File Exchange Pick of the Week

September 6th, 2007

MATLAB Basics video: Absolute and Logical indexing

This is one in a series of videos covering MATLAB basics. It is meant for the new MATLAB user.

This video covers how to use row and column notation to be able to pull a subset of data from a larger matrix. This is a basic skill that is required for anyone that is going to use MATLAB. This particular type of indexing is less intuitive than the mat(row,col) indexing, it deals with using a single value as an index into a matrix like mat(ind) and mat([0 1 0 0 1]). These are also very important techniques for indexing into MATLAB.

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By Doug Hull

12:13 UTC | Posted in MATLAB Basics, Video | Permalink | 5 Comments »

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5 Responses to “MATLAB Basics video: Absolute and Logical indexing”

naor replied on September 6th, 2007 at 17:51 UTC :

Doug, in the RSS feed of your blog, read in internet explorer 7, the videos never show. I don’t mean they don’t run, there are just lines of text with a single blank line where the videos should be. Is there a way to fix this?
Thanks,
naor (win32xp r2006a)
Dennis Olly replied on March 1st, 2008 at 19:12 UTC :

Hi Doug, very clear video, however, is it possible to do logical indexing directly to a submatrix, eg:

data=zeros(10,10,10);

here data(:,:,5) could be slice.
now let

mask=zeros(10,10);mask(5:5,8:8)=1;

Is it possible to use this mask to logically index the submatrix data(:,:,5)?

slice = data(:,:,5);
and then accessing slice(mask) is possible. But can this not be done directly, without extracting ‘slice’?

Thanks
Dennis
Doug replied on March 3rd, 2008 at 14:20 UTC :

It will be required to pull out the intermediate values to use that logical matrix or you will need to do some other indexing method.

Doug
Ronny replied on March 3rd, 2010 at 19:49 UTC :

Hi Doug,
I saw your video and I have a question, when I use “find” with a matrix with X,Y,Z or logical for extract valor from ranges, the new matrix is always a column vector and I need this matrix in similar format (x,y,z), how can I do?
Please let me know,
Thanks,
RM
Doug replied on March 8th, 2010 at 16:27 UTC :

@ronny
```
>> a = [2 0 3; 0 4 0]

a =

     2     0     3
     0     4     0

>> b = (a>1)

b =

     1     0     1
     0     1     0

>> c = find(a>1)

c =

     1
     4
     5
```
Try this. If you must use find, you could always make a zeros matrix of the same size and set the indices that you find to 1.