An Introduction To Python - Python Midterm Review

An Introduction To Software
Development Using Python
Spring Semester, 2015
Midterm Review

What’s In Your Python Toolbox?
print() math strings I/O IF/Else elif While For
Lists

A Closer Look At Our First Program
• # My first Python program.
– This is a comment – it’s for you, not the computer
– Comments begin with # and are not statements.
• print("Hello, World!")
– displays a line of text, namely “Hello, World!”.
– We call a function named print and pass it the information to be
displayed.
– A function is a collection of programming instructions that carry
out a particular task.
– It is part of the Python language.

Let’s Talk About Variables…
• Variables are reserved memory locations to store values. This means that
when you create a variable you reserve some space in memory.
• Variables in Python consist of an alphanumeric name beginning in a letter or
underscore. Variable names are case sensitive.
• Python variables do not have to be explicitly declared to reserve memory
space. The declaration happens automatically when you assign a value to a
variable. The equal sign (=) is used to assign values to variables.
• The operand to the left of the = operator is the name of the variable and the
operand to the right of the = operator is the value stored in the variable.
• Examples:
counter = 100
miles = 1000.0
name = "John"
Image Credit: ClipArt Best
Python variable
names can have
unlimited length –
but keep them short!

Let’s Talk About Python Math!
Normal Stuff
Weird Stuff
5%2 = 1

Modulo (%) Is Your Friend
• A man has 113 cans of Coke. He also has a group of boxes that can hold 12 cans
each. How many cans will he have left over once he’s loaded all of the boxes?
• On a military base the clock on the wall says that the time is 23:00. What time of
day is this?
• My friend has 10,432 ping pong balls that he needs to put into storage. My car can
transport 7,239 balls. How many will be left after I leave?

What Comes First?
Precedence Rules
• The precedence rules you learned in algebra apply during the evaluation of
arithmetic expressions in Python:
– Exponentiation has the highest precedence and is evaluated first.
– Unary negation is evaluated next, before multiplication, division, and
remainder.
– Multiplication, division, and remainder are evaluated before addition and
subtraction.
– Addition and subtraction are evaluated before assignment.
– With two exceptions, operations of equal precedence are left associative,
so they are evaluated from left to right. Exponentiation and assignment
operations are right associative, so consecutive instances of these are evaluated from right to
left.
– You can use parentheses to change the order of evaluation
• "PEMDAS", which is turned into the phrase "Please Excuse My Dear Aunt
Sally". It stands for "Parentheses, Exponents, Multiplication and Division, and
Addition and Subtraction".
3**2
-5
2+3*4/6
sum = 2+3
2+3*4/6
sum = 3**2**5
(2+3)*4/5
Image Credit: www.pinterest.com

Precedence Rules: Examples
• Simplify 4 + 32
• Simplify 4 + (2 + 1)2
• Simplify 4 + [–1(–2 – 1)]2
• Simplify 4( –2/3 + 4/3)
• Three people ate dinner at a restaurant and want to split the
bill. The total is $35.27, and they want to leave a 15 percent
tip. How much should each person pay?
Image Credit: pixgood.com

Not All Numbers Are Created Equal
• Integers are the numbers you can easily count, like 1,
2, 3, as well as 0 and the negative numbers, like –1, –2,
–3.
• Decimal numbers (also called real numbers) are the
numbers with a decimal point and some digits after it,
like 1.25, 0.3752, and –101.2.
• In computer programming, decimal numbers are also
called floating-point numbers, or sometimes floats for
short (or float for just one of them). This is because the
decimal point “floats” around. You can have the
number 0.00123456 or 12345.6 in a float.
Image Credit: www.clipartpanda.com

Mixed-Mode Arithmetic
and Type Conversions
• Performing calculations involving both integers and
floating-point numbers is called mixed-mode arithmetic.
• Conversions
– Determine type: type()
– Drop decimal: int()
– Round up / down: round()
– Change to decimal: float()
– Change to string: str()
Image Credit: personaltrainerbusinesssystems.com

Cool Kid Stuff:
Increment / Decrement / E-Notation
• Incrementing: sum = sum + 1
– sum += 1
• Decrementing: sum = sum – 1
– sum -= 1
• E-Notation
– 1,000,000 = 1 x 10**6 = 1e6
Image Credit: www.toonvectors.com

