Tips and Examples

• Studying for the Midterm with Flashcards
• Getting Your First IT Job
• Keep In Contact
• A Common Error

Review

• for Loops
• The range Function
• Setting the First and Last Values with range
• Setting the Increment with range
• Nested Loops

New Material

• Calculating a Running Total
• Augmented Assignment Operators
• Calculating Averages
• Sentinels
• Functions
• Using Functions in Writing Programs
• Two Types of Functions
• Function Names
• Defining a Function
• Calling a Function

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.

Readings

If you have the textbook you should read Chapter 5, Functions, from Starting Out with Python.

Solution to Homework 4

I have posted a solution to homework 4 here.

Let's take a look.

Homework 6

I have posted homework 6 here.

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

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.

Remember, the Midterm and Final determine 50% of your grade.

Questions

Are there any questions before I begin?

Tips and Examples

Studying for the Midterm with Flashcards

• 60% of the point on the Midterm come from Class Quiz Questions
• You can use the flashcards you create for these questions when studying for the Midterm
• But not all Class Quiz Questions will appear on the Midterm
• If the flashcard question covers a topic not in the Midterm Review ...
• you do not have to study it for the exam
• You should remove these flashcards from your collection

Getting Your First IT Job

• Students sometimes come to me asking for advice on how to get an IT job
• I have not had a job in industry for many years
• So I usually refer them to Career Services
• They provide a web site called Handshake where companies can find interns
• In IT, as in many fields, your first job is the stepping stone to your career in the field
• But it can be hard to find an internship or or a job when you have no IT experience
• If this is a situation you face, there are things you can do
• If you have a job, check with your current employer
• They must have computers somewhere and maybe you can help keeping them running
• If they have an IT Department ask if there is something you can do for them in your free time
• You can do something similar with local organizations like a church, temple, mosque or youth group
• They probably use a computer for some of the work they do or perhaps they need a web page
• Volunteer to do some IT work for them in return for a letter talking about the work you did
• You can cite this work in your resume
• Another place to find volunteer opportunities is Volunteer Match
• They have virtual opportunities that in many different areas
• Or perhaps you can find some open source project that needs help
• The Free Software Foundations is based in Boston and often needs volunteers
• Many people who are looking to hire people say they are having a hard time ...
• finding people who take their work seriously
• Whenever you get a job, no matter how menial ...
• be sure to do your best ...
• and take the work seriously
• The economy goes through cycles and sometimes jobs are hard to find
• Employers don't want people who don't give a damn
• Just get in the habit constantly looking for opportunities
• But above all don't give up

Keep In Contact

• When solving a problem, one of your best resources ...
• is the people you work with
• Sometimes you get you get too wrapped up in a problem ...
• and can't see something that is obvious to others
• Other times you need a function of technique ...
• that you have never used before
• Nobody in IT knows everything
• Of course you could always Google for an answer
• But someone you work with might be able to explain it ...
• saving you hours of searching
• All of us who work in IT are critically dependent on our peers
• This is why I urge all of you to keep in touch with those you meet in your classes ...
• or on the job
• Most of the jobs I have had in my life ...
• have come from leads from people I know
• Make sure you have the email or text address of everyone you study or work with ...
• and keep in regular contact
• Maybe go out for a beer with them one a month
• You won't regret it

A Common Error

• One of the most common errors in programming ...
• results from a simple spelling mistake
• When this happens you get an error message like this

  NameError: name 'miles' is not defined

• Often student get confused when they see this
• But a careful look at the code can show the problem

  min = int(input("Minimum: "))
  max = int(input("Maximum: "))
  
  print()
  
  print("Miles\tKillometers")
  print("---------------------")
  for miles in range(min, max + 1):
      km = round(miles * 1.60934)
      print(str(mile) + "\t" + str(km))

Review

for Loops

• The for loop in Python has the following format

  for VARIABLE_NAME in LIST_OF_VALUES:
      STATEMENT
      STATEMENT
      ...

