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Simple Question about Matlab

  1. Nov 21, 2005 #1
    Hi Guys,
    I just have a simple question about Matlab. When solving a quadratic equation two answers will be produced. If one is Negative what command can I use in Matlab to disregard it and continue further calculations with just the positive one?

  2. jcsd
  3. Nov 22, 2005 #2


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    The result should be returned as an array and you can access individual elements to determine the sign of each one. Write a subroutine/function to test the elements of the matrix and return a matrix with only postive elements.
  4. Nov 29, 2005 #3
    Thanks Tide. I tried what you said, writing a little if else routine to give me only the positive answer, put that led to another problem. It wouldnt work because the answer of the quadratic is a sym object and the routine will only work for an array. Do you know how i can convert it or find a way around this? I thought I would send you part of the code that i wrote:

    >> h = 500;% Input Value
    >> V = 55;% Input Value
    >> syms t;
    >> T = @(h,V)((V*sin(0))*t - 0.5*9.8*t^2 + h );
    >> T(h,V);
    >> T = ans;
    >> solve(T);
    >> T = ans
    >> if ans(1,1)> 0 & ans(2,1)>0
    >> T = ans
    >> elseif ans(1,1)< 0 & ans(2,1)>0
    >> T = ans(2,1)
    >> else T= ans(1,1)
    >> end

    Let me know what you think. Thanks for the help.
  5. Nov 30, 2005 #4


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    I don't have current access to MatLab so I can't fiddle with it to try and resolve the issue. I do have a clone with sparse documentation but I'll see if I can dig anything up. Any current ML users - feel free to jump in!
  6. Dec 5, 2005 #5
    Oh ok, well thanks Tide. I did find a way to convert from into an actual number, that is with the double command. however, this proved inconsequential as Matlab refuses to use this programming block when I call it up as a function giving me an error message that I have used the variable t (in code above) as a command/function. It seems to ignore the fact that I defined it as an symbolic object before. Funnily enough, when I run the routine on its own It works fine. If anybody has any ideas please let me know. Thanks.
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