logical Indexing

assume a as 3*2 matrix

a = [1 , 1 ; 2 , 2 ; 3 , 3];
a = a(a(:,2) > 1);

what i expect from the above statement is that I get a 2*2 matrix but instead i get 2 *1 matrix

the same thing when i try and define a logical array based on single column of a variable that is

flag = a(:,2) > 1
a = a(flag);

I can get around this but saying [flag;flag]; and a(flag) and then trying to get a back into its original dimensions but i feel stupid doing this, I guess i need to get some basics right.

Answer 1

[Oleg Komarov]

idx = a(:,2) > 1;
a   = a(idx,:)

with a(idx) - note no ,: - you're saying to take the elements indexed by idx.

with a(idx,:) you're saying to take the rows indexed by idx.

[Nikhil]

Thanks man, can I ask what is the logic behind the statement are we asking to do it for all columns when we say ",:" ?

[Oleg Komarov]

Take the rows indexed by idx and the all the columns.

Equivalent syntaxes:

- a(idx,1:end)

- a(idx, : )