IT 116: Introduction to Scripting

IT 116: Introduction to Scripting
Class 13

Microphone

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.

Solution to Homework 5

I have posted a solution to homework 5 here.

Let's take a look.

Homework 7

I have posted homework 7 here.

It is NOT due this coming Sunday.

Instead it is due Sunday, March 24th.

This is to give you time to study for the midterm.

And to give me time to score it.

Midterm

The Midterm exam for this course will be held on Tuesday, March 19th.

That is the first Tuesday after the Spring Break

The exam will be given in this room.

It will consist of questions like those on the quizzes along with questions asking you to write short segments of Python code.

60% of the points on this exam will consist of questions from the Ungraded Class Quizzes.

There will be 15 of these questions, worth 4 points each.

The other 40% of points will come from four questions that ask you to write a short segment of code.

There will be 4 of these questions worth 10 points each.

To study for the code questions you should be able to

Write an if/elif/else statement
Write a while loop
Write a for loop using range
Write a function that does not return a value

A good way to study for the code questions is to review the Class Exercises and homework solutions.

The last class before the exam, Thursday, March 7th, will be a review session.

You will only be responsible for the material in the Class Notes for that class on the exam.

You will find the Midterm review Class Notes here.

If for some reason you cannot take the exam on the date mentioned above you must contact me to make alternate arrangements.

The Midterm is given on paper.

I scan each exam paper and upload the scans to Gradescope.

I score the exam on Gradescope.

You will get an email from Gradescope with your score when I am done.

The Midterm is a closed book exam.

You are not allowed to use any resource other than what is in your head, while taking the exam.

Cheating on the exam will result in a score of 0 and will be reported to the Administration.

Remember your Oath of Honesty.

To prevent cheating, certain rules will be enforced during the exam.

Today's New Material Not on Midterm

The new material in today's class will not appear on the Midterm.

You will only be responsible for the material in the Class Notes for the next class.

They will cover Class Notes 2 through 12.

Quiz

There will be no graded quiz next week.

Course Requirements

Script Requirements for Functions

The new homework assignment requires you to define functions
When you write these function you must pay attention to the Homework Script Rules
Some of these requirements apply to functions
Every function must have a comment before the header which describes what the function does
All function definitions must must appear at the top of the script
The function body must contain no blank lines
All functions definitions must have a blank line between them
The code that runs the functions must appear at the bottom of the page ...
after all functions have been defined
There must be a blank line between the last function definition ...
and the code that runs the function

The code below shows what I mean

# this program prints some information about me
# and prints the address of our campus

# prints the address of UMB
def print_umb_address():
    print("University of Massachusetts at Boston")
    print("100 Morrissey Boulevard")
    print("Boston, Massachusetts   02125-3393")

# prints some information about me
def print_personal_info():
    print("Glenn Hoffman")
    print("Information Technology Program Director")
    print("Computer Science Department")
    print("University of Massachusetts at Boston")
    print("Glenn.Hoffman@umb.edu")
    print("McCormack 3-0201-22")

print("Some information about me")
print()
print_personal_info()
print()
print_umb_address()

Questions

Are there any questions before I begin?

Tips and Examples

Make Sure You Body Code is Not Indented

A Python program usually has two sections
- Function definitions
- Code that calls the functions
The function definitions must come first
I call the other section the body of the script
It can also be called the driver
This section comes after the last function definition
The code block of a function must be indented
Sometimes students continue indenting after the last function
If the body code is indented ...
it will not be executed when you run the program
That means nothing will happen when you run the code

Here is an example

def hello():
    print("Hello world!")

    hello()

When I run this code nothing happens
That's because the interpreter thinks that the call to hello is inside the function ...
and there is no body code to call the function

The Two Parts of a Function Definition

A function definition has the following format

def FUNCTION_NAME([PARAMETER][...]):
    STATEMENT
    STATEMENT
    ...

