IT 117: Intermediate Scripting

IT 117: Intermediate Scripting
Class 5

Tips and Examples

Read the Homework Assignments Carefully

New Material

Sequences
Dictionaries
Dictionary Literals
Empty Dictionaries
Getting Values from a Dictionary
Changing a Dictionary Value
Dictionaries Are Not Sequences
Looping Through a Dictionary
Tuples As Dictionary Values
When To Use a Dictionary

Graded Quiz

You can connect to Gradescope to take weekly graded quiz today during the last 15 minutes of the class.

Once you start the quiz you have 15 minutes to finish it.

You can only take this quiz today.

There is not makeup for the weekly quiz because Gradescope does not permit it.

Readings

If you have the textbook read Chapter 9, Dictionaries and Sets section 9.1, Dictionaries.

Homework 3

I have posted homework 3 here.

It is due this coming Sunday at 11:59 PM.

Graded Quiz

You can connect to Gradescope to take weekly graded quiz today during the last 15 minutes of the class.

Once you start the quiz you have 15 minutes to finish it.

You can only take this quiz today.

There is not makeup for the weekly quiz because Gradescope does not permit it.

Late Penalty

You will lose 2 points for every day that your assignment is late.

Every Unix file has a time stamp that changes every time the file is modified

I look at this time stamp to determine whether a submission is late.

If your assignment is not working by the due date, you can continue to work on it but you will lose a few points.

If your assignment is working, do not go back and change it, because that will change the time stamp and result in a late penalty.

Do Not Email Me about Missing Assignments

If you get an email from me saying an assignment is missing do not email me about this.

I get far too many emails.

Instead of sending me an email, fix the problem.

If you do not know how to fix it, post a question on Piazza or contact the Class Assistant.

I collect homework assignments and and check Class Exercises several times during the week.

I will check or collect your assignment later in the week.

Class Exercises

Scripts for each Class Exercise must have the correct name and be in the right directory or my scripts will not see them.

If my scripts do not see them, your score with be zero.

If you receive no email from me about your Class Exercise script, that means your script will get full credit, less any late penalty that applies.

Tips and Examples

Read the Homework Assignments Carefully

Homework assignments may ask you to do many things
So you may need to read the assignment more than once
A single reading might not be enough to understand what I want you to do
Take the time to read these documents slowly
This will help you understand all the details
If you don't, you will make mistakes and lose points
Technical work is detailed work
So you must learn to read documents with a lot of detail
If you do not understand something post a question on Piazza
NEVER GUESS at the meaning of something I have written

New Material

Sequences

Sequences are Python objects that hold more than one value
The values are stored one right after the other
So the elements in a sequence have a definite order
In IT 116 you encountered three types of sequences
- Lists
- Tuples
- Strings
A list contains a series of values that can be changed at any time
A tuple is also a series of values but a tuple is immutable
That means it cannot be changed
A string is a sequence of characters which cannot be changed
But you can make a new string from a previously created string
Strings also have special methods that only make sense when applied to characters
Such as capitalize

Dictionaries

Dictionaries can also contain multiple entries
But each entry has two parts
The first part is the key
The second is a value associated with that key
You use the key to get the value
An entry in a Python dictionary is a key-value pair

Dictionary Literals

When you create a list literal you enclose the individual values in square brackets, [ ]
And put commas between the values
```
numbers = [1, 2, 3, 4, 5]
```
With tuples the values are enclosed in parentheses, ( )
Again, commas separate the values
```
numbers = (1, 2, 3, 4, 5)
```
When we create strings we enclose them in single or double quotes
```
numbers = "12345"
name    = 'Bob'
```
In a dictionary literal we enclose the entries inside curly braces, { }, with commas between the entries

In each entry, there is a colon, :, between the key and the value

email_addresses = {"Chris":"chris57@gmail.com" , "Alan":"ahurley175@yahoo.com", "Frank":"frank27@hotmail.com"}

While keys are often strings they can be values of many different data types
So we could create a dictionary of integers and the words for the same value
```
integer_words = {1:"one", 2:"two", 3:"three", 4:"four", 5:"five"}
```

Empty Dictionaries

A special kind of dictionary literal is an empty dictionary
We often create a list by starting with an empty list and then add entries to it
Similarly, we often create a dictionary by starting out with an empty dictionary
Just as the empty list is represented by [ ]
And an empty tuple is represented by ( )
{ } is an empty dictionary

Getting Values from a Dictionary

How do we get the value associated with a key?

To get a value from a list we put an index inside square brackets, [ ]

>>> numbers
[1, 2, 3, 4, 5]
>>> numbers[0]
1
>>> numbers[1]
2
>>> numbers[4]
5

The square brackets are actually an operator
When used with lists the square bracket operator takes two operands
A list variable and an index
The operator gives us the value at that index position
You can use the square bracket operator with a dictionary
But we use the key not an index number inside the brackets
So to get the email address for "Chris" I would write
```
>>> email_addresses["Chris"]
'chris57@gmail.com'
```
And to get the email address for "Frank" I would write
```
>>> email_addresses["Frank"]
'frank27@hotmail.com'
```
We don't use indexes to get the values from a dictionary as we do with lists, tuples, and strings
Instead we use the key to get the value

