An “int” is a whole number. If you type one in Python the programme will recognise what it is, but it won’t do anything with it unless you tell it to.
# This is just a number
27## 27If you aren’t familiar with reading code, know that the first (grey) box contains the Python code that we wrote while the second (white) box contains the output of that code.
You can add two whole numbers together or do other arithmetic operations on them. Depending on the programme you are using to run Python you may or may not see any output (but even if you don’t see anything the addition will still have happened in the background).
# You can use numbers to do arithmetic
5 + 7## 12A “floating point number” is a number with a decimal point in it, so-called because the way Python stores these value in memory is reminiscent of numbers ‘floating’ around a decimal point.
# This is just a number
27.03## 27.03# You can use numbers to do arithmetic
5.128 + 7.8## 12.928A number is useless on its own. In order to use it you need to assign it to a variable:
# Assigning numbers to variables
x = 5
y = 7Notice that when you assign a number to a variable it no longer gets outputted to the console. Variable assignment suppresses output! Unless you are using an IDE that automatically suppresses output, in which case it makes no difference.
You can use these numbers by calling the variables:
# You can use variables to do arithmetic by calling them
# If you assign the answer to a third variable (z), the output will be suppressed:
z = x + y
# If you don't assign the answer to a variable, it will output to the console:
x + y## 12Here’s something that often confuses beginner coders: the statement x = x + 1. In mathematics, it doesn’t make any sense! There’s no value of x that will satisfy the equation! However, in programming, this statement is not an equation but an assignment - you are giving a variable x a value that is equal to that of an existing variable x plus one:
# Give the variable 'x' a value of six
x = 6
# Give the variable 'x' a value that is equal to the value of 'x', plus one
x = x + 1
# 'x' should now equal seven. Check that it does by calling it and outputting its value to the console:
x## [1] 7Note: there are rules that have to be followed when making variable names:
Additionally, there are a few conventions that programmers tend to adhere to:
The following are GOOD examples of variable names:
# Lowercase
people = 34
# Some things, especially acronyms, are only ever written in uppercase in
# English. So it wouldn't make sense to code them in lowercase:
BMI = 22
# Use underscores to separate words
final_answer = 100
# A person's height in centimetres
height_cm = 170“Strings” are characters that have been ‘strung’ together to form words. You can write a string by using single or double quotation marks, or sets of three double or single quotation marks:
# These are strings, representing text
my_string = "Hello, World!"
my_string = 'Hello, World!'
my_string = """Hello, World!"""
my_string = '''Hello, World!'''If you want to create a string that is more than one line long you have to use one of the triple-quotation-mark methods:
my_string = """Hello,
World!"""
my_string = '''
Hello, World!
'''Note: a number can be written as a string, but if you do this it will be interpreted by your program as text, not as a number! This is something that confuses many beginner coders:
# This is a string, so you will not be able to use it to do maths!
x = '5'
# If you try to run the following code you will get an error (because you are trying to add a number to a letter,
# which doesn't make sense!):
# x + 2
# Remember, numbers must NOT have quotation marks in order to be used as numbers
x = 5
x + 2## 7These are binary “trues” and “falses”:
passed_test = True
largest_prime_number = FalseNote that the values “True” and “False” above are not strings because we have not used quotation marks. They are treated as their own object type by Python.
Booleans become useful when we start to use “if statements” later on. Essentially, they are used to test things: we can ask Python if something is true or not.
Nulls are, as the name suggests, equal to nothing. While it may seem weird that you can set a variable to nothing it becomes useful when you’re dealing with missing data.
In Python, you have two different null characters:
None is the most basic: something has been defined but no value has been assigned to itNaN or nan (‘not-a-number’) means that something has been defined as a number but no value has been given to it. If you are using something called Pandas to import data, it will use NaN for all missing data. Note that NaN does not exist in Python’s standard library, you need to import it from a library called “Numpy”:from numpy import NaN
nothing = None
not_a_number = NaNAs the name suggests, a ‘list’ is a collection of objects. These can be numbers, strings, Booleans or a mixture, and are created by using square brackets:
# A list of integers
a = [4, 5, 6, 7, 8]
# A list of floating-point numbers
b = [4.1, 5.6, 6.0, 7.3, 8.2]
# A list of strings
c = ['One', 'Two', 'Three']
# A list of Booleans
d = [True, False, True, True, False]
# A list of different data types
e = [1, 2.3, 'Hello, World', False]The items in a list are known as its elements.
