If/Else and While Loops

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If/Else andWhile Loops

• Ethan Cerami
• New York University

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This Week

• Tuesday
• Basics of If/Else Statement
• Relational Operators
• Increment/Decrement Operators
• Thursday
• While Loops
• Amazon.com Program

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If Statement

• Control Structure a structure that controls the
  flow of a program.
• If Statement
• If some condition is true, do this.
• Otherwise, do something else.
• Every programming language has some form of an if
  statement.

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Basic If Syntax

• Basic If Example
• if (grade gt 60)
• printf (You passed!)
• Basic If/Else Example
• if (grade gt 60)
• printf (You passed!)
• else
• printf (You failed!)

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Understanding Flow Charts

• Flow Chart a visual tool that helps you
  understand the flow of your program.
• You can think of flow charts as pipes of water
• You pour water in the top.
• Water pours through the pipes and is pulled
  downward

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Flow Chart Basics
Diamonds Represents conditions
Rectangles represent statements of work. For
example printf()
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If/Else Flow Chart
                               
 
True
False
grade gt60
Print You passed
Print You failed
 
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Understanding Blocks of Code

• If you want to run several lines of code
  together, you must include them within a block of
  code.
• Blocks of code are denoted with
• For example
• if (grade gt 60)
• printf ("You passed!!!")
• printf ("\nCongratulations!")

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Understanding Indentation

• Note that everything within the block of code
  should be indented. This helps you see the block
  at a quick glance.
• Avoid writing code like this
• if (grade gt 60)
• printf ("You passed!!!")
• printf ("\nCongratulations!")
• This is valid C code, but it is not easy to view
  the block. Hence, it is considered bad style.

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Example

• / Introduction to If/Else Statements /
• include ltstdio.hgt
• include ltconio.hgt
• main ()
• int grade
• printf ("Enter your course grade ")
• scanf ("d", grade)
• if (grade gt 60)
• printf ("You passed!!!")
• printf ("\nCongratulations!")
• else
• printf ("You failed!")
• getch()

Note the indentation
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Relational Operators
Operator Meaning
gt Greater than
lt Less than
gt Greater than or equal to
lt Less than or equal to
Equal to
! Not Equal to
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Testing for Equality

• To test for equality, use the operator.
• For example
• if (grade 100)
• printf (Perfect Score!)
• To test for inequality, use the ! operator.
• For example
• if (grade ! 100)
• printf (Not perfect!)

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Equality v. Assignment

• A very common mistake
• if (grade 100)
• printf (Perfect Score!)
• In this case, we are using a single character.
  (We really want to use )
• Nonetheless, this is perfectly valid code. But
  what will it actually print?

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Understanding Truth

• In C (unlike life) truth is very simple.
• 0 is False
• Anything else is True
• For example
• -1 is true
• 299 is true
• O is false

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For Example

• / Understanding Truth /
• include ltstdio.hgt
• include ltconio.hgt
• main ()
• int x -100
• int y 299
• int z 0
• if (x)
• printf ("x d\n", x)
• if (y)
• printf ("y d\n", y)
• if (z)
• printf ("z d\n", z)
• getch()

This is a completely valid program. Output x
-100 y 299 Note z is not printed because the
if condition fails.
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Equality v. Assignment

• Given
• if (grade 100)
• printf (Perfect Score!)
• This statement does the following
• Grade is assigned the value 100.
• Because 100 is true, the condition is always
  true.
• No matter what the student grade, it always says
  Perfect Score!

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Amazon.com Series

• Over the next couple of weeks, we will take a
  single program and make it more and more
  complicated.
• Each class we will add some new feature to the
  program.
• Examples are the best way to learn how to
  program.
• Make sure to study these programs!

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Amazon.com Series

• The Amazon.com Series will simulate the
  purchasing of books.
• For example
• Prompt you for all the books you want to buy.
• Calculate the total sales.
• Print out a receipt.

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Amazon.com Version 1.0

• Feature Set
• Prompt the user for 3 digit order
• Prompt the user for cover price
• Prompt the user for the of books
• Calculate the total sales
• If sales gt 100.00 send via Federal Express
• else send via U.S. Mail

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Program Snapshot

• / E-Commerce Program for Amazon.com, Version
  1.0 /
• / Created by Ethan Cerami, 1998 /
• include ltstdio.hgt
• include ltconio.hgt
• main ()
• int order_number, number_books
• float price, sales
• / Print Introduction /
• printf ("Welcome to Amazon.com!\n")
• printf ("Earth's Biggest Bookstore.\n\n")
• / Get User Input /
• printf ("Enter the three digit book order
  number ")
• scanf ("d", order_number)
• printf ("Enter the cover price ")
• scanf ("f", price)

Now that we are working on bigger programs, its
hard to display them within Powerpoint(!) So,
please bring your Annotated Program packet.
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Program, Page 1

• / E-Commerce Program for /
• / Amazon.com, Version 1.0 /
• / Created by Ethan Cerami, 1998 /
• include ltstdio.hgt
• include ltconio.hgt
• main ()
• int order_number, number_books
• float price, sales

Variable Declaration Note that price and sales
are defined as floats.
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Program, Page 2
Prompt user for book order, cover price, and
number of copies.

• / Print Introduction /
• printf ("Welcome to Amazon.com!\n")
• printf ("Earth's Biggest Bookstore.\n\n")
• / Get User Input /
• printf ("Enter the three digit book order number
  ")
• scanf ("d", order_number)
• printf ("Enter the cover price ")
• scanf ("f", price)
• printf ("Enter number of copies you would like to
  order ")
• scanf ("d", number_books)

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Program, Page 3
Use an if/else statement to determine the
shipping option.

• / Calculate Total Sales /
• sales price number_books
• printf ("\nThank you. Your total order is
  .2f\n", sales)
• / Determine Shipping Method /
• if (sales gt 100.0)
• printf ("Shipping Your order will be shipped
  via Federal Express.")
• else
• printf ("Shipping Your order will be shipped
  via U.S. Mail.")
• printf ("\n\nPress any key to exit...")
• getch()

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Reminder

• All the programs contained in the Annotated
  Programs packet are available on the web site.
• You can therefore easily download them and try
  them on your own.
• Learning by example is a great technique.

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Amazon.com, Version 2.0

• Feature Set (everything from Version 1.0 plus)
• Prompt the user for a payment option (e.g.
  Mastercard, Visa, American Express.)
• Use the if short-cut operator

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Program, Page 1

• / Get Credit Card Information /
• printf ("Amazon.com accepts the following payment
  options\n")
• printf ("1. American Express\n")
• printf ("2. Mastercard\n")
• printf ("3. Visa\n")
• printf ("Please enter a payment option ")
• scanf ("d", payment_option)

Here, we are prompting the user for a
payment option. This is stored in the
payment_option integer variable.
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Program, Page 2

• / Print Credit Card Confirmation /
• printf ("This account balance will be charged to
  ")
• if (payment_option 1)
• printf ("American Express.\n")
• else if (payment_option 2)
• printf ("Mastercard.\n")
• else if (payment_option 3)
• printf ("Visa.\n")

Here, we use a series of if/else statements to
print out the payment option.
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Program, Page 3

• C Provides a shortcut if/else operator
• printf ("Shipping Your order will be shipped
  via ")
• printf ("s", (sales gt 100.0) ? "Federal
  Express." "U.S. Mail.")

s is the format specifier for text strings.
The conditional Statement.
True Option
False Option
? short- cut operator