The first line is called the function header
The code block below the header is the function body
The header tells you what you need to know ...
to use the function through a function call
It tells you the name of the function ..
and it's parameters
The function body contains the statements that run when the function is called

Review

Local Variables

Any time you create a variable inside a function ...
you are creating a local variable
These variables are created in a special chunk of memory ...
that comes into existence when the function is called ...
and disappears when the function exits

The function's bit of RAM disappears ...
when the function finishes
So do all it's variables
Outside the function, the local variable does not exist
Code outside the function cannot see it
Local variables defined in one function are invisible to other functions
This means we can use the same variable name ...
in more than one function
Another way of saying this is that the scope of a local variable ...
is the function in which it is defined

Parameters

Parameters are a special kind of local variable
They are defined in the function header inside the parentheses
They get their values from the corresponding arguments in the function call
Like other local variables, they disappear after the function ends

Attendance

New Material

Formatting Functions for This Course

The way information is presented affects how easily it can be read
Here is another version of a script you have seen before
It contains the same code as the original

But looks very different

def print_umb_address():
    print("University of Massachusetts at Boston")
    print("100 Morrissey Boulevard")
    print("Boston, Massachusetts   02125-3393")
def print_personal_info():
    print("Glenn Hoffman")
    print("Information Technology Program Director")
    print("Computer Science Department")
    print("University of Massachusetts at Boston")
    print("Glenn.Hoffman@umb.edu")
    print("McCormack 3-0201-22")
print("Some information about me")
print()
print_personal_info()
print()
print_umb_address()

Although this code works, it is hard to read
Writing code that works is not enough
You must also write code that can be understood
In most large companies there are coding standards
The standards specify the format for code
These standards are meant to improve consistency and readability
I want you to learn good habits when writing code
So all homework scripts must follow the Rules for Homework Scripts
If you do not, I will deduct points
I have added to these rules a few that apply only to functions
All scripts must have a comment at the top of the script
This comment must briefly describe what the script does
The functions must appear at the top of the file ...
after the script comment ...
and before the main body of the code
Python only requires that the function definition comes before the first call to the function
This means the following code will run properly ...

even though it is very hard to read

print("Some information about me")
print()
def print_umb_address():
    print("University of Massachusetts at Boston")
    print("100 Morrissey Boulevard")
    print("Boston, Massachusetts   02125-3393")
print_umb_address()
print()
def print_personal_info():
    print("Glenn Hoffman")
    print("Information Technology Program Director")
    print("Computer Science Department")
    print("University of Massachusetts at Boston")
    print("Glenn.Hoffman@umb.edu")
    print("McCormack 3-0201-22")
print_personal_info()

This is not what I want
Each function must be preceded by a comment that tells what the function does
These comments must be written using #s
There should be no blank lines after the comment

Function comments should look like this

# prints the address of UMB
def print_umb_address():

Not like this

# prints the address of UMB

def print_umb_address():

There should be no blank lines inside the function body

Don't do this

def print_umb_address():
    print("University of Massachusetts at Boston")

    print("100 Morrissey Boulevard")

    print("Boston, Massachusetts   02125-3393")

Do this instead

def print_umb_address():
    print("University of Massachusetts at Boston")
    print("100 Morrissey Boulevard")
    print("Boston, Massachusetts   02125-3393")

There must be a blank line between the end of one function
And the start of the next

In other words I should see this

# prints the address of UMB
def print_umb_address():
    print("University of Massachusetts at Boston")
    print("100 Morrissey Boulevard")
    print("Boston, Massachusetts   02125-3393")

# prints some information about me
def print_personal_info():
    print("Glenn Hoffman")
    print("Information Technology Program Director")
    print("Computer Science Department")
    print("University of Massachusetts at Boston")
    print("Glenn.Hoffman@umb.edu")
    print("McCormack 3-0201-22")

