need help Writing a function that returns roots of a quadratic

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steve guy 2013-2-17

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This is my plan on how to write it (don't have software at my disposal currently). I am learning matlab so understand there may be quite a few problems.

Function [x]=myquad(a,b,c)
x=[r1 r2];
d=sqrt((b.^2)-4.*a.*c)
if a==0 & b>0
    r1=-c/b;
elseif a==0 & b==0
    r1=0;
elseif a~=0 & d==0
    r1=-b./(2.*a);r2=r1;
elseif a~=0 & d>0
    r1=(-b+d)./2a; r2=(-b+d)./2.*a;
elseif a~=0 & d<0
    r1=(-b+d.*i)/2a; r2=(-b-d.*i)./2.*a;
end
end

I need to follow this algorithm for an assignment.

THANKS!

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Walter Roberson 2013-2-17

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编辑：Walter Roberson 2013-2-17

Your first two cases do not appear to set r2 to anything.

your a=0 b=0 case is wrong

Your "di" should probably be "d*i"

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steve guy 2013-2-17

then that means my "d=0" should also be "d==0" in the code as well, right?

Azzi Abdelmalek 2013-2-17

Yes

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更多回答（2 个）

Azzi Abdelmalek 2013-2-17

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There are some mistakes

if a==0  %instead  of if a=0
elseif % instead of ifelse

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Walter Roberson 2013-2-17

Multiple assignment is not permitted. r1=r2=VALUE is not allowed. Use r1=VALUE; r2=r1

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Image Analyst 2013-2-18

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Why do you need all those "if"s at all? Why not just do

function [r1 r2]=myquad(a,b,c)
d = sqrt((b^2)-4*a*c); % Can be complex with no problem.
r1=(-b-d)/(2*a); 
r2=(-b+d)/(2*a);

MATLAB handles complex numbers and infinite numbers with no problem, so you don't need special if statements to do it manually.

Probably would make more sense to do that but I have to address all the different cases for an assignment. For instance if a user entered myquad(0,1,2) then I need it to say. "Not a quadratic, r1=-1/2".

do you think I could just write fprintf statements underneath the logical expressions for the different cases, in place of the formulas, and use what you suggested?