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Matlab if statements and extracting rows of data

  1. Oct 1, 2011 #1
    There may be a simple solution (I'm guessing so), but I've been trying for ages now and can't seem to figure it out... I have a matrix of data in Matlab, 4 columns by 1000 rows. I need to evaluate that data...
    I need to find answers in column 3 which lie between 0.9998 and 1. I want to extract all of these numbers, but keep them with columns 1 - 4 of my data... Column 3 represents the R^2 value and I want a new table where the R^2 value is over 0.9998, and need to extract the corresponding figures from the other columns, so I keep my 4 columns, but have less then the 1000 rows...
    I've tried using if statements, but can't seem to get a result out...
    Can anyone help?
     
  2. jcsd
  3. Oct 1, 2011 #2

    uart

    User Avatar
    Science Advisor

    It's a bit quirky looking, but say your matrix is called "M", then one way would be :

    M( M(:,3) >= 0.9998 , : )


    I'm assuming the data in column 3 is some sort of coefficient with a maximum value of 1, is that correct? Otherwise you'd have to test for <= 1 as well.
     
  4. Oct 1, 2011 #3
    You are correct
    Thank you :-)
     
    Last edited: Oct 1, 2011
  5. Sep 17, 2012 #4
    I was searching for a similar solution and found this page. this was an awesome help to me. I am super grateful to "uart"

    thanks a lot. thanks also to the person who asked the question

    Also, I was curious if you would recommend a site where I could learn such neat tricks
     
  6. Sep 17, 2012 #5
    I have files that look like following:

    % num vel error

    1 4 0.12
    2 6 1.23
    ..

    I want to extract "rows" according to the value of errors. for example, I only want raws which have low velocity errors. I will then use them to compute average-velocity. Can you please help what's the best way to proceed
     
  7. Sep 19, 2012 #6
    % Ok, so say your matrix looks like this:

    x=[1,2,0.1;3,4,0.5;5,6,0.9;7,8,0.76;9,10,1.2]

    x =

    1.0000 2.0000 0.1000
    3.0000 4.0000 0.5000
    5.0000 6.0000 0.9000
    7.0000 8.0000 0.7600
    9.0000 10.0000 1.2000

    % Define what you want to say is low velocity error (so <=0.8 for example as I'm not sure of your units)

    good=(x(:,3)<=0.8) %based on % num vel error (as you mentioned in your question with col 3= velocity error - if velocity error is in a different column then substitute the 3 in (:,3))

    % This defines a new variable "good" which is a logical statement which gives a value of 1 the rows where velocity error is <=3 - basically puts your data into a binary

    % to get the rows out and keep the matrix you just type:

    x(good,:)

    ans =

    1.0000 2.0000 0.1000
    3.0000 4.0000 0.5000
    7.0000 8.0000 0.7600

    % or call it something new like:

    y=x(good,:)

    % You could also just do it like this to avoid creating the "good" binary code... this just joins those lines of code into 1 (but I think it helps illustrate the point showing it in both ways):

    y=x(x(:,3)<=0.8,:)


    % Hope this helps!!
     
  8. Sep 19, 2012 #7
    yes, this definitely helps !
    thanks a lot !
     
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