Dictionaries

Programming basics

Dictionaries are sometimes found in other languages as “associative memories” or “associative arrays”. Unlike sequences, which are indexed by a range of numbers, dictionaries are indexed by keys.

It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary). A pair of braces creates an empty dictionary: {}. Placing a comma-separated list of key:value pairs within the braces adds initial key:value pairs to the dictionary; this is also the way dictionaries are written on output.

The main operations on a dictionary are storing a value with some key and extracting the value given the key. It is also possible to delete a key:value pair with del. If you store using a key that is already in use, the old value associated with that key is forgotten. It is an error to extract a value using a non-existent key.

Creating a dictionary

# The code below creates a dictionary 
# Please note the KEY:VALUE construction

studentEmail = {'Alice':'alice@google.com', 'Bob':'bob@facebook.com', 'Charlie':'charlez@microsoft.com', 'Daniel':'Danny@aswarsaw.org'}

# Bob's email can be called by:

studentEmail['Bob']

# (in a list, we would need to do something like: studentEmail[1] )

Accessing a dictionary

# If you want to print a dictionary (kind of ugly) you can simply type:

print(studentEmail)

# However, it is far more common to retrieve an element by it's key
# The instructions below accesses Bob's email address:

print(studentEmail['Bob'])

Inserting into a list

# if we want to add onto a list (append) we could simply use the append method. The code below appends wild board onto the end of our list.

polishAnimals.append('wild boar')

# if we wanted to replace a certain element, we could simply overwrite it. Below we are replacing the 2nd item of our list with a new animal:

polishAnimals[1] = 'Stork'

Deleting an element from a list

# There are a few different ways to remove an element from a list. The first way is to call the remove method.
# The remove method works by finding the name of an element.

polishAnimals.remove('Moose')

# another way to remove an element from a list is to call the pop method. Pop-ing removes an index and return the value you removed. 
# the line below removes the 2nd element from our list and makes it available for us to use. 

polishAnimals.pop(1)

# we might use the pop method like this:

animalsDeleted = polishAnimals.pop[1]

# we can also use the del function.

del polishAnimals[2]

# to delete an entire list, call the clear method (this is only available in Python 3+

polishAnimals.clear

Standards

• Construct algorithms using pre- defined sub-programmes, one- dimensional arrays and/or collections.

Give a sequence of brief answers with no explanation.

Develop information in a diagrammatic or logical form.