Tips and Examples

• Studying for the Midterm with Flashcards

Review

• Strings and String Literals
• Characters
• Comments
• Variables
• Creating Variables with Assignment Statements
• Expressions
• Numeric Data Types and Literals
• The input Function
• Performing Calculations
• Decimal and Integer Division
• Exponent Operator
• Remainder Operator
• Operator Precedence
• Grouping with Parentheses
• Escape Sequences
• Concatenation Operator
• Boolean Expression
• Relational Operators
• Precedence of Relational Operators
• Control Structures
• Code Blocks
• if Statements
• if-else Statements
• Indentation in the if-else Statement
• if-elif-else Statements
• Logical Operators
• Precedence of Logical Operators
• Loops
• While Loops
• Infinite Loops
• for Loops
• The range Function
• Running Totals
• Augmented Assignment Operators
• Functions
• Two Types of Functions
• Defining a Function
• Calling a Function
• Local Variables
• Parameters

Homework 7

I have posted homework 7 here.

It is NOT due this coming Sunday.

Instead it is due Sunday, March 27th.

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

And to give me time to score it.

Midterm

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.

Today's class will be a review session.

You will only be responsible for the material in the Class Notes for today's class on the exam.

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.

Quiz 5

Let's look at the answers to Quiz 5.

No Class Exercise or Class Quiz

Today is a review session.

There will no Class Exercise or Class Quiz today.

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

Review

Strings and String Literals

• Whenever we write a value directly inside a Python script it is called a literal
• Here are some examples

  5
  6.47
  True

• In programming, text values are called strings
• A string is just a sequence of characters
• When we write something like

      print("Hello world!")

• we have created a string literal
• In Python, string literals are enclosed in either single quotes, ' or double quotes, "

  >>> team = "Red Sox"
  >>> name = "Glenn Hoffman"

Characters

• Everything in a computer's memory is a binary number
• Binary number are written in powers of 2 and use a string of 1s and 0s
• To represent a character we need a table of numbers and the character they represent
• Nowadays we use a character table called Unicode
• Unicode represents all the characters in most of the world's writing systems

Comments

• A comment is text written inside a program
• That is not read by the interpreter
• Comments describe how the program works
• Any text on a line after a hash mark, #, is ignored by the interpreter

  >>> print("Hello") # but ignore this text
  Hello

Variables

• A variable is a place in memory with a name that stores a single value
• A variable in Python can hold a value of any type

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  =  EXPRESSION

• Here is an example

  name = "Glenn Hoffman"

Expressions

• An expression is anything that can be turned into a value
• There are four different kinds of expressions
  • Literal
  • Variables
  • Calculations with operators
  • Function calls
• Expressions are used on the right hand side of an assignment statement ...
• and as arguments in a function call

Numeric Data Types and Literals

• Python recognizes two types of numbers
  • Integers (int)
  • Decimals ( float)
• The integers are the whole numbers
• They can be positive and negative
• Decimals can also be positive and negative
• A Python variable can hold many different types of values
  • Strings
  • Integers
  • Floats
  • Booleans
• Values of each type ares stored in the computer's memory in a different format
• The technical term for each of these different representations is a data type

The input Function

• The input function gets a value from the user
• When the interpreter comes across input in a script it stops running ...
• and prints the text in its argument as a prompt
• input waits for the user to type some text
• When the user hits Enter or Return, input returns the string to the function call

  >>> name = input("Name: ")
  Name: Glenn
  >>> name
  'Glenn'

• The value returned by input is always a string
• If you want the value to be a number ...
• you must use a conversion function
• To convert the variable number to an integer we can use the int function

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

Performing Calculations

• Operators are special characters that act upon one or more values ...
• and perform some action
• Usually the action creates a new value
• The values that the operators work on are called operands
• Python has operators for basic arithmetic
  
  

  Character	Operation	Description
  +	Addition	Adds two numbers
  −	Subtraction	Subtracts one number from another
  *	Multiplication	Multiplies one number by another
  /	Division	Divides one number by another and gives the result as a decimal number
  //	Integer division	Divides one number by another and gives the result as an integer
  %	Remainder	Divides one number by another and gives the remainder
  **	Exponent	Raises a number to a power

Decimal and Integer Division

• Python has two division operators
  • /
  • //
• / works like ordinary division

  >>> 4 / 2
  2.0
  >>> 4 / 5
  0.8

• / always returns a float
• // division always return a whole number (int)
• Throwing away the decimal part

  >>> 10 // 3
  3
  

Exponent Operator

• ** is the exponent operator
• The first operand is raised to the power of the second operand

  >>> 3 ** 2
  9
  >>> 3 ** 3
  27
  >>> 3 ** 4
  81

Remainder Operator

• When we perform long division we get two results
  • A quotient
  • A remainder
• If we divide 17 by 5 we get a quotient of 3 and a remainder of 2
• Integer division, //, gives us the quotient

