Representing data: Variables and Types

Learning Goals

Numeric types

• Integer (whole number) - int()
• Float (decimal value) - float()

Text

• String (text) - str()
• Text in Python is represented by string objects, composed of individual characters that are enclosed between the matching single or double quotes.
• Usually, a “character” is a single symbol, such as ‘a’, ‘1’, or ‘$’ but there is a handful of characters that are “escaped” and are made up by two characters (e.g., \n and \t)
• \n is a newline character; technically, it consists of two characters (\ and n) but together are considered by the Python interpreter as a single character. Same with a tab character \t.
• A space, tab, or newline is called whitespace.

Assume year is 2021, and budget is set to "$1,200,000". What is the output of the following? print("The budget in", year, "was", budget)

Numbers vs. text

The string '2021' (with quotes) is fundamentally different from the integer 2021 (without quotes). The ‘2021’ string is a sequence of the characters '2', '0', '2', and '1' arranged in a certain order, whereas 2021 represents the integer value two-thousand twenty-one. You can do arithmetic with the numeric types but they do not work with text.

What do you get if you run the following? Why?

print('2021' * 2)
print(2021 * 2)

Collections

• Lists

names = [] # same as list()
names.append(‘Sarina’)
names.append(‘Lia’)
names.sort()
print(names)

# first name on the list
names[0]
# second name on the list
names[1]
print(“Name 1”, names[0])
print(“Name 2”, names[1])

• Dictionaries
• Sets

A list and a dictionary

Important notes and terminology:

• When creating a list, we need to use square brackets, which are [ and ].
  • An empty list is created by assigning an empty set of brackets to its corresponding variable name: test_list = [ ]
  • The different values/elements in a list are separated by commas ,
    • test_list = ['Enterprise', 3.14159]
  • The first element in a list is at the position/index zero, i.e., test_list[0]
• When creating a dictionary, we need to use curly braces, which are { and }.
  • An empty dictionary is created by assigning an empty set of braces to its corresponding variable name: test_dict = { }
  • The different elements in a dictionary are stored as the key : value pairs, which are separated by commas ,
    • test_dict = {'Enterprise' : 3.14159 }
  • The dictionary doesn’t have a built-in order of its elements, i.e., we cannot access its first element using an index
  • To access a value of an element in a dictionary, we need to know its key, which is accessed using square brackets indexed by key
    • test_dict['Enterprise']

Check your understanding

• What’s the difference between characters and strings?
• What’s the difference between lists and dictionaries?