One of the most infamous facts about Python is that it uses zero-indexing. This means that it starts counting from 0 as opposed to from 1. Thus, when we want to refer to an element in a list (known as indexing the list) we need to use the number one lower than it’s actual position.
Indexing a list is done by using square brackets around the index number of the element you want and appending that to the list’s variable name:
# Create a list of strings
ls = ['how', 'now', 'brown', 'cow']
# Get the first element (which is at index 0)
ls[0]## 'how'# Get the second element (which is at index 1)
ls[1]## 'now'A loop is a piece of code that runs multiple times (ie it runs in a loop) and a for loop is a loop that runs once for each element in a list:
ls = ['how', 'now', 'brown', 'cow']
# Execute a 'print' command once for each element in the above list
for element in ls:
print(element)## how
## now
## brown
## cowThe above code printed each element of the list in turn, as opposed to the following code which prints all the elements in one go:
ls = ['how', 'now', 'brown', 'cow']
# Execute a 'print' command once on the whole list
print(ls)## ['how', 'now', 'brown', 'cow']Functions are bits of code that can be used again and again just by calling the function’s name. You can tell that a function is being used because it will have round brackets - ().
The first function you should know about is the print() function. This function takes a string and outputs it (prints it) to the console:
print('Hello, World!')## Hello, World!See what happened there? We called the function print() by using its name and round brackets. Then we told it what to print - the string 'Hello, World!'. This string is the argument of the function. You pass an argument to a function.
Let’s look at a different function, type(). This can take any piece of data as an argument and will return (ie output) the type of data that the argument is. We can then print that answer to the console with the print() function. Take a look:
# An "int" is an integer - a whole number
print(type(7))## <class 'int'># A "float" is a floating point number - it has a decimal point
print(type(7.0))## <class 'float'># An "str" or string is text
print(type('Hello, World!'))## <class 'str'># A "bool" or Boolean is a logical value
print(type(True))## <class 'bool'># A numeric null is stored as a number
print(type(NaN))## <class 'float'># A general null is its own data type
print(type(None))## <class 'NoneType'># A list is a list of objects
print(type([1, 2, 3]))## <class 'list'>Often, objects of one data type can be converted into a different data type:
# Start with a number
x = 15
# Convert it into a string
x = str(x)
# Check that it is indeed now a string
print(type(x))## <class 'str'># Now take a string that is the text version of a number
y = '15'
# Convert it to an integer
y = int(y)
# Check that it is indeed now an integer
print(type(y))## <class 'int'>Obviously, only strings that are made up of digits can be converted into numbers. The string "Hello" cannot be converted into anything else!
2 Comments
If you’re brand new to Python and coding, the first thing to know about is comments.
These are lines in your program that do absolutely nothing - they are ignored when your program is run. But they are very useful for making notes for yourself and, especially, for others who will have to read your code.
You write a comment by starting your line with a hash “#”:
You can also write a multi-line comment using three sets of quotation marks (double or single) at the start and at the end of your text:
Technically, using three sets of quotation marks like this doesn’t create a comment as such, it creates a string that is simply not used anywhere (see the “Strings” section below). However, this is still used as a recognised method for commenting; it has even been endorsed by the creator of Python.
Don’t underestimate the power of comments! Just because your code makes sense to you now while you’re writing it doesn’t mean it always will. It’s surprisingly common for someone to re-open a piece of code they haven’t worked on for a considerable period of time only to have completely forgotten what their thought process was. So use your comments as notes to yourself! Similarly, if someone else needs to use your code for something they will need to work out what it does; an almost impossible task if you don’t explain yourself…