Not this

# prints the address of UMB
def print_umb_address():
    print("University of Massachusetts at Boston")
    print("100 Morrissey Boulevard")
    print("Boston, Massachusetts   02125-3393")
# prints some information about me
def print_personal_info():
    print("Glenn Hoffman")
    print("Information Technology Program Director")
    print("Computer Science Department")
    print("University of Massachusetts at Boston")
    print("Glenn.Hoffman@umb.edu")
    print("McCormack 3-0201-22")

There must also a blank line between the last function definition
And the rest of the script

Default Values for Parameters

The function call must supply a value for each parameter
This can be very annoying if the function has many parameters
But sometimes one of the parameters has a certain value ...
that is used most times the function is called
Let's look at an example
Here is a function that converts feet and inches into inches ...

or centimeters

# converts feet and inches to inches or centimeters
def convert(ft, in, units):
    total_inches = 12 * ft + in
    if units == "inches":
        print(total_inches)
    else:
        print(total_inches * 2.54)

When we call this function we must supply three arguments
```
>>> convert(6, 5, "inches")
77
```
America is the only major country that does not use the metric system
In this country the third argument would almost always be "inches"
Having to enter a third argument which is almost always the same ...
is very annoying
Fortunately, Python gives you an alternative
Python lets you define a function with a default value
A default value is a value that is supplied automatically ...
if it is left out

The format for writing a function with default values is

def  FUNCTION_NAME([PARAMETER, ...], PARAMETER=DEFAULT_VALUE [, PARAMETER=DEFAULT_VALUE]):
    STATEMENT
    ...

Notice that the default values must come at the end of the parameter list
You can have as many defaults as you want
But defaults must come after the parameters that do not have defaults

Let's rewrite the previous function to use a default value

# converts feet and inches to inches or centimeters
def convert(ft, in, units="inches"):
    total_inches = 12 * ft + in
    if units == "inches":
        print(total_inches)
    else:
        print(total_inches * 2.54)

Most calls to the function will have only two arguments
```
>>> convert(6, 5)
77
```
But we can also supply the third argument when we need it
```
>>> convert(6, 5, "cm")
195.58
```
With this particular function we can go a step further
What if the measurement had no inches value?
Only feet?
You would still have to provide a second value
```
>>> convert(6, 0)
72
```

Why not give the second argument a default value of 0?

def convert(ft, in=0, units="inches"):
    total_inches = 12 * ft + in
    if units == "inches":
        print(total_inches)
    else:
        print(total_inches * 2.54)

Now we can call the function with only one argument
```
>>> convert(6)
72
```

Named Arguments

Python has an unusual feature called named arguments
Though the textbook calls this feature "keyword arguments"
It let's you call a function with arguments in any order
But you have to use the parameter name when you do it
You write the name of the parameter ...
followed by = ...
followed by the value
So instead of writing
```
>>> convert(6, 3, "cm")
190.5
```

We can write

>>> convert(units="cm", feet=6, in=3)
190.5

When we use named arguments we can list the arguments in any order
But this is not the reason this feature is useful
Many functions have multiple default values
Lets say the function had three parameters with default values
If you wanted to override the default for the last of these parameters
You would have to supply values for all three parameters ...
even if you were OK with the default values of the first two
With named parameters you only need to override the default value ...
for the one parameter you need to change
Several classes ago I showed you how to change the way print works
I showed you how you could turn off the automatic newline character ...
that print automatically adds to its argument
You do this by adding an argument of the form
```
end=VALUE
```
to print
You can also change the space that print automatically adds between its arguments ...
to some other character or string
You do this by adding an argument of the form
```
sep=VALUE
```

For example

>>> a = 5
>>> b = 6
>>> c = 7
>>> print(a, b, c)
5 6 7
>>> print(a, b, c, sep=",")
5,6,7

Both end and sep are parameters to print that have default values
There may be others ...
but you do not have to worry about these others ...
if you use named arguments

