IT 116: Introduction to Scripting

IT 116: Introduction to Scripting
Class 4

Microphone

Homework 2

I have posted homework 2 here.

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

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.

The quiz will be available from the Gradescope web site but only for the last 15 minutes of the class.

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

If you do not take the quiz when it is offered you cannot take it at another time because Gradescope does not permit this.

If you fail to take the quiz but have a valid excuse, send me an email.

Gradescope

In order to take the quiz on Gradescope, you need to sign in to my Gradescope section.

Sometime during the next few days I will enter all your UMB email addresses into Gradescope.

You will receive an email from Gradescope.

What happens next depends on whether you have 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.

Recorded Class Meetings

You will links to the recording of class meetings here .

You will also find this link on the class web page.

Zoom Office Hours

You will find the link to the Zoom Office Hours meetings here .

You will also find this link on the class web page.

Questions

Are there any questions before I begin?

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
Right click on the directory
This will bring up a menu that will let you create a 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 in one directory on your personal machine
This will make it easier to move them to FileZilla
I would suggest you create a python directory in your Documents directory

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 ...
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
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 work 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

Review

Output

The output of Python scripts is a stream of text
You can create output in Python using print
To print my name to the screen, I would write
```
print("Glenn Hoffman")
```
This line of code is a Python statement
A statement is one or more lines of code that performs a complete action
It is like a sentance in Python
The interpreter runs your script statement by statement
print is a function
A function is a series of Python statements which has a name and performs some 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
Statements of this type are called function calls

Literals

A value that is written directly into the code is called a literal
In the statements below, the values after the = are literals
```
name  = "Bill Smith"
total = 0
rate  = .05
```

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 just text, not a number, it is called a string
When we write a string directly inside the code it is a string literal
You can use either single (') or double (") quotes when writing a string literal
But you must use the same type of quote at the end of a string ...
that you used at the beginning

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
In a many computer languages you have to specify what kind of value the variable will hold
This has to be done before you can use a variable
So if you declared the variable rate to be an integer ...
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 it some time later

Creating Variables with Assignment Statements

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
```
To create the variable name, and give it my name as a value, I would write
```
name = "Glenn Hoffman"
```

Attendance

New Material

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
```
VARIABLE_NAME  =  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)
print(round(5.3))
```

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 words that have special meaning to Python as variable names
- Variable names cannot have spaces
- The first character of a variable name must be a letter, a through z and A through Z, or an underscore, _
- After the first character you can use letters, digits or the underscore
- Uppercase and lowercase letters are different
Words that have special meaning to Python are called keywords

You can find a list of Python keywords in 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.
>>>  help
Type help() for interactive help, or help(object) for help about object.
>>>  help()

Welcome to Python 3.5's help utility!

If this is your first time using Python, you should definitely check out
the tutorial on the Internet at http://docs.python.org/3.5/tutorial/.

Enter the name of any module, keyword, or topic to get help on writing
Python programs and using Python modules.  To quit this help utility and
return to the interpreter, just type "quit".

To get a list of available modules, keywords, symbols, or topics, type
"modules", "keywords", "symbols", or "topics".  Each module also comes
with a one-line summary of what it does; to list the modules whose name
or summary contain a given string such as "spam", type "modules spam".

help> keywords

Here is a list of the Python keywords.  Enter any keyword to get more help.

False               def                 if                  raise
None                del                 import              return
True                elif                in                  try
and                 else                is                  while
as                  except              lambda              with
assert              finally             nonlocal            yield
break               for                 not                 
class               from                or                  
continue            global              pass

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, you should use the underscore character, _, to separate words
Like this
```
course_id
student_name
first_name
```
Another way to combine multiple words in a variable name is to capitalize the first letter of each new word
Like this
```
courseId
studentName
firstName
```
This technique is called camel case
I prefer to use underscores since I think they are more readable
But you are free to use either technique
You must do something to make the start of each word in name stand out
I don't want to see
```
courseid
studentname
firstname
```
They are not easy to read
This is much better
```
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
Changing the data type of a value assigned to a variable ...
causes no problems in Python
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
input is used in an assignment statement like this
```
VARIABLE_NAME = 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

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'>

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'

In the code above we used two assignment statements
One to get a value from the user using input ...
and another to convert the value into the right data type

We can combine both operations into a single assignment statement

>>> number = int(input("Please enter an integer: "))
Please enter an integer: 67
>>> type(number)
<class 'int'>

Why does this work?
The call to input is an expression
The int function takes one argument and that argument can be any expression ...
like a call to input
We can nest as many function calls within other function calls ...
as we like
As long as each returns a value of the proper data type

Studying

Some Students Are Having a Hard Time

Tips and Examples

Review

New Material

Studying

Homework 2

First Graded Quiz

Gradescope

Recorded Class Meetings

Zoom Office Hours

Questions

Tips and Examples

Creating Directories with FileZilla

Keeping Your Scripts Organized

Using FileZilla to Fix Problems

Use Flash Cards to Prepare for Graded Quizzes

Review

Output

Literals

Strings and String Literals

Characters

Unicode

Variables

Creating Variables with Assignment Statements

Attendance

New Material

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

Studying

Class Exercise

Class Quiz

Reading Numbers with the `input` Function