IT 116: Introduction to Scripting

IT 116: Introduction to Scripting
Class 3

Microphone

Trouble? Questions?

Has anyone had any trouble doing something for this class or have any questions?

Readings

If you have the textbook you should read Chapter 2, Input, Processing and Output, from the textbook, Starting Out with Python.

Homework 2

I have posted homework 2 here.

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

Let's take a look at it.

Today's Class

Today I will talk about the basic components of the Python language.

And how they are used to write programs

General Advice

Do You Have Enough Time to Do the Work for This Course?

Many of you work, either part or full time
This cuts down on the time you have to study
Technical courses are filled with details where accuracy is very important
They also introduce concepts which may take some time to fully understand
Such courses require a significant amount of study time ...
to learn the material and do well in class
This studying should not be done on one day ...
but spread out over the days of the week
You should not be taking this course ...
if you can't study several hours a week for it
Too many students rush to finish a degree ...
without doing the work to really learn the material
These people are setting themselves up for professional failure
If you try to do too much ...
you may end up accomplishing too little
Today is the last day of Add/Drop ...
when dropping the class won't mean losing money
Be honest with yourself
Looking at all your other commitments ...
do you have enough time for the work I will demand of you?

Course Work

Class Quizzes

From now on, each class will have a Class Quiz
The Class Quiz is ungraded
It is designed to help you remember the material covered in class
Here is how it works
Go to the listing of Class Quizzes you will find here
Click on the link for the day's quiz
Get a blank piece of paper
Write the answers to the questions on the paper
Go to the listing of Class Quiz Answers you will find here
Check your answers
Do not give me the paper with your answers
Class quizzes are not graded
They are designed to help you learn the material covered in class

First Graded Quiz

The first Graded Quiz will be given on Tuesday next week
I will pass out the exam papers after my lecture
Questions on the Graded Quiz come from the Class Quizes
Usually the quizzes from the previous week
Unless I tell you otherwise ...
you should expect a Graded Quiz each week on Tuesday
You will be able to see your scores on Gradescope

Homework and it116 Directories

When you first log in, you will be in your home directory
This directory belongs to your account ...
and no other account can make changes to it
The name of this directory is your Unix account name
If you registered for this class, you should see a it116 entry here
It looks like a directory ...
but it is really something called a link
If you use cd to enter it116 ...
you will be inside your IT 116 class directory
In Class Exercise 2 your were told to create two directories
An hw directory for homework scripts
And an ex for Class Exercises
For each new assignment you must create a new directory inside either hw pathname ex
So today you will create an ex3 directory inside your ex directory
The files you create for this exercise should be created in this directory

Tips and Examples

Some Skills Only Come With Work

This course requires you to learn and understand a number of concepts
You will find a list of them here
You must know them on a deep and fundamental level ...
so that you can use them to accomplish a task
It is not enough to memorize a definition ...
though that is a first step
Each of these concepts is a tool
And the best way to learn a tool is to use it
That is why there are so many assignments in this class
You will be writing small programs for this course
When you write them, think of the concepts you are learning
And what other things you can do with them
You might want to play around with a concept
Write a little something of your own ...
to better see how they work
Understanding comes with effort
The more you work with a concept ...
the more you can do with it

Class Exercises

There is a Class Exercise for every class ...
except the review sessions for the Midterm and Final exams
Each Class Exercise must be in a separate directory inside the ex directory
The ex directory must be inside your it116 directory
If you do not finish the script in class you can finish it later
All Class Exercises are due the following Sunday at 11:59 PM
I check your Class Exercises 4 times a week
The last check is on the weekend after they are due
If there is a problem with your script, or it is missing ...
I will send an email to your UMB email address
If the exercise is not complete or it has an error you will lose points
I have have written test scripts for each of the Class Exercises
I use these scripts to test your code
You can run these scripts to test what you have written
But you have to connect to pe15.cs.umb.edu to do this
At the bottom of every Class Exercise is the command you run to test your script
When you run this command you will be asked for your username
After you enter this an hit return, the tests will bw run

