IT 116: Introduction to Scripting

IT 116: Introduction to Scripting
Class 4

Microphone

Questions? Issues?

Does anybody have any questions before I begin?

Homework 2

I have posted homework 2 here.

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

Today's Class

In today's class I will talk more about important concepts in writing Python programs.

I will also talk more about variables and data types.

Then I will show you how to input data into a Python script.

Make a Study Schedule

One of the most important thing you can do to do better in class ...
is to make a study schedule ...
and keep to it
A consistant study routine is the single most important thing you can do ...
to improve your grade
You should set aside regular chunks of time for this course ...
and stick to it as best you can
The semester is long - 14 weeks
Most of you have many responsibilites
Conflicting demands on your time can cause you to lose focus
Having a regular schedule can keep you on track ...
and prevent panic attacks before exams
Time is a finite resource
Once you have wasted an hour ...
you can never get it back
If you find your schedule is not working ...
make some changes
But do not use that as an excuse to abandon a schedule
There is an old saying
If you fail to plan ...
you are planning to fail

Course Work

First Graded Quiz

The first graded quiz will be given on Tuesday next week
The question on the quiz are taken from the ungraded quizzes ...
of the previous week
You should expect a Graded Quiz each week on Tuesday
If you do not take the quiz when it is offered ...
you cannot take it at another time
If you miss a quiz but have an excuse, send me an email

Gradescope

In order to receive your scores in Gradescope ...
you need to sign in to my Gradescope section
You do not have to register with Gradescope
In the next few days I will enter all your UMB addresses into Gradescope
You will then receive an email from Gradescope
What happens next depends on whether you are already registered with Gradescope
If you are new to Gradescope
- Click the "Set your password" link
- Confirm your password
- Click Save
- You will then be taken to the page for your course
If you are already registered with Gradescope
- Click the link to the course
- At the login screen enter your UMB email address and your Gradescope password
- If you have forgotten you password, click the "Forgot your password"link
You will find more information about Gradescope here.

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

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

Creating Directories with FileZilla

You need to create a new directory for each Class Exercise and homework assignment
You can do this in FileZilla
First connect to pe15.cs.umb.edu
Then drill down to either the ex or hw directory
Double click on the directory
Right click on the blank area of the bottom panel
This will bring up a menu
Select "Create directory"
You can also move files and directories in FileZilla by clicking and dragging

Keeping Your Scripts Organized

Organization is an important part of IT work
You should keep all the scripts for this class in one directory
Go to your Documents directory and create a python directory
Then create all your Python scripts in this directory
This will make it easier to find them in FileZilla ...
when you need to move them to pe15.cs.umb.edu

Using FileZilla to Fix Problems

You may receive an email from me saying I cannot find an assignment
If you know you created it, it's probably in the wrong directory ...
or the file has the wrong name
Either way, you can use FileZilla to fix it
If a file is in the wrong place, click and drag it to the right directory
If the file has the wrong name right click on it to bring up a menu ...
and select Rename

Use Flash Cards to Prepare for Graded Quizzes

To studying for the graded quizzes you need to study the Class Quizzes for the previous week
The best way to do this is to use flash cards
You take an index card and write the question on one side ...
and the answer on the other
You should make a set of flash cards for each Class Quiz
Label them with the Class Number
And put a rubber band around them
I did this when taking courses for my Masters degree
I would go through them on my way to class on the T
There are programs that you can use to create virtual flash cards
But it is better if you write your own cards
The simple act of writing each card, helps you remember the material
You can also use flash cards in reverse
Turn them over, read the answer and guess the question
This can be helpful when studying for the midterm or final

Recorded Class Meetings

All lectures for this class are recorded
Use this resource if you miss a class ...
or don't quite understand something mentioned in class
You will find links to the recordings of class meetings here

