How to save specified data from a matrix?

hello, everyone.

I am wondering how can I delete specified data from a data matrix. For example, if I have a matix like this:

m=rand(100,5);

How can I save a matrix that satisfy this : for each row, each element is smaller than a specified number such as 0.200.

I know I can use indice, eg. ind=find(m(:,2:5)< 0.2)

but I can't save it by using m1=m(ind,:)

I guess the problem is the number of rows is changing when I use m1=m(ind,:).

does anyone know how can i achieve this?

Thanks.

Answer 1

[Walter Roberson]

Your line

ind=find(m(:,2:5)< 0.2)

is not correct. You need

ind=find(all(m(:,2:5)< 0.2,2))

[Lei]

hello Walter, Thx, I see. But how can I save the data if it satisfy this: ind=find((m(:,2:5)< 0.2)) when I use m2=m(ind,:); there's an error ??? Index exceeds matrix dimensions.

[Walter Roberson]

m(:,2:5)< 0.2 returns a two-dimensional matrix, so find() on it is going to return the linear indices relative to the two-dimensional matrix. You then try to use those linear indices as if they were column indices.

Your question asked "for each row, each element is smaller than a specified number such as 0.200" which is only true if all() of the comparisons are true along the row. When all() is used specifying row processing, the number of logical values produced would be the same as the number of rows, and find() applied to that is going to give row indices, which would be fine to pass in to the first index of m. Thus my answer find(all(m(:,2:5)< 0.2,2)) which includes the "all" step.

[Lei]

Hello, Walter. I see what you mean! My problem is no matter whether I use ind=find((m(:,2:5)< 0.2)) or find(all(m(:,2:5)< 0.2,2)), I do not know how to save this data in the workspace. ind=find(whatever) is to extract the data I need to save ( I just used a simple example,but your suggests are absolutely right). Now the problem is that I do not know how to save it!

[Walter Roberson]

As you had above,

m1=m(ind,:);

and then save m1

[Lei]

hello, Walter,

This one (m1=m(ind,:);) does not work! As I mentioned above.

try to tun this

m=rand(100,5);
ind=find(m(:,2:5)< 0.2);
m1=m(ind,:);

The first two lines are ok, but the third line has an error!

it says '??? Index exceeds matrix dimensions.'

Thats why I am asking for help here!

any other suggestions? thx a lot

[Walter Roberson]

m=rand(100,5);
ind=find(all(m(:,2:5)< 0.2,2))
m1=m(ind,:);

Do not omit the all() !

[Lei]

I see what you mean now!!! Thx a lot. But what if I want save the data meets this criteria: for each row, any of the element in this row is bigger than a number (0.2), then I save the data. Thats why I used this ind=find(m(:,2:5)< 0.2);. In this case, I cant save the data by using m1=m(ind,:);. What you code was doing is that all of the elements in that row are smaller than 0.2 Do you have any idea how can I do this???

[Lei]

probaably ind=find(any(M(:,2:5)>54,2));

Answer 2

[Babak]

if your matrix m is 100 by 1, you can do this:

 m(m<0.2)

gives you a new matrix that is smaller or equal size to the original matrix.

For a 100 by 5 m, I think you can write a for loop on the different columns of m,

 a = m<.2;
 for j=1:5
   m(a(:,j))  % gives you the elemets of m(:,j) that are less than 0.2
 end

[Lei]

hello Babak, Thx for your help,I tried your code. but it seems does not work.