how to write a function for quadratic equation?

I wrote this into matlab but it doesn't work where express x1, x2, it express only one sol in d>=0 but ploting is work in d>=0. and it's error in d<0

why it doen't work and how to fix it?

function [x1,x2] = f(a,b,c)
if a==0
x1=-c/b;
x2=-c/b;
plot(x1,0,'gx',x2,0,'gx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
end
d=b^2-4*a*c;
if (d>=0)&&(~(a==0))
x1=((-b-sqrt(d))/(2*a));
x2=((-b+sqrt(d))/(2*a));
plot(x1,0,'rx',x2,0,'rx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
elseif a==0
x1=-c/b;
x2=-c/b;
plot(x1,0,'gx',x2,0,'gx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
else
[x1 x2]=[]
end

[Dyuman Joshi]

Why have you written the code for condition a==0 twice?

You have to include the code for condition d<0 as well.

[jun]

where include and how?,

i include this statements

"else if (d<0)&&(a=0)

[x1 x2] = []

else

[x1 x2] = []" which are next of condition 'a==0',

but it works f(1,0,-4)=-inf

[Dyuman Joshi]

Do you want to return empty values for if any/both of the two conditions - a==0 , d<0?

[jun]

i want d<0 => empty value return

but a=0&d<0 => x1=x2=-c/b return that i want

Answer 1

[VBBV]

if (d>=0) & (~(a==0))  

use binary operator &

[VBBV]

[x1 x2] = f(4.1,10,4)

d = 34.4000

x1 = -1.9348

x2 = -0.5042

function [x1,x2] = f(a,b,c)
if a==0
x1=-c/b;
x2=-c/b;
plot(x1,0,'gx',x2,0,'gx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
end
d=b^2-4*a*c
if (d>=0)&&(~(a==0))
x1=((-b-sqrt(d))/(2*a));
x2=((-b+sqrt(d))/(2*a));
plot(x1,0,'rx',x2,0,'rx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
elseif a==0
x1=-c/b;
x2=-c/b;
plot(x1,0,'gx',x2,0,'gx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
else
x1 = [];
x2 = [];
end
end

Missing end keyword in the function

[jun]

Maybe in your matlab, besides plotting, does the output of exactly what the values ​​of x1 and x2 are?

I'm using octave now, and your code only displays the result of x1. but ploting is well

it's octave problem..?

[VBBV]

It displays both x1 and X2 values with red markers on plot.

[VBBV]

Since a is not zero in demonstrated case function does not execute first if end condition

[jun]

I'm not good at English, so I'll ask again, In my matlab in octave ver, when f(1,0,-4), it doesn't come out as x1=-2 and x2=2, but derived as ans=-2. However, in plotting, both solutions work well. Does your matlab yield x1=-2,x2=2?

[VBBV]

[x1 x2] = f(1,0,-4)

d = 16

x1 = -2

x2 = 2

function [x1,x2] = f(a,b,c)
if a==0
x1=-c/b;
x2=-c/b;
plot(x1,0,'gx',x2,0,'gx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
end
d=b^2-4*a*c
if (d>=0)&&(~(a==0))
x1=((-b-sqrt(d))/(2*a));
x2=((-b+sqrt(d))/(2*a));
plot(x1,0,'rx',x2,0,'rx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
elseif a==0
x1=-c/b;
x2=-c/b;
plot(x1,0,'gx',x2,0,'gx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
else
x1 = [];
x2 = [];
end
end

[VBBV]

x1 and x2 values are -2 and 2 from above result. What do you get as result ?

[jun]

result appers x1, but figure graph is well,

[Dyuman Joshi]

Why are you repeating a piece of code? It's redundant.

[jun]

Are you right about 'a==0'? If 'a=0', an error like this is displayed

[Dyuman Joshi]

Let me highlight it. See the green parts, both are same.

I would suggest you to remove the top one and edit last else as

elseif d<0

function [x1,x2] = f(a,b,c)
%{
if a==0
x1=-c/b;
x2=-c/b;
plot(x1,0,'gx',x2,0,'gx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
end
%}
d=b^2-4*a*c
if (d>=0)&&(~(a==0))
x1=((-b-sqrt(d))/(2*a));
x2=((-b+sqrt(d))/(2*a));
plot(x1,0,'rx',x2,0,'rx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
%{
elseif a==0
x1=-c/b;
x2=-c/b;
plot(x1,0,'gx',x2,0,'gx')
hold on
fplot(@(x) a*x.^2+b*x+c)
hold off
%}
else
x1 = [];
x2 = [];
end
end