How to plot all contour lines in one plot?

Giiven a function f(x) I need to Plot the level sets contour lines where 𝑓(x) = −4.34, −4.3, −4.2, −4.1, −4, −3, −2, −1, 0.

Plot all of them in one graph

Answer 1

[Star Strider]

Specify them as a single levels vector —

[X, Y, Z] = peaks(50);

figure
contour(X, Y, Z, [-4.34, -4.3, -4.2, -4.1, -4, -3, -2, -1, 0], 'ShowText',1)
colormap(turbo)

.

[Jorge Arturo Clares Pastrana]

Thnakyou.

z = -4*x - 2*y - x^2 + 2*x^4 - 2*x*y + 3*y^2;

when i input the following equation for z it outputs a blank post do you know why?

[Star Strider]

First, the function needs to be vectorised so that it can do element-wise calculations. Otherwise, it could be because for the values of ‘x’ and ‘y’ you are using, ‘z’ has no values in the range of the desired contours. Also, ‘x’ and ‘y’ must be matrices, not vectors, although I suspect that would throw a specific error.

Use your values for ‘x’ and ‘y’ in something like this example code —

z = @(x,y) -4*x - 2*y - x.^2 + 2*x.^4 - 2*x.*y + 3*y.^2;            % Vectorised Anonymous Function For 'z'
x = linspace(-5, 5, 150);
y = linspace(-5, 5, 150);
[X,Y] = ndgrid(x, y);
Z = z(X,Y);

figure
contour(X, Y, Z, [-4.34, -4.3, -4.2, -4.1, -4, -3, -2, -1, 0], 'ShowText',1)
colormap(turbo)
axis([-1  2    -1  2])

.

Answer 2

[Jorge Arturo Clares Pastrana]

Thnakyou.

z = -4*x - 2*y - x^2 + 2*x^4 - 2*x*y + 3*y^2;

when i input the following equation for z it outputs a blank post do you know why?