• Python's for loop works differently from for loops in other computer languages
• The for keyword is followed by a variable
• Next comes the in keyword followed by a list of values
• The first value in the list is assigned to the loop variable
• It keeps that value while passing through the code block
• When execution of the code block ends ...
• the next value in the list is assigned to the loop variable
• This continues until all values are used ...
• at which point the loop ends
• Here is is a for loop that prints the powers of 2

  $ cat powers_of_2.py 
  # this program uses a for loop to print the powers of 2
  # from 1 to 10
  
  for number in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]:
      value = 2 ** number
      print("2 to the power", number, "is", value)
  
  $ python3 powers_of_2.py 
  2 to the power 1 is 2
  2 to the power 2 is 4
  2 to the power 3 is 8
  2 to the power 4 is 16
  2 to the power 5 is 32
  2 to the power 6 is 64
  2 to the power 7 is 128
  2 to the power 8 is 256
  2 to the power 9 is 512
  2 to the power 10 is 1024

• The Python for loop is different from the for loop in other languages ...
• which only give integer values to the loop variable
• You can do everything with a Python for loop ...
• that you can do with the for loop in other languages
• But the Python for loops allows you to do more
• In Python the loop variable can be of any data type

The range Function

• Python provides the range function to make it easy to create a list of integers
• The range function creates a special kind of list ...
• that only contains integers
• When range is run with one argument, the list of integers starts with 0 ...
• and ends with one less than the argument

  >>> for number in range(5):
  ...   print(number)
  ... 
  0
  1
  2
  3
  4

Setting the First and Last Values with range

• The range function can also be run with two arguments
• The first argument is the first value in the list
• The second argument is one more than the last number in the list
• So if I wanted to print the numbers from 1 to 10 I would write

  >>> for number in range(1, 11):
  ...     print(number)
  ... 
  1
  2
  3
  4
  5
  6
  7
  8
  9
  10

Setting the Increment with range

• If you call the range function with one or two arguments ..
• each value in the sequence is 1 more than the previous number
• You can change this by giving range a third argument
• The third argument is called the step value
• It tells range how much to add to the previous integer ...
• to get the next number in the sequence
• We can use this feature to print the even numbers up to 10

  >>> for number in range(2, 11, 2):
  ...  print(number)
  ... 
  2
  4
  6
  8
  10

Nested Loops

• Just as you can have an if statement inside another if statement
• You can have a loop inside another loop
• This is called a nested loop
• When you nest for loops the name of each loop variable must be different ...
• or you will get very strange results
• Let's write a script to create a multiplication table

  $ cat times_table.py 
  # This script creates a times table using nested for loops
  
  for row in range(1, 11):
      for column in range(1, 11):
          entry  = row * column
          print(entry, end="\t")
      print()
  
  $ python3 times_table.py 
  1   2   3   4   5   6   7   8   9   10  
  2   4   6   8   10  12  14  16  18  20  
  3   6   9   12  15  18  21  24  27  30  
  4   8   12  16  20  24  28  32  36  40  
  5   10  15  20  25  30  35  40  45  50  
  6   12  18  24  30  36  42  48  54  60  
  7   14  21  28  35  42  49  56  63  70  
  8   16  24  32  40  48  56  64  72  80  
  9   18  27  36  45  54  63  72  81  90  
  10  20  30  40  50  60  70  80  90  100

• With each pass through the outer loop the loop variable row gets a new value
• It keeps that value while the inner loop variable column changes from 1 to 10

Attendance

New Material

Calculating a Running Total

• If we wanted to add a series of numbers
• We might use the following algorithm

  set a variable to 0
  for a number in a list of numbers
      add the number to the variable
  print the result

• Here is code to add the numbers from 1 to 10

  >>> total = 0
  >>> for number in range(1, 11):
  ...     total = total + number
  ... 
  >>> print(total)
  55

• Each time we go through the loop a new value is added to total
• A total that is updated as new numbers are encountered is called a running total
• A good example of a running total in everyday life is a bank balance
• Your bank balance is the running total of all deposits and withdrawals ...
• from your bank account
• Running totals are calculated using
  • A variable that holds the running total
  • A loop adding each new number
• The variable that holds running total is called an accumulator
• Before you enter the loop, the accumulator must be set to 0
• Let's say you wanted to add a list of numbers entered by the user
• We have to know when the last number is entered
• There are many ways to do this
• One way is to ask the user how many numbers will be entered ...
• before entering the loop
• Like this

  $ cat add_many_numbers.py 
  # adds a series of numbers entered by the user
  # uses a for loop to do this after asking the user
  # for the number of entries to be added
  
  entries = int(input("How many entries? "))
  total = 0
  for entry_no in range(entries):
      number = int(input("number: "))
      total = total + number
  print("Total", total)
  
  $ python3 add_many_numbers.py 
  How many entries? 5
  number: 34
  number: 54
  number: 123
  number: 345
  number: 55
  Total 611

• The numbers in blue are user entries
• Notice that we did not use the value of the loop variable
• We simply wanted to run the loop a certain number of times
• I'll show you a much better way to do this below