Say Hello To Strings!
• Your computer programs will have to process text
in addition to numbers.
• Text consists of characters: letters, numbers,
punctuation, spaces, and so on.
• A string is a sequence of characters.
• Example: “John Smith”
Image Credit: imgarcade.com

How Do We Deal With Strings?
• Strings can be stored in variables:
answer = “a”
• A string literal denotes a particular string:
“Mississippi”
• In Python, string literals are specified by enclosing a
sequence of characters within a matching pair of either
single or double quotes.
“fire truck” or ‘fire truck’
Image Credit: imgarcade.com

Strings and Characters
• Strings are sequences of Unicode characters.
• You can access the individual characters of a
string based on their position within the
string.
• This position is called the index of the
character.
Image Credit: www.fontspace.com
0 1 2 3 4
First Last

String Indexes
• name = “TILEZ”
• len(name) = 5
• name[0] = “T”, name[4] = “Z”
0 1 2 3 4

All Characters Have Numbers
• A character is stored internally as an integer value. The
specific value used for a given character
is based on its ASCII code (or Unicode code).
• Python provides two functions related to character encodings.
– The ord function returns the number used to represent a given
character. ord(“A”) = 65
– The chr function returns the character associated
with a given code. chr(65) = “A”

Input and Output
• When a program asks for user input, it should first print a message that
tells the user which input is expected. Such a message is called a prompt.
In Python, displaying a prompt and reading the keyboard input is
combined in one operation.
– first = input("Enter your first name: ")
• The input function displays the string argument in the console window
and places the cursor on the same line, immediately following the string.
– Enter your first name: █
Image Credit: blogs.msdn.com

Numerical Input
• The input function can only obtain a string of text from the
user.
• To read an integer value, first use the input function to obtain
the data as a string, then convert it to an integer using the int
function.
numBottles = input("Please enter the number of bottles: ")
bottles = int(numBottles)
bottlePrice = input("Enter price per bottle: ")
price = float(bottlePrice)
Image Credit: www.canstockphoto.com

Formatted Output
• To control how your output looks, you print a formatted string and then provide the values
that are to be “plugged in”.
• If the value on the right of the “%” is a string, then the % symbol becomes the string format
operator.
• The construct %10.2f is called a format specifier: it describes how a value should be
formatted.
– The 10 specifies the size of the field, the 2 specified the number of digits after the “.”.
– The letter f at the end of the format specifier indicates that we are formatting a floating-point value. Use d for an
integer value and s for a string;

Formatted Output
• To specify left justification for a string, add a minus sign
before the string field width:
title1 = "Quantity:“
title2 = "Price:"
print("%-10s %10d" % (title1, 24))
print("%-10s %10.2f" % (title2, 17.29))
• The result is:
Quantity: 24
Price: 17.29
Image Credit: www.theindianrepublic.com

Python Format Specifier Examples

Say Hello To The “IF” Statement
• In Python, an IF statement is used to
implement a decision.
• When a condition is fulfilled, one set of
statements is executed. Otherwise, another
set of statements is executed

Tabs
• Blockstructured code has the property that nested statements are indented by one
or more levels:
if totalSales > 100.0 :
discount = totalSales * 0.05
totalSales = totalSales − discount
print("You received a discount of $%.2f" % discount)
else :
diff = 100.0 − totalSales
if diff < 10.0 :
print("purchase our item of the day & you can receive a 5% discount.")
else :
print("You need to spend $%.2f more to receive a 5% discount." % diff)

Relational Operators
Equality
Testing
Requires
2 “=“
Note: relational operators have a lower precedence than arithmetic operators

Examples Of Relational Operators

What Is A “Nested Branch”?
• It is often necessary to include an if statement
inside another. Such an arrangement is called
a nested set of statements.
• Example:
Is the
club
full?
Arrive at
the club
Are you
on the
VIP list?
Wait in car
Go right in
Wait in line
Y
Y
N
N

Example of a “Nested Branch”
if (peopleInClub < maxPeopleInClub) :
if (youName == VIPListName) :
goRightIn
else :
waitInLine
else :
waitInCar

Problems With “Super Nesting”
• Difficult to read
• Shifted too far to the right due to indentation
Image Credit: www.clipartbest.com

A Better Way: elif
If (classScore >=90) :
print(“You got an A!”)
elif (classScore >= 80) :
print(“You did ok, you got a B!”)
elif (classScore >=70) :
print(“So-so, you got a C”)
elif (classScore >= 60) :
print(“Oh –oh, you got a D”)
else :
print(“Dang it, you got an F”)
Image Credit: jghue.blogspot.com
Note that you have to test the more specific conditions first.

