IT 116: Introduction to Scripting

IT 116: Introduction to Scripting
Class 3

Microphone

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.

First Graded Quiz

The first graded quiz will be given on Tuesday next week.

The questions from the graded quiz are taken from the ungraded quizzes of the previous week.

Unless I tell you otherwise, you should expect a graded quiz each week.

The quiz will be available on Gradescope on Tuesday.

Login Failure

If you enter the wrong password 5 times when connecting to pe15 you will be temporarily blocked from logging in to this machine.

This lock out will only last for 2 hours.

After 2 hours, you should be able try again.

If you have forgotten or misremembered your password, you can change it.

Go to https://portal.cs.umb.edu/password/

Recorded Class Meetings

You will find links to the recordings of class meeting here.

Use of Storage Space on the CS LAN

All of you will have an account on the Computer Science Department network.

With this account you get a home directory.

There is a limit on the total size of the files in your home directory.

So you should not store there anything you do not need for your classwork.

Do Not Post Anything From this Class Online

There are many web site that claim to "help" students learn new material.

As far as I am concerned sites like Chegg and CourseHero get their money by helping students cheat on their assignments.

Posting ANYTHING on such sites is a violation of your Oath of Honesty and will be reported to the Provost's Office.

This includes quiz and exam questions as well as homework assignments and Class Exercises.

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 quiz you need to take.

Answer all the questions on a piece of paper.

Go to the listing of Class Quiz Answers you will find here.

Check your answers.

Questions

Are there any questions before I begin?

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 the 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 must be finished before 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 two points for each day it is late
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!
==================================

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 always 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 two ways you can do this
- Post a question in the class discussion area
- Email the Class Assistant
DO NOT EMAIL ME with your question
I have over 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 from the ssh client window
Copy both what you typed, and what happened after you hit Enter

Something like this

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

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
And type the name of your file into your Piazza entry
This way I can run the code myself
And use various debugging techniques

Review

Running Python in Interactive Mode

There are two ways of using Python
- Interactively
- Writing Python scripts

If you enter python3 on the Unix command line and hit Enter you will find yourself using Python's interactive mode

$ python3
Python 3.5.2 (default, Nov 17 2016, 17:05:23) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

In Python interactive mode, you can type Python statements directly at the command line
Whenever you see >>>, Python is ready for your input
In interactive mode, you can use the Python interpreter as a calculator
```
>>> 5 + 6
11
>>> 6 - 5
1
>>> 3 * 5
15
>>> 4.1 / 2
2.05
```
To leave the interactive mode you must type
```
>>> exit()
```
If you have a Mac or a Linux machine you can hit Control D to quit

Python Scripts

A Python script is just a text file that contains a series of Python statements
Here is a Python script that prints a simple message
```
print("Hello world!")
```
To run this file on Unix or a Mac I need to use it as an argument to the Python interpreter, python3
```
$ python3 hello.py 
Hello world!
```
All Python script files must have the extension .py

Attendance

New Material

Output

The output of Python scripts is a stream of text
You can create output in Python using the print function
Here is a one line Python program that prints my name
```
$ cat name.py 
print("Glenn Hoffman")
```
Look what happens when I run this script on the Linux machine pe15
```
$ python3 name.py 
Glenn Hoffman
```
Whatever text is inside the parentheses after print will be printed to the screen
The single line of code in this script is a Python statement
A statement is one or more lines of code that performs a complete action
A statement is like a sentence
It stands by itself
A Python script consists of one or more statements
The Interpreter executes these statements one by one ...
in the order in which they appear
When you are in interactive mode the Python interpreter waits ...
until you have completed a statement ...
and hit Enter or Return
Then it swings into action
Every time the Python interpreter comes across a statement it does something
print is a function
A function is a series of previously written Python statements
Each function has a name and performs a specific task
To run a function you write the name of the function followed by parentheses
The parentheses may be empty
But they usually contain one or more values called arguments
The statement consisting of the name of the function and the parentheses with arguments is a function call
Most functions need arguments to do their work
When we write
```
print("Hello world!")
```
The argument is "Hello world!"
Functions can have zero, one, or many arguments
The author of the function decides how many arguments it needs

Literals

Programs often calculate values
But some values don't need to be calculated
We know that there are 7 days in a week and 24 hours in a day
When we write programs that need specific values
We write these values directly inside the script
A value that is written directly into the code is called a literal
In the print statement above, "Hello world!" is a literal
When I write
```
print(5)
```
5 is a literal

Strings and String Literals

All computer programs use different types of values
The values can be words, numbers or other things
When the value is text, not a number, it is called a string
A string is just a sequence of characters
A string can be calculated
Or we can write the string directly inside the code
When we write a string directly inside the code we have created a string literal
String literals must be enclosed in either single quotes, ' ...
or double quotes, "
It does not mater which one you use
But you must use the same type of quote at the end of a string ...
that you used at the beginning
If you don't you will get an error message
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

Triple Quotes

There is another way to write string literals in Python
But we will not be using it in this class
This method involves using three quotes at the beginning and end of the string
You can use either three single quotes or three double quotes
But whatever quote characters you use a the beginning have to be used at the end
You can't mix quotes
If you use triple quotes to enclose a string literal ...
you can use both single and double quotes freely inside the string itself
```
>>> print('''He's crying "Help!"''')
He's crying "Help!"
```
Triple quote can hold a string literal that runs across several lines
```
>>> print("""Line 1
... Line 2
... Line 3""")
Line 1
Line 2
Line 3
```
... is the way the Python interpreter tells you it is waiting for more

You can't do this with ordinary quotes

>>>  print("Line 1
  File <stdin>, line 1
    print("Line 1
              ^
SyntaxError: EOL while scanning string literal

This error message is saying that the interpreter can't find the end of the string

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 number in RAM is translated 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")
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 them inside the computer
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
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
If you used 17 bits to represent every character, text documents would be extremely large
But the numbers that represent Unicode characters can be represented 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
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 act on ...
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, 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
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
```
VARIABLE_NAME  =  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