need help Writing a function that returns roots of a quadratic

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!

Answer 1

[Walter Roberson]

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"

[steve guy]

There is no r2 in that case because the function would not be a true quadratic. Do I just assume the user knows that so they only use one variable to store the output when calling the function? or is there a way for matlab to tell the user this?

[Walter Roberson]

If you define an output and do not assign any value to it, then MATLAB will give you an error message. You have to assign something, even if it is the empty array.

The usual way of handling this is to define the function to return only one variable, and set that one variable to be a vector containing all of the roots -- so only one element if there is only one root.

[steve guy]

I just edited it to that way. Thanks for the help. do you see anything else wrong with the code?

also what is the difference between a==0 and b==0 having 2 equal signs where as it is ok if d=0 has just one. Does it have to do with a and b being user inputs?

[Azzi Abdelmalek]

a==0 is a logical expression, it can be true or false while d=0 is an affectation.

Try this in windows command

a=10     % it's an affectation the result is 10
a==20    % it's a logical expression, the result is 0 (false)
a==10    % it's a logical expression, the result is 1 (true)

and

d=10   % it's an affectation the result will be 10

[steve guy]

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

[Azzi Abdelmalek]

Yes

Answer 2

[Azzi Abdelmalek]

There are some mistakes

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

[Walter Roberson]

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

Answer 3

[Image Analyst]

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.

[steve guy]

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?

[Image Analyst]

Sure.