Book Image

Applying Math with Python

By : Sam Morley
Book Image

Applying Math with Python

By: Sam Morley

Overview of this book

Python, one of the world's most popular programming languages, has a number of powerful packages to help you tackle complex mathematical problems in a simple and efficient way. These core capabilities help programmers pave the way for building exciting applications in various domains, such as machine learning and data science, using knowledge in the computational mathematics domain. The book teaches you how to solve problems faced in a wide variety of mathematical fields, including calculus, probability, statistics and data science, graph theory, optimization, and geometry. You'll start by developing core skills and learning about packages covered in Python’s scientific stack, including NumPy, SciPy, and Matplotlib. As you advance, you'll get to grips with more advanced topics of calculus, probability, and networks (graph theory). After you gain a solid understanding of these topics, you'll discover Python's applications in data science and statistics, forecasting, geometry, and optimization. The final chapters will take you through a collection of miscellaneous problems, including working with specific data formats and accelerating code. By the end of this book, you'll have an arsenal of practical coding solutions that can be used and modified to solve a wide range of practical problems in computational mathematics and data science.
Table of Contents (12 chapters)

Basic plotting with Matplotlib

Plotting is an important part of understanding behavior. So much can be learned by simply plotting a function or data that would otherwise be hidden. In this recipe, we will walk through how to plot a simple function or data using Matplotlib.

Matplotlib is a very powerful plotting library, which means it can be rather intimidating to perform simple tasks with it. For users who are used to working with MATLAB and other mathematical software packages, there is a state-based interface called pyplot. There is also an object-orientated interface, which might be more appropriate for more complex plots. The pyplot interface is a convenient way to create basic objects.

Getting ready

Most commonly, the data that you wish to plot will be stored in two separate NumPy arrays, which we will label xand yfor clarity (although this naming does not matter in practice). We will demonstrate plotting the graph of a function, so we will generate an array of x values and use the function to generate the corresponding y values. We define the function that we will plot as follows:

def f(x):
return x*(x - 2)*np.exp(3 - x)

How to do it...

Before we can plot the function, we must generate the x and y data to be plotted. If you are plotting existing data, you can skip these commands. We need to create a set of the x values that cover the desired range, and then use the function to create the y values:

  1. The linspace routine from NumPy is ideal for creating arrays of numbers for plotting. By default, it will create 50 equally spaced points between the specified arguments. The number of points can be customized by providing an additional argument, but 50 is sufficient for most cases:
x = np.linspace(-0.5, 3.0)  # 100 values between -0.5 and 3.0
  1. Once we have created the x values, we can generate the y values:
y = f(x)  # evaluate f on the x points
  1. To plot the data, we simply need to call the plot function from the pyplot interface, which is imported under the plt alias. The first argument is the xdata and the second is the y data. The function returns a handle to the axes object on which the data is plotted:
plt.plot(x, y)
  1. This will plot the y values against the x values on a new figure. If you are working within IPython or with a Jupyter notebook, then the plot should automatically appear at this point; otherwise, you might need to call the plt.show function to make the plot appear:
plt.show()

If you use plt.show, the figure should appear in a new window. The resulting plot should look something like the plot in Figure 2.1. The default plot color might be different on your plot. It has been changed for high visibility for this book:

Figure 2.1: Plot of a function produced using Matplotlib without any additional styling parameters

We won't add this command to any further recipes in this chapter, but you should be aware that you will need to use it if you are not working in an environment where plots will be rendered automatically, such as an IPython console or Jupyter Notebook.

How it works...

If there are currently no Figure or Axes objects, the plt.plot routine creates a new Figure object, adds a new Axes object to the figure, and populates this Axes object with the plotted data. A list of handles to the plotted lines is returned. Each of these handles is a Lines2D object. In this case, this list will contain a single Lines2D object. We can use this Lines2D object to customize the appearance of the line later (see the Changing the plotting style recipe).

The object layer of Matplotlib interacts with a lower-level backend, which does the heavy lifting of producing the graphical plot. The plt.show function issues an instruction to the backend to render the current figure. There are a number of backends that can be used with Matplotlib, which can be customized by setting the MPLBACKEND environment variable, modifying the matplotlibrc file, or by calling matplotlib.use from within Python with the name of an alternative backend.

The plt.show function does more than simply call the show method on a figure. It also hooks into an event loop to correctly display the figure. The plt.show routine should be used to display a figure, rather than the show method on a Figure object.

There's more...

It is sometimes useful to manually instantiate a Figure object prior to calling the plot routine—for instance, to force the creation of a new figure. The code in this recipe could instead have been written as follows:

fig = plt.figure()  # manually create a figure
lines = plt.plot(x, y) # plot data

The plt.plot routine accepts a variable number of positional inputs. In the preceding code, we supplied two positional arguments that were interpreted as x values and y values (in that order). If we had instead provided only a single array, the plot routine would have plotted the values against their position in the array; that is, the x values are taken to be 0, 1, 2, and so on. We could also supply multiple pairs of arrays to plot several sets of data on the same axes:

x = np.linspace(-0.5, 3.0)
lines = plt.plot(x, f(x), x, x**2, x, 1 - x)

The output of the preceding code is as follows:

Figure 2.2: Multiple plots on a single figure, produced using a single call to the plot routine in Matplotlib

It is occasionally useful to create a new figure and explicitly create a new set of axes in this figure together. The best way to accomplish this is to use the subplots routine in the pyplot interface (refer to the Adding subplots recipe). This routine returns a pair, where the first object is Figure and the second is an Axes object:

fig, ax = plt.subplots()
l1 = ax.plot(x, f(x))
l2 = ax.plot(x, x**2)
l3 = ax.plot(x, 1 - x)

This sequence of commands produces the same plot as the preceding one displayed in Figure 2.2.

Matplotlib has many other plotting routines besides the plot routine described here. For example, there are plotting methods that use a different scale for the axes, including the logarithmic x or y axes separately (semilogx or semilogy, respectively) or together (loglog). These are explained in the Matplotlib documentation.