Global Variables

Parameters and variables declared inside a function are local variables
Local variables cannot be seen outside the function
But what about the variables declared outside any function?
These variables live in the outermost memory space
This gives them special properties
Variable declared outside of functions can be seen inside any function
These variables are called global variables

Consider this script

def cheer_3():
    print("Go " +  team) + "!")

team = "Red Sox"
cheer_3()

In this example, team is a global variable
When the interpreter runs this script it first stores the function definition
Then it creates the variable team

Now it calls the function cheer_3
Which creates the function's memory space

The function has no parameters

But it does not need any since it can see the global variable team
In the enclosing memory space for the script

You can think of functions as looking at the enclosing memory ...
through a one way mirror
Functions can see the variables declared in the body of the program
But the body of the program cannot see variables declared inside the function
If I try to run of cheer_3 without having defined team outside the function

I will get an error

>>> cheer_3()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in cheer
NameError: name 'team' is not defined

For this reason, it is not a good idea to use global variables inside functions
If you forget to define the global variable ...
the function will not work
Useful functions are often put into modules
They can then be imported into any program as needed
Functions in a module should not depended on global variables ...
defined inside the script
If you did and forgot to define the global variable in the script ...
the module function would not work

Variable Shadowing

What if a local variable has the same name as a global variable?

In that case the Python interpreter will use the value of the local variable

>>> def cheer_4():
...     team = "Red Sox"
...     print("Go", team, end="!\n")
... 
>>> team = "Pats"
>>> cheer_4()
Go Red Sox!

This is called variable shadowing

Global Constants

Some things in life, and in programming, never change
- The number of days in the week
- The number of months in a year
- The first date of the year
- The ratio of the circumference of a circle to its diameter (π)
For this reason, some languages have special variables called constants
A constant is a variable whose value cannot be changed
Python does not have constants
But you can use a global variable for this purpose
You just give it value ...
and remember not to change that value
It is customary to write the names of constants in ALL_CAPITALS
This serves as a reminder never to change their values
Global variables used as constants should be declared once ...
at the top of the program ...

like this

# computes paychecks

DAYS_IN_WEEK = 7

def .....

You will sometime find globals in modules

Pausing Execution of a Script

Sometimes you need to pause a script ...
that is printing a lot of output
You don't want the output to scroll off the top of the screen
A script should try to limit its output to the size of the window
To do this you could use a call to input ...
but not assign the result of the call to a variable
Just use a call to input to stop the program ...
until the user hits Enter
```
input("Hit Enter to see more")
```
The interpreter always stops when it come to a statement with input
It resumes when the user hits Enter or Return

Functions that Return Values

Functions that return values can be used in assignment statements
```
>>> result = round(8.765) 
>>> result
9
```
Functions return a value using a return statement
Return statements have the form
```
return  EXPRESSION
```
An expression is anything that the Python interpreter can turn into a value
A return statement does two things
- It ends the execution of the function
- It sends a value back to the function call
A return statement always causes the function to quit
So any code after the return statement will never be executed
Here is a new version of the cheer function that returns a value
```
>>> def cheer_5(team):
...    return "Go " + team + "!"
```
To see the output we have to put a call to the function inside a call to print
```
>>> print(cheer_5("Red Sox"))
Go Red Sox!
```
Since cheer_5 returns a value
A call to cheer_5 is an expression
You can always use a function call as the argument to a print

Returning Multiple Values

A Python return statement can return more than one value
All you have to do ...
is follow the return keyword with a comma separated list of expressions
```
return EXPRESION, EXPRESSION [, EXPRESSION, ...]
```

Using this feature I can write a function to get both the first and last name

>>> def get_full_name():
...     first_name = input("Please enter your first name: ")
...     last_name = input("Please enter your last name: ")
...     return first_name, last_name
...

