An Introduction To Software Development Using Python

Spring Semester, 2015

Class #22:Dictionaries

What Is A Dictionary?

• A dictionary is a container that keeps associations between keys and values.

• Keys are unique, but a value may beassociated with several keys.

John

Mike

Ann

Mary

$15,000

$12,000

$30,000

Keys Values

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What Is A Dictionary?

• The dictionary structure is also known as a map because it maps a unique key to a value.

• It stores the keys, values, and the associations between them.

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Creating Dictionaries

• Each key/value pair is separated by a colon.

• You enclose the key/value pairs in braces,just as you would when forming a set.

• When the braces contain key/value pairs, theydenote a dictionary, not a set.

• The only ambiguous case is an empty {}. By convention, it denotes an empty dictionary, not an empty set.

• You can create a duplicate copy of a dictionary using the dict function:oldSalaries = dict(salaries)

salaries = { “John": 15000, “Ann": 30000, “Mike": 12000, “Mary": 15000 }

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Accessing Dictionary Values

• The subscript operator [] is used to return the value associated with a key.

• The statement:

print(“Ann’s salary is", salaries[“Ann"])

prints 30000

• Note that the dictionary is not a sequence-type container like a list.

• Even though the subscript operator is used with a dictionary, you cannot access the items by index or position.

• A value can only be accessed using its associated key

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Searching For Keys

• The key supplied to the subscript operator must be a valid key in the dictionary or a KeyError exception will be raised.

• To find out whether a key is present in the dictionary, use the in (or not in) operator:

if “Ann" in salaries : print(“Ann’s salary is", salaries[“Ann”])else : print(“Ann’s salary is not in my list.”)

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Default Value

• Often, you want to use a default value if a key is not present. For example, if there is no salary for Mike, you want to get an average salary instead.

• Instead of using the in operator, you can simply call the get method and pass the key and a default value. The default value is returned if there is no matching key.

number = salaries.get(“Mike", 17000)print(“Salary: " + number)

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Adding and Modifying Items

• You can change a dictionary’s contents after it has been created.

• You can add a new item using the subscript operator [] much as you would with a list: salaries["Lisa"] = 25000

• To change the value associated with a given key, set a new value using the [] operator on an existing key: salaries["Lisa"] = 17000

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Another Way To Create A Dictionary

• Sometimes you may not know which items will be contained in the dictionary when it’s created.

• You can create an empty dictionary like this:salaries = {}

• and add new items as needed:salaries["John"] = 15000salaries["Ann"] = 30000salaries["Mike"] = 12000salaries["Mary"] = 15000

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Removing Items

• To remove an item from a dictionary, call the pop method with the key as the argument: salaries.pop(“Mike")

• This removes the entire item, both the key and its associated value.

• The pop method returns the value of the item being removed, so you can use it or store it in a variable: mikesSalary = salaries.pop(“Mike")

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Avoiding Removal Errors

• If the key is not in the dictionary, the pop method raises a KeyError exception.

• To prevent the exception from being raised, you can test for the key in the dictionary:

if "Mike" in salaries : contacts.pop("Mike")

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Traversing a Dictionary• You can iterate over the individual keys in a dictionary using a

for loop: print(“Salaries:") for key in salaries : print(key)

• The result of this code fragment is shown below: Salaries: John Ann

Mike Mary

• Note that the dictionary stores its items in an order that is optimized for efficiency, which may not be the order in which they were added.

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Different Ways Of Doing The Same Thing

• Lists– prerequisites = [“COP 2271c”, “Introduction to

Computation and Programming”, 3]– print(values[5]) # Prints the element at index 5

• Sets– cheesePizza = {“Creamy garlic”, “Parmesan sauce”,

“Cheese”, “Toasted Parmesan”}– if "Toasted Parmesan" in cheesePizza :

• Dictionaries– salaries = {"John": 15000, "Ann": 30000, "Mike":

12000, "Mary": 15000 }– print("Ann’s salary is", salaries["Ann"])

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What’s In Your Python Toolbox?

print() math strings I/O IF/Else elif While For

DictionaryLists And/Or/Not Functions Files ExceptionSets

What We Covered Today

1. Creating Dictionaries

2. Accessing Dictionaries

3. Searching Dictionaries

4. Removing Items

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What We’ll Be Covering Next Time

1. External Libraries

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