While Loops, For Loops and Switch Statements

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While Loops,For Loops andSwitch Statements

• Ethan Cerami
• New York University

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This Week

• Tuesday
• Pre-Post Increment Operators
• While Loops
• Amazon.com, Version 3.0
• Thursday
• For Loops
• Switch Statements
• Logical Operators

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I. Assignment Statements

• Given the following
• x 2
• x x 1
• printf (x d, x)
• There are actually several ways to rewrite this
  more concisely.

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Short Cut Operator

• One option is to use the operator
• x 2
• x 1
• printf (x d, x)
• There are similar operators for , -, /
• x x 5 is equivalent to x 5
• x x 5 is equivalent to x - 4
• x x / 5 is equivalent to x / 5

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Increment Operator

• A second option is to use an increment operator
• x Post-Increment Operator
• x Pre-Increment Operator
• Both operators will increment x by 1, but they do
  have subtle differences.

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Pre v. Post Increment

• PostIncrement Operator (x)
• use the current value of x in the expression.
  Then, increment by 1.
• PreIncrement Operator (x)
• Increment x by 1. Then, use the new value of x in
  the expression.

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How about a real example?

• / Preincrementing v. PostIncrementing /
• include ltstdio.hgt
• main ()
• int c5
• printf ("d\n", c)
• printf ("d\n", c)
• printf ("d\n\n", c)
• c5
• printf ("d\n", c)
• printf ("d\n", c)
• printf ("d\n", c)
• getchar()

Post Increment
Output 5 5 6 5 6 6
Pre Increment
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II. While Loops

• While Loop Keep repeating an action while some
  condition remains true.
• Examples
• Every Stairmaster Machine contains a while loop.
• while the person is still climbing, keep
  displaying the status, e.g. number of stairs
  climbed, calories burned, etc.
• Keep prompting for book orders until the user is
  done.

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Parts of a While Loop

• Every while loop will always contain three main
  elements
• Priming initialize your variables.
• Testing test against some known condition.
• Updating update the variable that is tested.

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Simple While Loop
Index 1 Index 2 Index 3 Index 4 Index
5 Index 6 Index 7 Index 8 Index 9 Index
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• include ltstdio.hgt
•  
• main ()
• int index 1
•   while (index lt 10)
• printf ("Index d\n", index)
• index

1. Priming
2. Test Condition
3. Update In this case, you can use either the
pre or post increment operator.
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While Loop Flowchart
1. Priming Set index1
2. Test index lt 10
TRUE
3. Update Index
FALSE
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Infinite Loop

• Infinite Loop A loop that never ends.
• Generally, you want to avoid these!
• There are special cases, however, when you do
  want to create infinite loops on purpose.
• Common Exam Questions
• Given a piece of code, identify the bug in the
  code.
• You may need to identify infinite loops.

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Infinite Loop Example 1

• include ltstdio.hgt
•  
• main ()
• int index 1
•   while (index lt 10)
• printf ("Index d\n", index)

Index 1 Index 1 Index 1 Index 1 Index
1 forever
Here, I have deleted part 3 the index
statement.
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Infinite Loop, Example 2

• include ltstdio.hgt
•  
• main ()
• int index 1
•   while (index gt 0)
• printf ("Index d\n", index)
• index

Index 1 Index 2 Index 3 Index 4 Index
5 forever
Here, I have changed Part 2 the test condition.
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Why this is important

• Or, how I crashed the NYU Computer System

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Amazon.com, Version 3.0

• New Features
• Users can now enter more than one book.
• Program keeps track of a running total.
• Uses a while loop to get the work done.

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Understanding Sentinels

• