How do you crop a Matrix?

I have a 770x770 matrix that I want to cut down to a 324x324 matrix. How would I cut off the bottom 446 rows and rightmost 446 columns?

Answer 1

[Jos (10584)]

You mean the number of dimensions of M is unknown, but Nkeep is known? Use subsref:

M = randi(10,4,3,4,5) ; % arbitrary dimensions
Nkeep = 2 
S = struct('type','()','subs',{repmat({[1:Nkeep]},1,ndims(M))})
M2 = subsref(M, S)

Answer 2

[dpb]

Pretty simply, just save what you want...

Nkeep=324;  % size to keep
M=M(1:Nkeep,1:Nkeep);

You can, of course, set the unwanted rows, columns = [], but that takes two steps instead of just one:

M(:,Nkeep+1:end)=[];  % columns for all rows, result is rectangular
M(Nkeep+1:end,:)=[];  % now the rows for remaining columns, result is square now

[Jurgen]

How do you generalize this for N dimensional matrices, assuming unknown or large matrix ndims? The only thing I can think of involves eval.

[dpb]

Not much support for templates and the like in procedural code...you might be able to get creative with sub2ind and friends, but likely if it is truly amorphic data sizes/shapes that must be handled it's likely to come down to eval at the bitter end, agreed.