Real quick one, passing through a matrix backwards or just flip it.

During a for or while loop I would like to start at the back of the matrix and cycle to the beginning. Or do I just flip it using a(end:-1:1,:) or flipud(a). I understand flipud but can you briefly explain the syntax for a(end:-1:1,:).

Thanks AD

Answer 1

[Fangjun Jiang]

a=rand(8,9);
b=a(5:end,5:end)

"end" here is the length of that dimension.

1:end is the same as [1,2,3,...,end]

end:-1:1 is the opposite. It is the same as [end, end-1, end-2, end-3, ... 1].

Answer 2

[Walter Roberson]

Cycling the loop backwards

for K=size(A,1):-1:1

can be quite efficient, and seldom requires more memory than working forwards (indeed, moving forwards can be less efficient if you have not preallocated a matrix.)