How Do You Do Things
Over And Over Again?
• In Python, loop statements repeatedly execute instructions until a
goal has been reached.
• In Python, the while statement implements such a repetition. It has
the form:
while condition :
statement1
statement 2
• As long as the condition remains true, the statements inside the
while statement are executed.
Image Credit: etc.usf.edu
Body
Condition

How Can I Loop For A Given Number
Of Times?
• You can use a while loop that is controlled by a
counter:
counter = 1 # Initialize the counter.
while counter <= 10 : # Check the counter.
print(counter)
counter = counter + 1 # Update the loop variable
Note: Some people call this loop count-controlled.
Image Credit: www.dreamstime.com

The For Statement
The for loop can be used to iterate over the contents of any container, which is an
object that contains or stores a collection of elements. Thus, a string is a container
that stores the collection of characters in the string.

What’s The Difference Between While
Loops and For Loops?
• In the for loop, the element variable is
assigned stateName[0] , stateName[1] , and so
on.
• In the while loop, the index variable i is
assigned 0, 1, and so on.

The Range Function
• Count-controlled loops that iterate over a range of integer values are very
common.
• To simplify the creation of such loops, Python provides the range function
for generating a sequence of integers that can be used with the for loop.
• The loop code:
for i in range(1, 10) : # i = 1, 2, 3, ..., 9
print(i)
prints the sequential values from 1 to 9. The range function generates a
sequence of values based on its arguments.
• The first argument of the range function is the first value in the sequence.
• Values are included in the sequence while they are less than the second
argument
Image Credit: www.best-of-web.com

You Can Do The Same Thing With
While And For Loops
for i in range(1, 10) : # i = 1, 2, 3, ..., 9
print(i)
i = 1
while i < 10 :
print(i)
i = i + 1
Image Credit: www.rgbstock.com
Note that the ending value (the second argument to the range function) is not included
in the sequence, so the equivalent while loop stops before reaching that value, too.

Stepping Out…
• By default, the range function creates the
sequence in steps of 1. This can be changed
by including a step value as the third
argument to the function:
for i in range(1, 10, 2) : # i = 1, 3, 5, ..., 9
print(i)
Image Credit: megmedina.com

Review: The Many Different Forms Of
A Range

Secret Form Of print Statement
• Python provides a special form of the print function that prevents it
from starting a new line after its arguments are displayed.
print(value1, value2, end="")
• By including end="" as the last argument to the first print function,
we indicate that an empty string is to be printed after the last
argument is printed instead of starting a new line.
• The output of the next print function starts on the same line where
the previous one left off.

4 Steps To Creating A Python List
1. Convert each of the names into strings by surrounding the
data with quotes.
2. Separate each of the list items from the next with a comma.
3. Surround the list of items with opening and closing square
brackets.
4. Assign the list to an identifier (movies in the preceding code)
using the assignment operator (=).
“COP 2271c” “Introduction to Computation and Programming” 3
“COP 2271c”, “Introduction to Computation and Programming”, 3
[“COP 2271c”, “Introduction to Computation and Programming”, 3]
prerequisites = [“COP 2271c”, “Introduction to Computation and Programming”, 3]
COP 2271c Introduction to Computation and Programming 3
Image Credit: Clipart Panda

How To Access A List
• A list is a sequence of elements, each of which has an
integer position or index.
• To access a list element, you specify which index you
want to use.
• That is done with the subscript operator ([] ) in the
same way that you access individual characters in a
string.
• For example:
print(values[5]) # Prints the element at index 5
Image Credit: imgkid.com

Access Your List Data Using The Square
Bracket Notation
print (prerequisites[0]) COP 2271c
print (prerequisites[1]) Introduction to Computation and Programming
print (prerequisites[2]) 3
Image Credit: Clipart Panda

You Can Create
Multidimensional Lists
• Lists can hold data of mixed type.
• But it gets even better than that: lists can hold
collections of anything, including other lists.
• Simply embed the inner list within the enclosing
list as needed.
multiDim = [[123],[456],[789]] =
1 2 3
4 5 6
7 8 9
Image Credit: www.rafainspirationhomedecor.com

