How to retain only the positive root of a quadratic equation?

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Rinu
Rinu 2014-8-15
评论： Walter Roberson 2016-9-19
I was wondering if there is any Matlab function that would allow me to retain only the positive root of a quadratic equation. I want to use that function along with the 'roots' function to solve n number of quadratic equations to get n number of positive roots.
x=zeros(1,n)
for i=1:n
x(i)=function(roots([a(i) b(i) c(i)]))
end

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采纳的回答

Guillaume
Guillaume 2014-8-15
To only get the positive elements of array a:
>>a = [-2.5 0 2.5 -3.5 1];
>>apos = a(a>=0)
apos =
0 2.5000 1.0000
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Walter Roberson
Walter Roberson 2016-9-19
Joel Matthew commented on the Answer:
did you read the question ? he is asking for finding the positive root not an element in an array
Walter Roberson
Walter Roberson 2016-9-19
Joel,
roots() returns a vector of values. roots() is an already known step. The question is therefore to write a function which examines a vector of values and returns the positive (real) values from the vector. So where the original poster had written
x=zeros(1,n)
for i=1:n
x(i)=function(roots([a(i) b(i) c(i)]))
end
using "function" as a stand-in for the name of the function to do the desired selection, that could be written as
x=zeros(1,n)
for i=1:n
x(i)=Extract_positive(roots([a(i) b(i) c(i)]))
end
function apos = Extract_positive(a)
apos = a(a > 0);
which is code that only has to worry about elements of an array.

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Evan
Evan 2014-8-15
编辑：Evan 2014-8-15
help imag
So, if you wanted to return only roots without complex parts:
R_all = roots([1 1 0 1]);
R_real = R_all(~imag(R_all))
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Evan
Evan 2014-8-15
编辑：Evan 2014-8-15
Sorry! I would say I misread your question, but I originally used the word "positive" as well! My only excuse, then, is that it's a Friday. ;)

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