Here is how it looks for ex2.py

$ ~ghoffman/it116_test/ex02.sh
Unix username: ghoffman
++++++++++++++++++++++++++++++++++
Error check
----------------------------------
----------------------------------
++++++++++++++++++++++++++++++++++
### Looking for: Hello world
----------------------------------
Hello world!
==================================

The first test is the Error check, which looks for mistakes in your code
If you see nothing between the two dashed lines, no errors were detected
If the output of your script is missing something ...
or produces incorrect output you will hear a beep ...
and see an error message

Getting Help

This course makes you do things that you may not have done before
Whenever you do something new ...
there is a good chance you will make a mistake
When this happens, the first thing you should do is reread the instructions
Mistakes often happen when we read too fast
If you cannot fix the problem in a reasonable amount of time ...
ask for help
There are a number of ways you can do this
- Post a question in the class discussion area
- Email the Class Assistant
- Ask other students in the class
- Come to Zoom Office Hours
DO NOT EMAIL ME with your question
I have close to 100 student each semester
With that workload I must be very careful how I spend my time
I would rather respond to your question on Piazza than in an email
Only one student will see an email response
But an answer on Piazza can be seen by the entire class
Only send me an email when you have an issue that affects you and you alone
Like being out of the country due to a family emergency

Describing a Problem

You can't get help with a problem unless you describe it well
Statements like the following are useless when asking for help
- It does not work
- I get an error when I run the code
You have to describe the specific circumstances that caused the problem
The best way to do this is with the actual text of the error mesage

Something like this

>>>  print "Hello World"
SyntaxError: Missing parentheses in call to 'print'. Did you mean print("Hello World")?
>>>

Also, be sure you upload your code to the correct directory on pe15.cs.umb.edu
That way I can make a copy of the code ...
and use various debugging techniques

Don't Use Screenshots in Piazza

Many students paste screenshots of code into Piazza
This is not a good idea
I have a hard time reading some of the screen shots
And I can't copy the code into the Python interpreter to test it
Instead, copy the code to your class directory on pe15.cs.umb.edu ...
and type the name of your file into your Piazza entry
The only time you paste a screenshot into Piazza ...
is when you are having problems with FileZilla

Login Failure

If you enter the wrong password when connecting to pe15 ...
you will be blocked from logging in for 10 minutes
Wait 10 minutes and try again
If you have forgotten or misremembered your password, you can change it
Go to https://portal.cs.umb.edu/password/
If you have trouble with your account send an email to operator@cs.umb.edu

Review

IDLE Has Two Different Parts

IDLE is a single program ...
that has two different components
The first part allows you to write a single Python statement ...
hit Enter or Return ...
and have the Python interpreter run that statement
This happens in the first window you see when you open IDLE
In this window you are talking directly to the Python interpreter
The second part is the editor
In this window you write your Python programs ...
and save them to a directory on your computer
After you have saved them ...
you run them in the interpreter window ...
to see if they work properly

Attendance

New Material

Statements

Just like as a paragraph is broken up into sentences ...
a Python program is broken up into statements
A statement is one or more lines of code ...
that performs a complete action
So a statement is like a sentence in Python
There are number of different types of statements in Python ...
some of which we will discuss today
A Python program is a collection of statements
These statements are contained in a text file ...
whose name must always end with .py
Since the CPU that runs our machines cannot understand text ...
Python programs are run using the Python interpreter
The interpreter is a program that runs through your script ...
working statement by statement ...
translating each statement into the machine language that the CPU understands
Python programs are often called scripts ...
since Python is a scripting language
How do you run a Python script?
You can create a text file using the editor window in IDLE ...
and then use the Run Module item in the Run menu
You can also run a script on the command line
On my Mac or on pe15 I would type
```
$ python3 hello.py 
Hello world!
```

