How to define which variable is x and which variable is y axis?

I have an equation like "2*c^2 - 4*b = 0" with b ranging from 0 to 10. I want to plot a graph of b verses c with b on the x axis. How would I tell the program to plot b on the x axis and c on the y axis?

Answer 1

[Mischa Kim]

John, simply use

 plot(b,c)

The first argument is the x the second the y axis. Or, if you want to show both solution branches

 b = 0:0.1:10;
 c = sqrt(2*b);
 plot(b,c,b,-c)

Answer 2

[Star Strider]

First, a bit of algebra to create ‘c’ as a function of ‘b’, then it’s simply another plot:

b = linspace(0,10);
c = [-sqrt(2*b);  sqrt(2*b)];

figure(1)
plot(b, c)
grid

[Star Strider]

The ‘V’ for your ‘new’ equation is complex, so it’s easier to simply evaluate it at each ‘b’ and plot it:

F = @(b) [pi+atan(b./(1-b)).^(1/2)+atan((b+2).*(1-b)).^(1/2)]./(1-b).^(1/2);
b = linspace(-100,100.500);
plot(real(F(b)),b, '-r',  imag(F(b)),b, '-g')
xlabel('V(b)')
ylabel('b')
legend('Re(V)', 'Im(V)')
grid

[John]

Ok thank you very much for this!

[Star Strider]

My pleasure!

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[Andrew Reibold]

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Answer 3

[John]

Thanks that clears up a lot however if I can't separate b into one term is there a way to plot it. This is the actual equation:

V=[pi+arctan(b/(1-b))^(1/2)+arctan((b+2)(1-b))^(1/2)]/(1-b)^(1/2)

I want V to be the x axis from 0 to 15. I want to solve for b and plot the results on the y axis.

Answer 4

[Matt J]

 f = @(V,b)V-(pi+atan(b./(1-b)).^(1./2)+atan((b+2).*(1-b)).^(1./2))./(1-b).^(1./2);

 ezplot(f,[0,1.1,15,100]);

[Mohammad Abouali]

he wants 0<V<15 so ezplot(f,[0,15]); with f being defined as you suggested.

and that is still not correct, since in those range the function f returns a complex number and what is being plotted is the norm of that complex number.

[Mohammad Abouali]

Yes, with x being V and y being b. so xmin=0 xmax=15

and again put b=1.1 and check what your V turns out to be.

ff=@(b) (pi+atan(b./(1-b)).^(1./2)+atan((b+2).*(1-b)).^(1./2))./(1-b).^(1./2);

V would be a complex number

He wants 0<V<15 on x axis

[Matt J]

Right you are (although I think it is the real part and not the norm that is being plotted). Here's a modification that fixes that issue,

    function doPlot

        ezplot(@(V,b) f(V,b),[0,15,0,1]);

        function fval = f(V,b)

            fval = V-(pi+atan(b./(1-b)).^(1./2)+atan((b+2).*(1-b)).^(1./2))./(1-b).^(1./2);

            fval=fval/(imag(fval)==0);

        end

    end