Sketching phase portrait in julia

When working with Julia, it is often necessary to sketch phase portraits to visualize the behavior of a system. In this article, we will explore three different ways to accomplish this task.

Option 1: Using the Plots.jl package

The Plots.jl package provides a high-level interface for creating visualizations in Julia. To sketch a phase portrait using this package, we can follow these steps:

using Plots

function phase_portrait(f, x_range, y_range)
    x = range(x_range[1], x_range[2], length = 20)
    y = range(y_range[1], y_range[2], length = 20)
    plot(x, y, f, xlabel = "x", ylabel = "y", legend = false)
end

f(x, y) = [y, -x]
phase_portrait(f, (-2, 2), (-2, 2))

In this code snippet, we define a function phase_portrait that takes a vector field function f and the ranges for the x and y axes. We then create a grid of points using the range function and plot the vector field using the plot function from Plots.jl.

Option 2: Using the DifferentialEquations.jl package

The DifferentialEquations.jl package provides a comprehensive suite of tools for solving differential equations in Julia. To sketch a phase portrait using this package, we can follow these steps:

using DifferentialEquations, Plots

function phase_portrait(f, x_range, y_range)
    x = range(x_range[1], x_range[2], length = 20)
    y = range(y_range[1], y_range[2], length = 20)
    u0 = [(x, y) for x in x, y in y]
    prob = ODEProblem(f, u0, (x_range[1], x_range[2]))
    sol = solve(prob)
    plot(sol, xlabel = "x", ylabel = "y", legend = false)
end

f(u, p, t) = [u[2], -u[1]]
phase_portrait(f, (-2, 2), (-2, 2))

In this code snippet, we define a function phase_portrait that takes a vector field function f and the ranges for the x and y axes. We create a grid of initial conditions using a list comprehension and solve the corresponding ordinary differential equation using the ODEProblem and solve functions from DifferentialEquations.jl. Finally, we plot the solution using the plot function from Plots.jl.

Option 3: Using the PyPlot.jl package

The PyPlot.jl package provides a Julia interface to the popular matplotlib library in Python. To sketch a phase portrait using this package, we can follow these steps:

using PyPlot

function phase_portrait(f, x_range, y_range)
    x = range(x_range[1], x_range[2], length = 20)
    y = range(y_range[1], y_range[2], length = 20)
    X, Y = meshgrid(x, y)
    U = f(X, Y)[1]
    V = f(X, Y)[2]
    quiver(X, Y, U, V)
    xlabel("x")
    ylabel("y")
end

f(x, y) = [y, -x]
phase_portrait(f, (-2, 2), (-2, 2))

In this code snippet, we define a function phase_portrait that takes a vector field function f and the ranges for the x and y axes. We create a grid of points using the range function and compute the corresponding vector field components using the meshgrid function. Finally, we plot the vector field using the quiver function from PyPlot.jl.

After exploring these three options, it is clear that the best choice depends on the specific requirements of the problem at hand. If simplicity and ease of use are the main priorities, Option 1 using the Plots.jl package is a great choice. However, if more advanced functionality and flexibility are needed, Options 2 and 3 using the DifferentialEquations.jl and PyPlot.jl packages, respectively, provide more comprehensive solutions.