Solving Quadratic Equations in Matlab: How to Disregard Negative Solutions

[Brandt]

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?

Thanks.

 

[Tide]

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 positive elements.

 

[Brandt]

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 wouldn't 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.

 

[Tide]

Brandt,

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!

 

[Brandt]

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.