Augmented Assignment Operators

• The program above contains the line

  total = total + number

• This statement does the following
  • Gets the current value of the variable total
  • Adds the value of number to the current value of total
  • Assign this new value to total
• This sort of statement occurs frequently in programs
• So most languages provide a special operator += ...
• to use in such situations
• We can use it to rewrite the line above as

  total += number

• This operator does three things
  • Gets the current value of the variable on the left
  • Add the value of the expression of the right to it
  • Assigns this new value to the variable on the left
• Using this new operator we can rewrite add_many_numbers.py like this

  $ cat add_many_numbers_2.py 
  # adds a series of numbers entered by the user
  # uses a for loop to do this after asking the user
  # for the number of entries to be added
  
  entries = int(input("How many entries? "))
  total = 0
  for entry_no in range(entries):
      number = int(input("number: "))
      total += number
  print("Total", total)
  
  $ python3 add_many_numbers_2.py 
  How many entries? 5
  number: 48
  number: 243
  number: 53
  number: 175
  number: 65
  Total 584

• The += operator makes the code shorter ...
• which reduces the risk of errors
• An operator like this is called an augmented assignment operator ...
• because it does more than assign a value to a variable
• Python has an augmented assignment operator ...
• for every arithmetic operator

  Operator	Example	Equivalent To
  +=	num += 5	num = num + 5
  -=	num -= 5	num = num - 5
  *=	num *= 5	num = num * 5
  /=	num /= 5	num = num / 5
  //=	num //= 5	num = num // 5
  %=	num %= 5	num = num % 5
  **=	num **= 5	num = num ** 5

• Here are some examples

  >>> number = 5
  >>> number += 1
  >>> number
  6
  >>> number -= 1
  >>> number
  5
  >>> number *= 3
  >>> number 
  15
  >>> number = 7
  >>> number /= 2
  >>> number
  3.5
  >>> number = 7
  >>> number //= 2
  >>> number
  3
  >>> number = 7
  >>> number %= 2
  >>> number
  1
  >>> number = 2
  >>> number **= 3
  >>> number
  8

Calculating Averages

• We use a running total when calculating averages
• The average is the total divided by the number of values
• Here is an example

  $ cat average.py 
  # this program asks the user how many numbers
  # they have to enter, then performs a running
  # total and computes the average
  
  entries = int(input("How many entries? "))
  total = 0
  for entry_no in range(entries):
      number = int(input("number: "))
      total +=  number
  average = total / entries
  print("Average", average)
  
  $ python3 average.py 
  How many entries? 10
  number: 10
  number: 9
  number: 8
  number: 7
  number: 6
  number: 5
  number: 4
  number: 3
  number: 2
  number: 1
  Average 5.5

Sentinels

• In the script above we had to ask the user for the number of values
• Before entering the loop
• This is not a good idea
• It forces that user to count the values before entering them
• In other words, we are asking the user to do extra work
• But computers are supposed to make our work easier
• A better way to know when to stop is to use a special value
• The program keeps adding entries to the total
• Until the user enters this special value
• This special value is called a sentinel
• Here is a program to calculate the average using 0 as a sentinel

  $ cat average_3.py 
  # this program averages a series of numbers entered
  # by the user using a sentinel to indicate the
  # end of input
  
  sentinel = 0
  total    = 0
  entries  = 0
  print("Enter numbers when prompted")
  print("When you are done, enter 0")
  number = int(input("number: "))
  while number != sentinel:
      total   += number
      entries += 1
      number   = int(input("number: "))
  average = total / entries
  print("Average", average)
  
  Enter numbers when prompted
  When you are done, enter 0
  number: 5
  number: 4
  number: 3
  number: 2
  number: 1
  number: 0
  Average 3.0

• We had to make a number of modifications to make this change
  1. Tell the user the value to used as a sentinel
  2. CHANGE the for loop to a while loop
  3. Create the variable entries initialized to 0
  4. Increment entries each time through the loop
  5. Ask for the first number before entering the loop
• To initialize a variable is to give it its first value
• But we have to be careful when choosing a sentinel value
• The value cannot be one of the numbers we are averaging
• This could happen if we are calculating average temperatures
• Because 0 is a perfectly good value for temperature
• We need a number that would never occur in our list of values
• We can do this with temperatures ...
• but we have to know some physics
• No Celsius temperature can be below -273 degrees
• So we could use -500 as a sentinel value
• Why use -500 when -274 would do?
• Because it is easier to remember 500 than 274