Playing With Lists: Append
• Add teacher: Dr. Jim Anderson
• Add year: 2015
Note: Python lists can contain data of mixed types. You can mix strings with
numbers within the same Python list. You can mix more than just strings and
numbers -- you can store data of any type in a single list.
prerequisites = [“COP 2271c”, “Introduction to Computation and Programming”, 3]
Image Credit: ClipArt Best

Playing With Lists: Pop
• Allows you to remove the item that is at the
end of a list
prereqs[course].pop()
CHM 2045 , Chemistry 1 , 3 , Dr. Anderson , 2015X
Image Credit: blog.poolproducts.com

Playing With Lists: Remove
• Allows you to specify which list item you want
to remove no matter where in the list it is
located
prereqs[course].remove("Dr. Anderson")
CHM 2045 , Chemistry 1 , 3 , Dr. AndersonX

Playing With Lists: Insert
• Allows you to add an item to a list in a
specified location on the list
prereqs[course].insert(2,"Really Hard")
CHM 2045 , Chemistry 1 , Really Hard , 3
Image Credit: ekskavatör

Playing With Lists: Extend
• Allows multiple list items to be added to an
existing list
prereqs[course].extend(["Dr. Anderson", "2015"])
CHM 2045 , Chemistry 1 , Really Hard , 3 , Dr. Anderson , 2015

Appending Elements
• Start with an empty list
goodFood=[]
• A new element can be appended to the end of the list with the append
method:
goodFood.append(“burgers”)
• The size, or length, of the list increases after each call to the append
method. Any number of elements can be added to a list:
goodFood.append(“ice cream”)
goodFood.append(“hotdog”)
goodFood.append(“cake”)

Inserting an Element
• Sometimes, however, the order is important and a new element has to be
inserted at a specific position in the list. For example, given this list:
friends = ["Harry", "Emily", "Bob", "Cari"]
suppose we want to insert the string "Cindy" into the list following the fist
element, which contains the string "Harry". The statement:
friends.insert(1, "Cindy")
achieves this task
• The index at which the new element is to be inserted must be between 0 and the
number of elements currently in the list. For example, in a list of length 5, valid
index values for the insertion are 0, 1, 2, 3, 4, and 5. The element is inserted
before the element at the given index, except when the index is equal to the
number of elements in the list. Then it is appended after the last element:
friends.insert(5, "Bill")
This is the same as if we had used the append method.
Image Credit: www.crazywebsite.com

Finding An Element
• If you simply want to know whether an element is present in a list, use the in
operator:
if "Cindy" in friends :
print("She's a friend")
• Often, you want to know the position at which an element occurs. The index
method yields the index of the fist match. For example,
friends = ["Harry", "Emily", "Bob", "Cari", "Emily"]
n = friends.index("Emily") # Sets n to 1
• If a value occurs more than once, you may want to find the position of all
occurrences. You can call the index method and specify a starting position for the
search. Here, we start the search after the index of the previous match:
n2 = friends.index("Emily", n + 1) # Sets n2 to 4
Image Credit: www.theclipartdirectory.com

Removing an Element
• Pop
– The pop method removes the element at a given position. For example, suppose we start with the
list
friends = ["Harry","Cindy","Emily","Bob","Cari","Bill"]
To remove the element at index position 1 ("Cindy") in the friends list, you use the command:
friends.pop(1)
If you call the pop method without an argument, it removes and returns the last element of the list.
For example, friends.pop() removes "Bill".
• Remove
– The remove method removes an element by value instead of by position.
friends.remove("Cari")

Let’s Talk About: Tables
• It often happens that you want to store
collections of values that have a two-
dimensional tabular layout.
• Such data sets commonly occur in financial and
scientific applications.
• An arrangement consisting of rows and
columns of values is called a table, or a matrix.
Image Credit: www.rafainspirationhomedecor.com

How To Create Tables
• Python does not have a data type for creating
tables.
• A two-dimensional tabular structure can be
created using Python lists.
• A table is simply a list in which each element is
itself another list

Accessing Elements Of A Table
• To access a particular element in the table,
you need to specify two index values in
separate brackets to select the row and
column, respectively.
medalCount = counts[3][1] = 0

An Introduction To Python - Python Midterm Review

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