If you use the [ ] operator on a key the doesn't exist you will get a KeyError exception

email_addresses["Waldo"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'Waldo'

In computer science, a dictionary is sometimes called an associative array or a map or a hash

Changing a Dictionary Value

You change an element in a list using [ ] and the assignment operator

Like this

>>> numbers
[1, 2, 3, 4, 5]
>>> numbers[0] = 83
>>> numbers
[83, 2, 3, 4, 5]

We can use the square bracket operators to do the same thing with dictionaries

>>> email_addresses
{'Alan': 'ahurley175@yahoo.com', 'Frank': 'frank27@hotmail.com', 'Chris': 'chris57@gmail.com'}
>>> email_addresses["Alan"] = "alan.hurley@gmail.com"
>>> email_addresses
{'Alan': 'alan.hurley@gmail.com', 'Frank': 'frank27@hotmail.com', 'Chris': 'chris57@gmail.com'}

Dictionaries Are Not Sequences

I started out this discussion by talking about sequences
But dictionaries are not sequences
Sequences store a single value
And the values are stored one right after the other
So you can pick out a specific value using an index
Which specifies its position in the sequence
Dictionaries use a data structure called a hash table
Hash tables minimize the time it takes to find the value for a key
The order of things in a hash table changes with time
As entries are created and deleted
In earlier version of Python 3 you could see this
Because the order in which dictionary entries were created
Was not the order in which they appeared when printed
This has changed
So dictionaries work different in the latest versions of Python
The order in which entries are made is now the order in which they are now printed
But dictionaries are still not sequence

Looping Through a Dictionary

The Python for loop has the following format

for LOOP_VARIABLE in LIST_LIKE_THING:
    STATEMENT
    STATEMENT
    ...

Where the LOOP_VARIABLE is assigned each value in the LIST_LIKE_THING

For each pass through the loop

>>> nonsense
('foo', 'bar', 'bletch')
>>> for word in nonsense:
...   print(word)
... 
foo
bar
bletch
>>> for number in [1, 2, 3, 4, 5]:
...    print(number)
... 
1
2
3
4
5
>>> for char in "Hello":
...    print(char)
... 
H
e
l
l
o

But dictionaries are not sequences
So you might think that dictionaries cannot be used with for loops

But you would be wrong

>>> for entry in email_addresses:
...    print(entry)
... 
Chris
Frank
Alan

In a for loop on a dictionary the loop variable does not get the value of the elements
Instead, each of the keys is assigned to the loop variable

And we can use these keys to get each value

>>>  for key in email_addresses:
...    print(key, "=", email_addresses[key])
... 
Chris = chris57@gmail.com
Frank = frank27@hotmail.com
Alan = ahurley175@yahoo.com

Tuples As Dictionary Values

What if I wanted to store more than one piece of information as the value for a given key?
For example, say I wanted to create a dictionary with data for each student in a class
The key would be the student ID
But I would need to store the following data
- First name
- Last name
- Unix username
- Email
Why first name and last name instead of just a student's name?
Because I want to produce alphabetical lists
To store this information do I need 4 dictionaries?
No
I can store all this in one dictionary using the student ID as the key
The value will be a tuple containing all data about the student
At the beginning of the semester I used to create a text file with data for each student

The file looked something like this

09329034 Jane        Adams      jadams   Jane.Adams001@umb.edu
03687480 Alexander   Smith      bigboy   Alexander.Smith002@umb.ed
05431692 Christopher Cannon     ccannon  Christopher.Cannon001@umb.edu
07511379 Joseph      Malloney   jmal     Joseph.Malloney003@umb.edu
07511379 Fatih       Jones      fjones   Fatih.Jones001@umb.edu
04175276 James       Reynolds   jr       jim.reynolds002@umb.edu

This is not real student data
Confidentiality prevents me from doing that

Here is a function that will read each line and return dictionary with the student information

def student_dictionary_create(filename):
    try:
        file = open(filename,"r")
    except:
        print("Cannot open", filename)
    else:
        data = {}
        for line in file:
            data_list  = line.split()
            student_id = data_list[0]
            first_name = data_list[1]
            last_name  = data_list[2]
            username   = data_list[3]
            email      = data_list[4]
            student_tuple = (first_name, last_name, username, email)
            data[student_id] = student_tuple
        return data

When I use this function and print the results

student_data = student_dictionary_create("student_data.txt")
for id in student_data:
    student_tuple = student_data[id]
    print(id, student_tuple[0], student_tuple[1], student_tuple[2], student_tuple[3])

I get

09329034 Jane Adams jadams Jane.Adams001@umb.edu
03687480 Alexander Smith bigboy Alexander.Smith002@umb.ed
05431692 Christopher Cannon ccannon Christopher.Cannon001@umb.edu
07511379 Fatih Jones fjones Fatih.Jones001@umb.edu
04175276 James Reynolds jr jim.reynolds002@umb.edu

When To Use a Dictionary

When should we use a dictionary instead of a list or tuple?
Lists and tuples are used when the elements are not particularly unique
If we create a list of quiz scores there is nothing special about a particular score
Generally I don't need to know the score for the 5th quiz
I need to have a place to put all the scores so I can do certain things
Like finding the average score
But every student is different
Each one has a unique identity
When you need to store information about unique things you should use a dictionary
Or when you need to associate two things
Like a name and an email address