When this function is called it will ask for two values ...

and return each to a separate variable

>>> fname, lname = get_full_name()
Please enter your first name: Glenn
Please enter your last name: Hoffman

You have to be careful when calling a function that returns more than one value
You need a variable on the left of the assignment statement ...
for every value the function returns
Giving more than one variable a value in an assignment statement is called multiple assignment

Standard Library

Computer languages come with functions that are always available
You don't have to do anything to use these functions
Here are some examples from Python
- print
- int
- float
- input
- range
They are called built-in functions
Built-in functions are actually part of the Python interpreter
But Python also comes with other useful functions
These functions are found in a set of modules called the standard_library
Each module in the standard library has functions and constants ...
that deal with a specific topic
The standard library modules are copied to your machine ...
when you install Python

For example, the math library contains many mathematical functions

log	Logarithm to base e
log10	Logarithm to base 10
sin	Sine
cos	Cosine
tan	Tangent
ceil	The first integer above the argument

It also has some constants

pi The ratio of the circumference of a circle to its diameter

e The base of the natural logarithms
To use a module you must import it with an import statement
It has the following form
```
import MODULE_NAME
```
The module name is the filename, without the .py
To import the math module you must first write
```
>>> import math
```

Calling a Module Function

Calling a function in a module is different from calling a built-in
When we use a built-in we simply type the function name ...
followed by the argument list in parentheses
```
>>> print("Hello")
Hello
```
But you have to do something different with a module function
Instead of the function name ...
you first type the module name ...
followed by a dot, . ...
followed by the function name
This is called dot notation

Here are some examples

>>> import math
>>> math.pi
3.141592653589793
>>> math.sin(math.pi)
1.2246467991473532e-16
>>> math.cos(math.pi)
-1.0
>>> math.ceil(4.3)
5
>>> math.floor(4.3)
4
>>> math.floor(-4.3)
-5
>>> math.ceil(-4.3)
-4

Creating Random Numbers

Many things in nature are random
- The number of dust particles in a room
- Whether a coin comes up heads or tails
- The number of leaves in my yard
- The number of water droplets in a cloud
Random events are things that cannot be exactly predicted
Like whether a batter will get a hit ...
or strike out
We often need random numbers when writing programs
We need them in writing
- Games
- Simulations
Would a game of solitaire be any fun ...
if the cards always appeared in the same order?
Simulations are extremely important in many sciences
Astronomers use simulations to study the evolution of galaxies
Climate scientists use simulations to see what global warming will bring
For both games and simulations we need random numbers
But we have a problem
Nothing that a computer does is random
If you give a program a certain input ...
you will always get the same output
The output of a program is completely predictable
There is nothing random about it
But the need for random numbers in computing is very great
So mathematicians have developed algorithms that create pseudorandom numbers
These algorithms create a sequence of numbers
In a truly random sequence, the numbers would never repeat
In a pseudorandom sequence the numbers eventually repeat
But the interval between repeats is so long ...
that they are "random" enough for most purposes
Functions that create these sequences are called pseudorandom number generators
Sometimes they are simply called random number generators ...
although that term in not really accurate

The random Module

One of the modules in the standard library is random
It contains several several functions that create pseudorandom numbers
The function randint takes two integers as arguments ...
and returns a pseudorandom integer
The two numbers specify the minimum and the maximum numbers created

We can call this function several times and each time it will return a different number

$ cat random_1.py 
# demonstrates the use of the randint function of the random module

import random

for i in range(10):
    print(random.randint(1,100))

$ python3 random_1.py 
89
98
93
73
32
40
63
100
76
80

$ python3 random_1.py 
66
49
1
29
63
17
91
3
70
5

We can use randint to simulate throwing dice

$ cat dice.py 
# this program emulates a throw of two dice

import random

die_1 = random.randint(1,6)
die_2 = random.randint(1,6)
print("You rolled", str(die_1), "and", str(die_2), "for a total of", str(die_1 + die_2))

$ python3 dice.py 
You rolled 4 and 3 for a total of 7

$ python3 dice.py 
You rolled 7 and 1 for a total of 8

$ python3 dice.py 
You rolled 5 and 3 for a total of 8

If we kept calling randint long enough it would eventually repeat
But we would have to call it a very large number of times

