1 Introduction
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1.1 What is a program?
A program is a sequence of instructions that tells a computer what to do – for example, read a file, calculate an average, or display a chart. Computers cannot understand human languages (at least not directly), so we write these instructions in a programming language: a language with a small vocabulary and strict grammar rules, designed to be precise and unambiguous.
In this book, we use Python, one of the most popular programming languages today. Python is relatively easy to read and learn, and it offers a huge collection of libraries (ready-made building blocks), including libraries for data analysis such as Pandas, which we will use extensively later in this book.
Programming means breaking a problem into small steps and expressing these steps in a programming language. Note that the computer follows our instructions exactly as written – so if a program does not do what we intended, it is usually because our instructions were not quite right, and it is our job to find and fix the mistake.
1.2 Hello, world!
Let’s start with the simplest Python program possible, printing a single line of text:
Here, we have called the built-in print function and provided (or passed) a piece of text (called a string in Python) to be printed out as its argument, placed inside the parentheses ( and ). Note that the text (the string) is enclosed in quotation marks.
The print function can take multiple arguments, separated by commas, and print them on the same line with spaces in between.
As you can see, the arguments of the print function do not have to be strings, we can use numbers too.
We can also call the print function without any arguments to print an empty line:
Note that we always need the parentheses () after print. Without them, we are just referring to the function itself, not actually calling it (i.e., not asking it to do anything).
Note for beginners: Think of a friend you want to ask to write something on a piece of paper. Without the parentheses, you are just thinking about that friend, but you have not actually asked them to do anything. Including the parentheses is like going to the friend and asking them to write something down. You also need to tell them what to write, whether it is a single piece of text or several pieces, and that is exactly what the arguments are for.
1.3 Using (interactive) Python as a calculator
When using interactive Python environments, such as IPython or Jupyter notebooks, entering and running a piece of code will automatically display the last calculated value.
This allows us to use Python as a calculator:
Note that we have not called print in this example. In interactive Python environments, the last calculated value is automatically shown as the output.
However, if we run:
we still see only the output of the last calculation. To display multiple values, we can use the print function.
Here’s another example, showing that in Python we can even “add” strings and “multiply” them by integers:
In these examples, 1, 3, 2, 7.5, "Testing, ", "testing, " and "can you hear me?" are literals, fixed values written directly in the program.
Exercise. Calculate the square root of 1524157875019052100. (Hint: you can search online how to do that in Python!)
Sample solution
# We have several options:
# Option 1
1524157875019052100 ** 0.5
# Option 2
import math
math.sqrt(1524157875019052100)
# Option 3
import numpy as np
np.sqrt(1524157875019052100)1.4 Using modules
Python comes with a large standard library, a collection of modules providing ready-to-use functions and other tools. To use a module, we first need to import it:
After import math, everything the module provides is available using the math. prefix.
If we only need a few specific functions, we can import them directly (no prefix needed):
We can also give a module a shorter alias with as. This is very common for data analysis libraries – you will soon get used to import pandas as pd and import numpy as np:
Beyond the standard library, there are hundreds of thousands of third-party packages (bundles of modules), including NumPy, Pandas and Matplotlib, which we will use later. In this online book, everything is preinstalled, so import is all you need. When using Python on your own computer (e.g., in Jupyter notebooks), third-party packages must first be installed, typically with the pip package manager (e.g., by running pip install pandas).
1.5 Programs are sequences of statements
The lines of code we have run so far are examples of statements, instructions to Python (and, in turn, to the computer). A program is a sequence of statements. Most typically, each statement is written on its own line, and Python executes them one after another, from top to bottom:
Python belongs to the family of imperative languages: programs are commands telling the computer what to do, step by step. In the following chapters, we will meet other kinds of statements: assignments, conditional statements, loops, function definitions, and more.