I think you are on the right path. Thank you for the kind words.

Doug

Great help again, the reason i choose MATLAB for my project was because i heard you had great support – i truely am impressed!

Just to make sure i’m on the right tracks – I can xlsread the file, returning the text and numeric arrays. STRMATCH the text array to identify which rows in the numeric array are NEW, UPDATE or DELETE). Then create a new mumeric array with just the matched row data?

eg.
newparametric = strmatch(‘new’, strvcat(2,:),’exact’)

returns each row with ‘new’ in column 2

Many thanks,

Waggy

It sounds like you want to read in only rows that meet a specific criteria. There are two ways to go about this.

* Read everything in, throw out what you do not need.

* Read in the text column, use that to decide what to read in.

Depending on the size of your data, you would make different decisions. I think it would be easier to code the first. You would likely use STRMATCH:

http://www.mathworks.com/access/helpdesk/help/techdoc/ref/strmatch.html

This video:

http://blogs.mathworks.com/videos/2007/09/06/matlab-basics-video-absolute-and-logical-indexing/

Especially at time 2:30 does something similar.

If it is impractical to read in all the data, then I would use the second method, get the indices you need to read in, and do them in a loop. You would again use the above video and STRMATCH to do this.

-Doug

I posted a comment recently with reguards to importing Excel data and handling it within an array. You pointed me to the above tutorials which have been a huge help.

I have a .xls file with samples (rows) containing various radar parameters (columns) most numeric however some text. I need to create a GUI (some great video tutorials i’ve viewed!) that will re-display the table data however filtered i.e. only the UPDATE sample (as identified by a text column). Is this possible within MATLAB or will i need to create a function to convert the text into a numerical reference value? I have looked into the XLSREAD function but couldn’t really decipher exactly how i could use it!

I am very new to MATLAB therefore i hugely appreciate your advice.

Many thanks,

Waggy

A grad student at Stanford (Sep Kamvar) asked me why his pagerank-style computations in MATLAB were so exceedingly slow when dealing with a matrix of dimension about 680,000. He showed me the code, and I saw x=A(i,:) in a loop. Aha! … and when it was fixed his run time was cut from maybe an hour to under a minute. For his matrix, x=A(i,:) takes 0.16 seconds, but x=A(:,i) takes 0.0015 seconds. If you’re curious, you can try it with his matrix at http://www.cise.ufl.edu/research/sparse/matrices/Kamvar/ .

The profiler is a great tool for finding hot spots like this. It won’t help you to figure out why x=A(i,:) is slow and x=A(:,i) is 100′s of times faster, though. It may seem a little mysterious unless you look into how sparse matrices are stored in MATLAB.

Other “gotcha’s” like this related to matrix indexing include, for full matrices and sparse alike:

x = [ ] ;
for i = 1:100000
x (i) = i ;
end

for which mlint is a great tool for suggesting a much faster alternative. The code above takes 50 seconds; using mlint’s suggestion and changing the first line to x=zeros(1,100000); cuts the time to 0.12 seconds (yes, of course I’m just doing x=1:100000; this is just an example).

These fractions add up though when you are in a loop that gets called many times. The profiler is very good at helping to identify these places. See my early videos for use of the profiler.

Doug

None of this comment applies to full matrices, just sparse. You don’t get sparse matrices unless you ask for them, so if you’re a MATLAB newcomer who also has no need for sparse matrices, then please ignore my comment.