Other random Functions

The random module has other pseudorandom functions
Each of them has advantages in different situations
The randrange function also returns an integer
But it gives you more options when specifying the range of values
While randint takes two arguments
The arguments to randrange work just like the arguments to range
When run with one argument randrange creates a pseudorandom sequence ...
with a minimum of 0 ...

and a maximum of one less than the argument

$ cat random_2.py 
# demonstrates the use of the randrange function of the random module
# with one argument

import random

for i in range(10):
    print(random.randrange(5))

$ python3 random_2.py 
1
3
2
0
1
1
2
2
0
4

Notice that all the numbers are between 0 and 4
When called with two arguments the argument specifies the minimum ...

and the second specifies one more than the maximum

$ cat random_3.py 
# demonstrates the use of the randrange function of the random module
# with two arguments

import random

for i in range(10):
    print(random.randrange(1, 6))

$ python3 random_3.py 
1
3
1
2
5
5
2
3
1
1

Here all the numbers fall between 1 and 5

When called with 3 arguments the third argument specifies the difference between any two possible values

$ cat random_4.py 
# demonstrates the use of the randrange function of the random module
# with three arguments

import random

for i in range(10):
    print(random.randrange(2, 11, 2))

$ python3 random_4.py 
6
6
4
6
8
10
10
4
8
2

The values are all even numbers between 2 and 10
Both randint and randrange return integers
If you want a decimal number, use random

It generates a random number between 0.0 and 1.0

$ cat random_5.py 
# demonstrates the use of the random function of the random module

import random

for i in range(10):
    print(random.random())

$ python3 random_5.py 
0.3366683809865726
0.6243291094221154
0.47182435685723234
0.3079697111617222
0.5048399470937616
0.11682962682702247
0.08597187089498437
0.10398098486344709
0.8667164814826076
0.15337564523669833

uniform takes two arguments and creates decimal numbers

The numbers lie between the two arguments which specify the range

$ cat random_6.py 
# demonstrates the use of the uniform function of the random module

import random

for i in range(10):
    print(random.uniform(1, 5))

$ python3 random_6.py 
3.987702460027857
2.776217942732739
2.890534381287354
1.5377836190792888
3.1461905324732022
1.010886780905638
1.4452549292653458
4.558908792372804
4.934363837349949
2.1135109420482228

Random Number Seeds

The algorithms used in random need a starting value
This starting value is called a seed
Each seed value generates a specific sequence of numbers
If you keep using the same seed value ...
you will get the same sequence of values
To prevent this from happening the functions in random use a trick
Instead of choosing the same seed value each time
They use the system time instead
Every computer keeps track of the time and date ...
in a variable called the system time
The system time counts seconds
So it is a very large integer that keeps changing
random functions will create different number sequences ...
each time they are run
But what if we wanted the sequence of numbers to be the same?
Let's say that you were writing a simulation of a complicated system
Say the weather over a continent
You run the simulation once and get a result
Then you wanted to change something about the simulation
Say you added calculations to take account of the presence of dust in the atmosphere
Dust helps rain clouds form
We would want to test this with the same sequence of as before
We can do this using the seed function
The seed function assigns a specific value to the seed
When we use the seed function to set the seed value to the same number ...
each time the program is run ...

we will get the same sequence of numbers

$ cat random_6.py 
# demonstrates the use of the seed function in the random module
# to generate the same series of numbers each time the program is run

import random

random.seed(67)
for i in range(10):
    print(random.randint(1, 100))

$ python3 random_6.py 
10
15
99
53
60
54
35
77
55
63

$ python3 random_6.py 
10
15
99
53
60
54
35
77
55
63

I use a call to seed in today's Class Exercise
So all your scripts will have the same output
This makes it easier for me to test

pi	The ratio of the circumference of a circle to its diameter
e	The base of the natural logarithms