Functions

• A function is a group of statements that has a name ...
• and performs a specific task
• The Python interpreter has a number of functions that are always available
• You don't have to do anything special to use them
• They are called built-in functions ...
• because they are defined inside the Python interpreter
• Python also allows you to define your own functions
• Most Python scripts contain functions that break up the work ...
• into smaller chunks of code
• If you define functions that you find yourself using over and over ...
• you can create Python files that contain these functions
• These files are called modules
• Modules are not scripts
• If you run them, nothing will happen
• But they contain functions that other scripts can use
• To use module functions inside your script you write an import statement

  import MODULE_NAME

• The module name is the file name without the .py extension
• The import statement must be at the top of the script ...
• before you make any calls to its functions
• The import statement loads the module code into memory ...
• so the interpreter can find the definitions when you script calls the functions
• Some modules are already on your machine
• They were loaded when Python was installed
• The interpreter knows where to find these modules
• If you create your own modules you have to let the interpreter know where to find them
• On Mac and Unix you do this by setting the PYTHONPATH system variable
• This variable tells the interpreter where to look for the modules you create

Using Functions in Writing Programs

• What is the first thing you should do if you are given a big job?
• You should break it down into smaller tasks
• And then work each one separately
• The textbook gives an example of a program to print a check
• This job breaks down into a number of smaller tasks
  • Get the hours worked
  • Get the employee's hourly rate
  • Calculate the gross pay
  • Calculate the overtime pay
  • Calculate the withholdings
  • Calculate the net pay
  • Print the check
• Breaking a big job into small jobs is called modularization
• It is one of the most important concepts in all of engineering
• Modularization is sometimes called "divide and conquer" ...
• or "separation of concerns"
• Without this we would never be able to do big jobs ...
• like build a bridge
• Breaking up a big task has several advantages
• We can assign each task to a different group
• This allows work to proceed in parallel
• Each group can work on its own assignment ...
• allowing many people to work on different parts of the project ...
• at the same time
• This makes the overall project work take less time
• In programming, each specific task should have its own function
• Programs that are broken up into functions are easier to read ...
• since each function each function is short ...
• and only does one thing
• This is particularly true if the function has a good name
• Modularized programs are often shorter ...
• because there is no repeated code
• You can use the same function over and over again ...
• but you only have to write it once
• Modularization makes programs easier to test
• Because you can test each function individually
• Modularization also makes it easier to work in teams
• If each function has a simple job to do ...
• one team member can work on it alone ...
• so many people can work on a project at the same time

Two Types of Functions

• There are two types of functions
  • Functions that return a value
  • Functions that do not return a value
• The textbook calls the second type of function a void function
• Conversion functions like str, int and float return a value

  >>> result = str(5)
  '5'
  >>> result = int("5")
  >>> result
  5
  >>> result = float("5")
  >>> result
  5.0

• An example of a function that does not return a function is print
• It causes something to be printed ...
• but it doesn't return a value

  >>> result = print("Hello world!")
  Hello world!
  >>> result
  >>>

Function Names

• Function names must abide by the same rules as variable names
  • 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 and A-Z, or an underscore, _
  • After the first character you can use letters, digits or the underscore
  • Uppercase and lowercase letters are distinct
• Its a good idea to use several words in a function name
• That makes it easier for the name to describe what the function does
• But you have to separate each word with an underscore, _
• Since functions do some work, most function names use verbs

• calculate_gross_pay
  get_hourly_rate
  calculate_overtime

Defining a Function

• A function definition has the following format

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

• A function is a statement that contains other statements
• So a function is a compound statement
• The first line of a function definition is called the function header
• There are four parts to the function header
  • The keyword def
  • The function name
  • The parameter list enclosed in parentheses
  • A colon, :, at the end of the line
• Parameters are variables used inside the function
• They get their values from the arguments in the function call
• Even if the function has no parameters it must still have parentheses
• Following the function header is a series of indented statements that form a code block
• Here is a function that prints the UMB address

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

• This function takes no arguments
• It is also a void function since it returns no value
• Notice that I put a comment before the function definition
• It describes what the function does
• All the functions you write in homework scripts must have such a comment

Calling a Function

• To run a function you write a function call
• A function call has two parts
  • The function name
  • A list of arguments enclosed in parentheses
• You must follow the function name with parentheses ...
• even if the function takes no arguments
• That is how the interpreter knows it is a function call ...
• and not a variable name
• Here is how we would call the print_umb_address function

  print_umb_address()

Class Exercise

Class Quiz