  >>> 17 // 5
  3

• And we can get the remainder with %

  >>> 17 % 5
  2

Operator Precedence

• Sometimes the order in which you use operators affects the result
• Python needs rules to decide which operators to use first
• The order in which operators are evaluated are called the rules of operator precedence
• The operator with the highest precedence is used first
• The precedence for Python's arithmetic operators is

  **	Exponentiation
  *   /   //   %	Multiplication, division and remainder
  +   -	Addition and subtraction

Grouping with Parentheses

• When we write

  2 + 3 * 5

• Python will multiply 3 times 5, before it adds 2
• If you want addition to be done first, you use parentheses

  >>> 2 + 3 * 5
  17
  >>> (2 + 3) * 5
  25

Escape Sequences

• The non printing characters
  • Space
  • Tab
  • Newline (Return or Enter)
• Are called whitespace characters
• You can add a space to a string just by hitting the space bar
• But how do you add a Tab?
• To include such characters in a string literal you need a special way to represent them
• You do this using two characters
• The first is a \ ...
• followed by another character
• Such combinations are called escape sequences
• The most commonly used escape characters in Python are

  Escape Character	Effect
  \n	Causes output to be advanced to the next line
  \t	Causes output to skip over to the next horizontal tab position
  \'	Causes a single quote mark to be printed
  \"	Causes a double quote mark to be printed
  \\	Causes a backslash character to be printed

Concatenation Operator

• Different data types use different operators
• Dividing two numbers make sense, but not dividing two strings
• The basic operation performed on strings is to join two strings into one
• This is called concatenation
• The concatenation operator in Python is +

  >>> team = "Red Sox"
  >>> cheer = "Let's go " + team
  >>> print(cheer)
  Let's go Red Sox

Boolean Expression

• A value that can only be True or False is called a boolean
• Python refers to this data type as bool
• A boolean expression is an expression whose only possible values are True and False
• There are two boolean literals
  • True
  • False
• Notice the capitalization

Relational Operators

• Boolean expressions are often created using relational operators
• Relational operators compare two values ...
• and return True or False depending on their relationship
• Here are the relational operators

  Operator	Meaning
  >	Greater than
  <	Less than
  >=	Greater than or equal to
  <=	Less than or equal to
  ==	Equal to
  !=	Not equal to

• Notice that we test for equality using == not =

Precedence of Relational Operators

• All the relational operators have lower precedence than the arithmetic operators
• So + will be evaluated before >
• The order of precedence is

  **	Exponentiation
  *   /   //   %	Multiplication, division and remainder
  +   -	Addition and subtraction
  >   <   >=   <=   ==   !=	Relational Operators

Control Structures

• Control structures are features of a language that allow the interpreter to take different paths through the code ...
• depending on the conditions when the code is run
• All control structures use code blocks

Code Blocks

• A group of statements that are only executed under certain conditions ...
• is called a code block
• All control structures use code blocks
• A code block in Python is created by indenting a group of statements
• All statements in a code block must be indented the same amount ...
• using the same characters ...
• either spaces or tabs
• It is best to use tabs

if Statements

• The if statement evaluates a boolean expression
• If it is True ...
• the statements in the code block will be executed
• It has the following format

  if BOOLEAN_EXPRESSION:
      STATEMENT
      STATEMENT
      ...

• If the boolean expression is True
• All the indented statements in the code block will be executed
• Otherwise the statements will be skipped

Attendance

if-else Statements

• An if statement will only run the statements in the code block ...
• if the boolean expression is True
• If the expression is not True ...
• the interpreter skips over the indented statements
• To write a Python statement that did one thing if the expression were True ...
• and something else if it were False ...
• you would use an if-else statement
• It has the following format

  if BOOLEAN_EXPRESSION:
      STATEMENT
      STATEMENT
      ...
  else:
      STATEMENT
      STATEMENT
      ...

• If the expression is True
• The statements in the code block following the if clause are executed
• Otherwise the statements in the code block after the else clause are executed

Indentation in the if-else Statement

• Both an if statement and and if-else statement must be indented properly
• An if-else statement has two clauses
  • An if clause
  • An else clause
• Both clauses must be indented the same amount
• If all the statements after the if and else are not exactly aligned
• You will get an error

  >>> if 4 > 3:
  ...   print("4 is ")
  ...     print("greater than 3")
    File "<stdin>", line 3
      print("greater than 3")
      ^

if-elif-else Statements

• An if-elif-else statement lets you test for many conditions
• Here is the format

  if BOOLEAN_EXPRESSION_1:
      STATEMENT
      ...
  elif BOOLEAN_EXPRESSION_2:
      STATEMENT
      ...
  elif BOOLEAN_EXPRESSION_3
      STATEMENT
      ...
  ...
  [else:
      STATEMENT
      ...]