Functions

The progrm hello.py consists of a single line
```
print("Hello world!")
```
print is a function
A function is a group of Python statements ...
that has been given a name ...
and does something useful
I like to think of a function as package of Python statements
You will be using a number of function in this class
And you will learn to write your own
When you want to ask a function to do something ....
you use a particular kind of statement ....
called a function call
The statement has two parts
- The function name
- Values inside parentheses ( )
The values are called arguments
A function call always has parentheses ...
even if there is nothing inside them

Data Types

The first computers were designed to work with numbers
But people soon learned that they can also deal with words
So the Python needs to tell the different between a number and a word
But there are two kinds of numbers that computers work with
Computers find it easy to work with integers
Intergers are the counting numbers ...
which can be both positive and negative
```
-5 -4 -3 -2 -1 0 1 2 3 4 5
```
But they need special circuits to work with decimals like 5.2345
Each of these three different types of data ...
are represented differently in the computer's memory
We call each of these different formats a data type
You can see these inside the IDLE interpreter window using the type function
You give type a value ...

And it returns a data type

>>> type(5)

>>> type(5.5)

>>> type("five")

We will come across different types later ...
But we start out with these three
- Integer
- Float (decimal)
- String
Float is short for floating decimal point

Strings

A string is a collection of characters
To write a string you must enclose the characters inside either single (') or double (") quotes
So "Bob" and 'Bob' are both strings
It doesn't matter which quote you use ...
as long as the quote at the begining and end are the same

If you mix the quotes you will get an error

>>> print("Hello')
  File "<stdin>", line 1
    print("Hello')
          ^
SyntaxError: unterminated string literal (detected at line 1)

Characters

Everything in a computer's memory is a binary number
Binary numbers use powers of 2 ...
and are written using the digits 1 and 0
The number 7 written in binary is
```
111
```
And 10 is written
```
1010
```
The electronics in a computer turns the number in memory ...
into the characters we see on the screen
This is done using a table where each character has its own unique number
We can see the number used to represent a character using the ord function
ord takes a character as its only argument

It returns the number representing that character

>>> ord("A")
65
>>> ord("a")
97
>>> ord("b")
98
>>> ord("c")
99

The chr function does the reverse
It takes a number as its argument
And returns the character represented by that number
```
>>> chr(100)
'd'
>>> chr(101)
'e'
>>> chr(102)
'f'
```

ASCII

To use strings on a computer ...
we need a table of all the characters ...
and a number that represents the charcter
In the early days of computing the most frequently used table was ASCII
ASCII stands for American Standard Code for Information Interchange
You will find the table for ASCII here
ASCII was developed in the United States for use with teletype machines ...
which were used to send written words over telegraph wires

ASCII had 128 entries which stood for 95 characters ...
and 33 control codes
Each character can be stored in 7 bits of memory
A bit is a binary digit
Lowercase and uppercase letters have different numeric values in ASCII
```
>>> ord("a")
97
>>> ord("A")
65
>>> 
```
The digits 0 to 9 are also characters

So each of them is included in the ASCII table

>>> ord("1")
49
>>> ord("2")
50
>>> ord("3")
51

ASCII also includes a number of symbols

>>> ord("#")
35
>>> ord("$")
36
>>> ord("%")
37
>>> ord("&")
38

Unicode

ASCII was good enough for English
But lacks characters used in many European languages ...
let alone Asian and African languages
Nowadays we use a new character table called Unicode
Unicode represents all the characters in most world writing systems
The first 128 Unicode characters are the ASCII characters
So ASCII is included in Unicode
The latest version of Unicode contains 128,237 characters
They cover 135 modern and ancient writing system
Most of the major world writing systems are contained in Unicode
But Chinese characters pose a challenge
Altogether there are over 50,000 Chinese characters
Though a good modern dictionary will rarely list over 20,000
An educated Chinese person will know about 8,000 characters
But you only need to know about 2-3,000 to read a newspaper
Unicode currently has 74,605 CJK characters which are used in writing
- Chinese
- Japanese Kanji
- Korean Hanja
- Vietnamese Chu Nom
A writing system is not the same thing as a language
Cuneiform was a writing system developed in the Near East around 3,200 BCE

It was used for writing on clay tablets
The following languages were written in cuneiform
- Sumerian
- Akkadian
- Eblaite
- Elamite
- Hattic
- Hittite
- Hurrian
- Luwian
- Urartian
- Old Persian
Unicode includes the Cueiform characters

Representing Unicode Values

There are so many Unicode characters that you can't represent all of them in 7 bits ...
as you can with ASCI
Some characters require 17 bits
But if you used 17 bits to represent every character ...
text documents would be extremely large
So the numbers that represent Unicode characters can be written in different ways
Each of these schemes is called an encoding
The most commonly used Unicode encoding is UTF-8
This is the encoding Python uses by default
UTF-8 uses a variable number of bytes to represent each Unicode character
Either 1, 2, 3 or 4 bytes
It uses the smallest number of bytes it can for each character
A byte is 8 bits
If you stick to English, each character is one byte long
You will find a listing on Unicode characters represented in UTF-8 here

Ranges of Characters in ASCII

The ASCII characters fall into certain ranges
The first 31 characters are control characters

The characters from 32 to 47 are symbols

32   (Space)
33 !
34 "
35 #
36 $
37 %
38 &
39 '
40 (
41 )
42 *
43 +
44 ,
45 -
46 .
47 /

The digits come next

Then some more symbols
```
58 :
59 ;
60 <
61 =
62 >
63 ?
64 @
```
Then the capital letters
```
65 A
...
90 Z
```
Some more symbols
```
91 [
92 \
93 ]
94 ^
95 _
96 `
```
The lower case letters
```
97 a
...
122 z
```
And the last few symbols
```
123 {
124 |
125 }
126 ~
```
Both Unix and Python are case sensitive
Which means that UPPERCASE and lowercase letters are totally different characters

Variables

What do you do with something you need to use ...
but not at this moment?
You put it somewhere it won't get lost ...
so you can find it later
Some old desk had slots to hold such things

The slots were call pigeon holes
In programming we often need to store a value that we will use later
To do this, we create a variable
A variable is a location in the computer's memory that has a name ...
and stores a single value

In some languages, such as Java, C and C++ ...
you must specify what type of value the variable holds
You must do this before you use the variable
And the type of value it holds cannot be changed
If you declared the variable number to be a decimal
You could not store a string in it
This is not true in Python ...
or other scripting languages like Perl and PHP
You can store a string in a Python variable one moment
Then store an integer in the same variable some time later

Creating Variables with Assignment Statements

In Python you create a variable by giving it its first value
This is done with an assignment statement
An assignment statement has the general form
```
LOOP_VARIABLE  =  VALUE
```
Whenever a word appears in ALL CAPITALS ...
it is a placeholder
In other words, it stands for a value that you have to supply
If I wanted to create the variable name and give it my name as a value I would write
```
name = "Glenn Hoffman"
```
If I wanted to define the variable rate with the value .03, I would write
```
rate = .03
```
Once you have defined a variable ...
you can change its value with another assignment statement
```
rate = .05
```
That is why they are called variables

More on Characters

In some computer languages, like C and Java ...
each type of quote has a special purpose
But not in Python
Why?
What if you wanted to print the following
```
What's for dinner?
```

If you enclosed the string inside single quotes you would get an error

>>> print('Hello world!')
Hello world!
>>> print('What's for dinner?')
  File "<stdin>", line 1
    print('What's for dinner?')
                ^
SyntaxError: invalid syntax

When the interpreter saw the ' in what's ...
it thought the string was over ...
and it had no idea what to do with the rest of the line
To get around this we can enclose the string in double quotes, ",
```
>>> print("What's for dinner?")
What's for dinner?
```
Similarly, if the string contained double quotes ...
we would enclose the string literal in single quotes
```
>>> print('He cried "Help!"')
He cried "Help!"
```
I tend to use doubles quotes because they are easier to see
But you should feel free to use either kind of quotes