Creating a diagram from polynomial

I defined a polynomial

P = [1 -2 4]

and I want to draw the shape of polynomial for the range between -10 to 10 with in the logarithmic diagram (only X axis). How can I do it?

Answer 1

[Walter Roberson]

P = [1 -2 4];
x = linspace(-10, 10, 500);
y = polyval(P, x);

lx = log10(x);
rlx = real(lx);
ilx = imag(lx);

plot3(rlx, ilx, y)
xlabel('real(log(x))'); ylabel('imag(log(x))'); zlabel('polynomial value')

You need to use a 3D plot for this because you want a log axes on a range that includes negative values, so you need some way to represent the component of the log of the negative numbers.

Answer 2

[Sulaymon Eshkabilov]

There are a few different ways to get this exercise done.

Compute the values of the polynomial P = [1 -2 4] at x = -10:dx:10 with some increment step size (dx) and then plot x vs. y using semilogx(). The code starts with these:

P = [1 -2 4];
dx = ...
x= -10:dx:10;
y = P(1)*x.^2+P(2)*x+P(3);
figure()
...
    

If you have a difficulty to undertand how to plot the computed values, use help.

>> help semilogx

Moreover, your plotted data will contain the positive values of x and the negative side will be ignored due to a log scale.