Review

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
A statement is like a sentence in Python
A Python program is a collection of statements
The Python interpreter works a script statement by statement
It translates each statement into binary machine language for the CPU
Once it has translated one statement ...
It moves on to the next
You can also run a script on the Unix command line ...
by typing python3 followed by the script name
Like this
```
$ 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 usefull
It's a package of statements
When you want to use a function you use a function call
The function call 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

Computers can work with different kings of data
- Integers
- Decimals
- Words
Each type of data must be stored in memory in its own format
The different formats allow the computer to work efficiently with these values
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)
<class 'int'>
>>> type(5.5)
<class 'float'>
>>> type("five")
<class 'str'>

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
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 number are written in powers of 2
And appear using the digits 1 and 0
To represent a character in the computer's memory, we need a number
We get that number from a table
The table contains a unique number for each character

Unicode

The table of numbers and characters that we use in Python is called Unicode
Unicode represents all the characters in most world writing systems
There are many ways to represent the numbers that make up Unicode
These representations are called encodings
Python uses the UTF-8 encoding

Variables

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 holds a single value
You create a variable with an assignment statement ...
which has the following format
```
			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"
```

Attendance

New Material

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 ...
these values are written 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

Expressions

Whenever we write programs we are dealing with values
We need values when we use an assignment statement to create a variable ...
and as arguments when we call a function
There are four ways that we can create a value in Python
We can write a value directly in the code using a literal
```
rate = .04
```
We can use a variable already defined
```
rate = old_rate
```
We can use a calculation involving operators
```
rate = old_rate * 3
```
Or we can use a function call
```
value = round(5.3)
```
Things like these are called expressions
An expression is anything that can be turned into a value
This means that the format for an assignment statement is really
```
LOOP_VARIABLE  =  EXPRESSION
```
There are four different kinds of expressions
- Literals
- Variables
- Calculations with operators
- Function calls
There are two places where expressions frequently occur
- Assignment statements
- As arguments in a function call
Whenever you need a value in your code you can use any of the four kinds of expression
```
print("Hello")
print(rate)
print(rate * 1.1)
```
pre>

Comments

The first thing we want a program to do is to work without error
But that is not enough
The program should also be readable
In other words, someone who knows Python should be able to read it ...
and get a general ideas of what it does
When you write a program you understand its different parts
But what if someone else needs to read the program to make changes?
What if you need to change the program a year from now?
There are many ways to make a program readable
But one of the most important is to use comments
A comment is text written inside a script that is not read by the interpreter
Comments describe how the program works to people reading the code
In Python, any text on a line after a hash mark, #, is ignored by the interpreter
```
>>> print("Hello") # but ignore this text
Hello
```
If you want to write a multiline comment, each line must begin with a #
Every homework script must have a comment at the top ...
that describes briefly what the program does

Variable Naming Rules

Computer languages are different from human languages
They have very rigid rules
In a human language there are many ways of saying the same thing
People learning a new language can make themselves understood ...
even if their grammar is not perfect
That's because people are flexible
But computers are not as flexible as people
When you write a Python program it has to be understood by the Python interpreter
The Python interpreter is a computer program
And it only understands what it has been programmed to understand
When you write a Python program you must follow certain rules ...
or your program will not work
Here are Python's rules for the names of variables
- You cannot use keywords as function names
- Function names cannot have spaces
- The first character of a function name must be a letter, a-z or A-Z, or an underscore, _
- After the first character you can use also use digits
- Uppercase and lowercase letters are different
Words that have special meaning to Python are called keywords
You can find a list of Python keywords here
Python is case sensitive
That means that uppercase letters and lowercase letters are completely different
So value and Value are different variables
Here are some examples of legal variable names
```
rate
rate_1
Rate_1
_rate
```

Here are some examples of illegal variable names

guns&roses      # can't use symbols in variable names
guns and roses  # can't use spaces in variable names
1st_try         # can't start a variable name with a digit

Although you can use UPPERCASE letters in variable names ...
it is better to use only lowercase letters
An underscore, _, can be used to separate words in a variable name
```
first_name = "Bob"
```

Choosing Good Variable Names

Just because a variable name is legal ...
doesn't mean its a good variable name
Consider the following code
```
d = s * t
```
Can you guess what this is calculating?
How about this
```
distance = speed * time
```
Both expressions use legal variable names
But the second is more informative than the first
It is more readable
The code you write makes sense to you right now
But will it still make sense a year from now?
The name of a variable should always describe the value it holds
So speed is a good variable name
But xena_the_warrior_princess is not
Sometimes you need more than one word ...
to describe the value contained in a variable
When this happens, use the underscore character, _ ...
