Dictionaries: Difference between revisions

Revision as of 06:58, 8 August 2017

Programming basics^[1]

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.^[2]

Creating a dictionary[edit]

# 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[edit]

# 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'])

# in addition, we have the following functions to access elements in a dictionary<ref>https://www.digitalocean.com/community/tutorials/understanding-dictionaries-in-python-3</ref>:

# dict.keys() isolates keys
# dict.values() isolates values
# dict.items() returns items in a list format of (key, value) tuple pairs

Inserting into a dictionary[edit]

# if we want to add into a dictionary we can simply type:

studentEmail['Bill'] = 'bmackenty@aswarsaw.org'

# if we wanted to replace a certain element, we could simply overwrite it. 

studentEmail['Bill'] = 'bmackenty@gmail.com'

Deleting an element from a dictionary[edit]

# The del function deletes an element from a list:

del studentEmail['Bill']

Do you understand this?[edit]

Standards[edit]

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

References[edit]

[1] ttp://www.flaticon.com/

[2] ttps://docs.python.org/3.6/tutorial/datastructures.html

[1]

[2]

@@ Line 55: / Line 55: @@
 </syntaxhighlight>
-== Deleting an element from a list ==
+== Deleting an element from a dictionary ==
 <syntaxhighlight lang="python3">
-# There are a few different ways to remove an element from a list. The first way is to call the remove method.
+# The del function deletes an element from a list:
-# The remove method works by finding the name of an element.
-polishAnimals.remove('Moose')
+del studentEmail['Bill']
-# 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