e-6 * randn(m,n); +b = ones(m, 1) + 1e-6 * randn(m,1); +norm(A \ b) +@end group +@end example + +@noindent +while Octave's minimum-norm values are around 3e-2, Matlab's results +are 50-times larger. For another issue, try this code: + +@example +@group +m = 5; +n = 100; +j = floor(m * rand(1, n)) + 1; +b = ones(m, 1); +A = zeros(m, n); +A(sub2ind(size(A),j,1:n)) = 1; +x = A \ b; +[dummy,p] = sort(rand(1,n)); +y = A(:,p)\b; +norm(x(p)-y) +@end group +@end example + +@noindent +It shows that unlike in Octave, mldivide in Matlab is not invariant +with respect to column permutations. If there are multiple columns of +the same norm, permuting columns of the matrix gets you different +result than permuting the solution vector. This will surprise many +users. + +Since the mldivide (\) and mrdivide (/) operators are often part of a more +complex expression, where there is no room to react to warnings or flags, it +should prefer intelligence (robustness) to speed, and so the Octave developers +are firmly of the opinion that Octave's approach for singular, under- and +over-determined matrices is a better choice that Matlab's + @item Octave extensions The extensions in Octave over @sc{Matlab} syntax are very useful, but might cause issues when sharing with @sc{Matlab} users.