In this introductory tutorial we will learn how to load a text file containing numerical data into a NumPy array, analyze it and finally plot it using Matplotlib. To write Python commands we will use interactive Python console IPython.

For Windows and Mac users the most convenient way to get IPython, NumPy and Matplotlib is to download the Canopy installation file. Depending on your operating system type, you will need either a 32-bit or 64-bit version. If you're unsure which version to download you can find the information by right click on My Computer and Properties in Windows. On Mac follow the instructions here. Alternatively Mac users experienced in using Terminal might wish to get a subset of packages using Macports instead of installing the whole Canopy file:

sudo port install py27-numpy py27-scipy py27-matplotlib py27-ipython

Canopy file contains much more packages than needed for this exercise, but if you decide to do the other one it will be useful.

Linux users can get the packages by entering the following command in the Terminal:

sudo apt-get install python-numpy python-scipy python-matplotlib ipython 

Our data is stored as a Google spreadsheet and you can download it here. To download the spreadsheet, click on File and Download as > Comma Separated Values. Comma Separated Values is another format of a simple text file, where values are separated by commas (surprising!). There's nothing special about a .csv format, you can think of it as a it's a bit nicer .txt file. Feel free to take a look at it using your favorite text editor.

The spreadsheet contains some made up data: in the first column we have listed ages and in the second column the number of electronic devices. Each row in the column corresponds to data from one person.

Start your IPython console and import the NumPy and matplotlib package:

import numpy as np
import matplotlib.pyplot as pl

Load the data with the following command:

data = np.genfromtxt('data - Sheet1.csv', delimiter=',', skiprows=1)

In this example, our data is stored in a file called data - Sheet1.csv and we tell NumPy function genfromtxt to read the data which is separated by commas to skip the first row which contains two strings: Age and Electronic devices. If at any point in the future we forget how to use this function, we can remind ourselves by using IPython's built in help function:

np.genfromtxt?

To make sure that our data is indeed what we had in the spreadsheet, print the data, print the data type and its dimensions by simply typing in the three following commands:

data
type(data)
data.shape

Notice that there is a dot . in the output, this means our numbers are stored as floating-point numbers. To write 1. is a lazy way of writing 1.0 and it's pretty common.

Executing data.shape returns the number of rows and number of colums. We say that our NumPy array data has two dimensions with the number of rows being the first, and the number of columns the second.

To access the first row, aka data from the first person:

data[0,:]

Remember, to access the first row we use the index 0 because indexing in Python starts with 0. : simply means give me everything in that row. If we want to know how many electronic devices this person has we type:

data[0,1]

How to get all the ages? Well, knowing that : gives us everything, this is easy:

data[:,0]

In IPython navigate using the cd (change directory) command to the folder where the downloaded .csv file is contained. To see in which folder you are at the moment use the pwd (print working directory). If you want to list all the files contained in the folder use ls.

So, we have our data ready and we would like to know who is the youngest, who is the oldest and what's the mean age. Well, that's easy:

ages = data[:,0]
min_age = np.min(ages)
max_age = np.max(ages)
mean_age = np.mean(ages)

print "The youngest person is ", min_age, " years old."
print "The oldest person is ", max_age, " years old."
print "On average, people are ", mean_age, " years old."

Obtaining the standard deviation is also easy:

np.std(data)

Let's sort the ages in ascending order:

np.sort(ages)

Let's say we are teleported in the future, year 2042 where everyone has twice as much devices as we have today. Based on our data, the number of devices in 2013 for a list of people are:

devices_now = data[:,1]

How to compute the number of devices devices_future in the future? First, let's initialize an empty NumPy array that has the same number of elements as devices_now:

devices_future = np.zeros(devices_now.shape)

One way is to use a for loop, to access each element in the array devices_now and multiply it by 2. That's the way we would multiply each element in a list if we used Python built-in lists that are initialized in a following way:

sample_list = [1, 2, 3, 4]

Ok, let's try that with the NumPy array:

for i in range(len(devices_now)):
  devices_future[i] = 2*devices_now[i]

print "Numbers of devices in 2042:", devices_future

Whoa, that's a lot of code! Well, NumPy comes to rescue here and we can replace the code by this line:

devices_future = 2*devices_now

Not only is this much nicer to read, but it's also much faster! Notice that if we were using built-in Python list of integers we would get something different if we used the same syntax. Here we're using range to create a list of 4 subsequent integers. Check this out:

a = range(4)
a*2

When not sure if dealing with lists or NumPy arrays, use type.

Let's make some nice simple plots. First, we can plot our two arrays:

pl.figure()
pl.plot(ages, label='Ages')
pl.plot(devices_now, label='Devices')
pl.xlabel('Person ID')
pl.title('Data')
pl.legend()
pl.show()

pl.show() is necessary to display your plot. Well, this looks nice but it's not so informative, let's try something else. How about plotting distribution of ages using histograms? We split our bins into equally spaced intervals of 10 years.

pl.figure()
bins = [0, 10, 20, 30, 40, 50, 60, 70, 80]
pl.hist(ages, bins, histtype='bar', rwidth=0.8)
pl.ylabel('Number of people')
pl.xlabel('Age')
pl.title('Distribution of ages')

Much better! Now, let's try to use some colors and NumPy's arange to generate the spacing of bins, instead of writing them out manually:

pl.figure()
bins = np.arange(0, 90, 10)
pl.subplot(1,2,1)
pl.title('Distribution of ages')
pl.hist(ages, bins, histtype='bar', rwidth=0.8, color=['crimson'])

pl.subplot(1,2,2)
pl.title('Distribution of devices')
bins_dev = np.arange(0, 30, 5)
pl.hist(devices_now, bins_dev, histtype='bar', rwidth=0.8, color=['burlywood'])

Try inspecting bins and bins_dev to convince yourself that those are the bins we wanted. What happens if you move the pl.title above the pl.subplot?

If you're done with this exercise and want to have more NumPy, here's something to play with: Use IPython's magic function %timeit to compare the execution time of the two above mentioned cases of array multiplication: the one using for loop and lists and the other with NumPy arrays. How long does it take to multiply an array with a scalar in a for loop? To spice it up a bit, don't be afraid to use bigger arrays and lists (e.g. N=1000, N=100 000 and N=100 000 000)!