Improved triu.m and tril.m

[Marco Caliari]

Dear maintainers,

although triu and tril are built-in functions in Matlab and then much 
faster, I discovered that it is possible to improve the m files in Octave, 
by eliminating the min/max evaluations inside the loops and selecting 
whether to replace numbers with zeros or zeros wih numbers.
The results, for my Octave 3.2.3, are:

> A=rand(100);
> i=randperm(200)(1)-100,tic,U=triu(A,i);,toc,tic,myU=mytriu(A,i);,toc,norm(U-myU)
i = -31
Elapsed time is 0.0221 seconds.
Elapsed time is 0.00397 seconds.
ans = 0
> i=randperm(200)(1)-100,tic,U=triu(A,i);,toc,tic,myU=mytriu(A,i);,toc,norm(U-myU)
i = -52
Elapsed time is 0.0224 seconds.
Elapsed time is 0.00114 seconds.
ans = 0
> i=randperm(200)(1)-100,tic,U=triu(A,i);,toc,tic,myU=mytriu(A,i);,toc,norm(U-myU)
i =  22
Elapsed time is 0.0174 seconds.
Elapsed time is 0.00495 seconds.
ans = 0
> i=randperm(200)(1)-100,tic,U=triu(A,i);,toc,tic,myU=mytriu(A,i);,toc,norm(U-myU)
i =  63
Elapsed time is 0.00873 seconds.
Elapsed time is 0.00261 seconds.
ans = 0
> i=randperm(200)(1)-100,tic,U=triu(A,i);,toc,tic,myU=mytriu(A,i);,toc,norm(U-myU)
i =  36
Elapsed time is 0.0146 seconds.
Elapsed time is 0.00413 seconds.
ans = 0

For me the speed-up is > 3. Enclosed, the patches wrt 3.2.3.

Best regards,

Marco

triu.m.diff
Description: Text Data

tril.m.diff
Description: Text Data