to separate words
Like this
```
course_id
student_name
first_name
```

Printing Multiple Values

The print function can accept more than one argument

>>> room_1 = 92
>>> print("My room number is", room_1)
My room number is 92

Notice that we used both a string literal ...
and a variable separated by a comma
Also notice that Python automatically put a space between the string ...
and the value of the variable

There is no limit to the number of arguments print will accept

>>> room_2 = 93
>>> print("My room number is", room_1, "and my friend is staying in", room_2)
My room number is 92 and my friend is staying in 93

You can also give print no arguments

In that case it will print a blank line

$ cat print_01.py 
# Prints three lines, one blank

print("Line 1")
print()
print("Line 2")

$ python3 print_01.py 
Line 1

Line 2

Numeric Data Types and Literals

A Python variable can hold many different types of values
- Strings
- Integers
- Decimals
Each type of value is stored in the computer's memory in a different format
So the binary number that represent the string "5" ...
is different from the binary number that represent the integer 5 ...
and the one that represent the decimal value 5.0
The technical term for each of these different representations is a data type
Any Python variable can hold any kind of value
When a string is stored in memory, its data type is str
When an integer is stored in memory, it has the data type int
When a decimal is stored, it has data type float
Float is short for floating point
Whenever you assign a value to a variable
Python can detect what data type it should use

You can see this using the Python function type

>>> type("5")
<class 'str'>
>>> type(5)
<class 'int'>
>>> type(5.0)
<class 'float'>

The type function also works on variables

>>> score = 85
>>> type(score)
<class 'int'>
>>> score = 85.0
>>> type(score)
<class 'float'>
>>> score = "85"
>>> type(score)
<class 'str'>

Notice that Python thinks that these three values ...
all have different data types
Even though we would think of them as all the same value

Why You Need to Know About Data Types

It might seem that data types are an unneeded complication
After all, you can give the print function any kind of value you want ...

and it will print the correct output

>>> value = "5"
>>> type(value)
<type 'str'>
>>> print(value)
5
>>> value = 5
>>> type(value)
<type 'int'>
>>> print(value)
5
>>> value = 5.0
>>> type(value)
<type 'float'>
>>> print(value)
5.0

But there are many situations where the data type is important
The data type of a value determines what you can do with it
You can divide an int or a float with another int or a float
```
>>> 30 / 5
6
>>> 30.0 / 5.0
6.0
```

But you can't divide strings

>>> "30" / "5"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for /: 'str' and 'str'

Or divide a string by an integer

>>> "6" / 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for /: 'str' and 'int'

Or add an integer to a string

>>> 5 + "6"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'

Or subtrack them

>>>  5 - "6"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'int' and 'str'

Conversion Functions

Python has functions that will convert from one data type to another
int() will try to convert a value into an integer
```
>>> int("5")
5
```
int() will also work with decimals
```
>>> int(5.1)
5
```

But there is a limit to what it can do

>>> int("five")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'five')

To convert integers or decimals into a string use the str() conversion function
```
>>> str(5)
'5'
>>> str(5.35)
'5.35'
```
There are conversion functions for all major data types

Reading Input from the Keyboard

A function gets the values it needs to do its job from its arguments
But how does a Python script get values from the user?
One way is to read input from the keyboard
This is done with the input function
input takes one argument
That argument is the text that will prompt the user for a value
This prompt tells the user what information is needed
input is used in an assignment statement like this
```
LOOP_VARIABLE = input(PROMPT)
```
When the Python interpreter comes upon a statement using input it
- Prints the prompt
- Waits for the user to type something and hit Enter
- Assigns the text entered by the user to the variable
The prompt string should make it clear what type of value is wanted

Here is an example

>>> team = input("Enter a team name: ")
Enter a team name: Red Sox
>>> print("Go", team)
Go Red Sox

The text in blue is entered by the user
Notice that there is a space at the end of the prompt string
There should be a gap between the prompt and the value entered by the user

Reading Numbers with the `input` Function

The input function gets input from the user ...
and turns it into a value that a Python program can use

The value produced by input function is always a string

>>> number = input("Please enter a number: ")
Please enter a number: 55
>>> type(number)
<class 'str'>

The text in blue is entered by the user
Remember, every value has a data type
If we want the value we get from input to be a different data type ...
we have to use a conversion function

To convert the value in number to an integer use int

>>>  number = int(number)
>>>  type(number)
<class 'int'>

To convert it to a decimal, we can use float

>>> number = float(number)
>>> type(number)
<class 'float'>

If you run a conversion function on a value that it can't handle ...

you will get an error

>>> number = input("Please enter a number: ")
Please enter a number: fifty-five
>>> number = int(number)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'fifty-five'

General Advice

Course Work

Tips and Examples

Review

New Material

Questions? Issues?

Homework 2

Today's Class

Make a Study Schedule

Course Work

First Graded Quiz

Gradescope

Class Quizzes

Homework and it116 Directories

Tips and Examples

Creating Directories with FileZilla

Keeping Your Scripts Organized

Using FileZilla to Fix Problems

Use Flash Cards to Prepare for Graded Quizzes

Recorded Class Meetings

Review

Statements

Functions

Data Types

Strings

Characters

Unicode

Variables

Attendance

New Material

Literals

Expressions

Comments

Variable Naming Rules

Choosing Good Variable Names

Printing Multiple Values

Numeric Data Types and Literals

Why You Need to Know About Data Types

Conversion Functions

Reading Input from the Keyboard

Reading Numbers with the input Function

Class Exercise

Class Quiz

Reading Numbers with the `input` Function