• The [ ] around the else clause means that it is optional
• The keyword elif is short for "else if"
• Notice that all the conditions line up vertically

Logical Operators

• Each data type has its own set of operators
• Logical operators work on boolean values ...
• and produce a boolean result
• The logical operators are
  • and
  • or
  • not
• The and operator returns True if both its operands are True

  >>> 5 > 4 and 4 > 2
  True

• If either of its two operands are False, and returns False

  >>> 5 > 4 and 4 < 2
  False

• It also returns False if both its operands are False

  >>> 5 < 4 and 4 < 2
  False

• The or operator returns True if either of its operands are True

  >>> 5 > 4 or 4 < 2
  True

• It only returns False if both of its operands are False

  >>> 5 < 4 or 4 < 2
  False

• The not operator takes only one operand ...
• and reverses its value
• So True becomes False

  >>> not 5 > 4
  False

• and False becomes True

  >>> not 5 < 4
  True

• To show how the logical operators work we use truth tables
• Here is the truth table for and

  p	q	p and q
  True	True	True
  True	False	False
  False	True	False
  False	False	False

• The truth table for or

  p	q	p or q
  True	True	True
  True	False	True
  False	True	True
  False	False	False

• And the truth table for not

  p	not p
  True	False
  False	True

Precedence of Logical Operators

• The logical operators, and, or and not have lower precedence than arthmetic operators
• They are also lower than the relational operators

  **	Exponentiation
  *   /   //   %	Multiplication, division and remainder
  +   -	Addition and subtraction
  >   <   >=   <=   ==   !=	Relational Operators
  not	Logical NOT
  and	Logical AND
  or	Logical OR

Loops

• A loop repeats the statements in its code block
• There are two types of loops
  • Loops controlled by counting
  • Loops controlled by some condition
• Counted loops repeat a certain number of times
• Conditional loops repeat as long as some condition is True

While Loops

• A while loop has the following format

  while BOOLEAN_EXPRESSION:
      STATEMENT
      STATEMENT
      ...

• The loop runs as long as the boolean expression is True
• The boolean expression is evaluated before entering the loop
• So it is possible that the statements in the code block will never be executed

Infinite Loops

• If the boolean expression in a while loop does not become False
• The loop will go on forever
• This is called an infinite loop
• On Unix, you stop an infinite loop by hitting Control C

for Loops

• The for loop has the following format

  for VARIABLE_NAME in LIST:
      STATEMENT
      STATEMENT
      ...

• The first value in the list is assigned to the loop variable
• The variable keeps that value while passing through the code block
• When all the statements in the code block are run ...
• the next value in the list is assigned to the loop variable
• This continues until all values are used

The range Function

• The range function is used to create a list of integers in a for loop
• It can take one, two or three arguments
• With one argument the list goes from 0 to one less than the argument

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

• With two arguments, the first gives the starting value ...
• and the second argument is one more than the last number

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

• The third argument is the step value
• It is what is added to each number to get the next

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

Running Totals

• A total that is updated as new numbers are encountered is called a running total
• A good example of a running total is a bank balance
• Your bank balance is the running total of all deposits and withdrawals from your account
• Running totals are calculated using a variable called an accumulator
• This is followed by a loop ...
• in which new numbers are added to the accumulator
• The accumulator must be set to 0 ...
• before you enter the loop
• Running totals are used when you calculate averages

  $ 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

• The values in blue are entered by the user

Augmented Assignment Operators

• Often we need to change the value of a variable by a specific amount
• If we wanted to add 5 to the variable num we could write

  number = number + 5

• Statements like this occur often
• So most computer languages have a special operator for this
• Instead of writing the statement above we could write

  number += 5

• This operator works as follows
  • Get the current value of the variable on the left
  • Get the current value of the expression on the right
  • Add the two values
  • Assign this new value to the variable
• An operator like this is called an augmented assignment operator
• Python has an augmented assignment operator for every arithmetic operator

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

Functions

• A function is a group of statements that has a name ...
• and performs a specific task
• Some functions are written inside the Python interpreter
  • print
  • int
  • round
• They are called built-in functions
• But you can define your own Python functions
• You can also create files called modules that contain a number of functions
• You can then use an import statement to bring these functions into any Python script

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
• The conversion functions like str, int and float return a value
• print does not return a value

Defining a Function

• A function definition has the following format

  def FUNCTION_NAME([PARAMETER, ...]):
      STATEMENT
      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
• The parameter list might be empty
• But the header must still have parentheses
• Following the function header is a code block
• The statements in the code block are executed when the function is called

Calling a Function

• To run a function you make a function call
• A function call has two parts
  • The function name
  • A list of arguments enclosed in parentheses
• The values inside the parentheses are called arguments
• The value of each argument is assigned to the corresponding parameter
• You must always have parentheses, ( ), after the function name ...
• even if the function takes no arguments
• Here is an example of a function call

  average = round(total/entries)

Local Variables

• A variable created inside a function is a local variable
• Local variables only exists while the function runs
• When the function stops running ...
• all local variables disappear
• A local variable is invisible outside the function
• 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
• The section of a script where a variable is defined is called its scope
• The scope of a local variable is the code block in which it is defined

Parameters

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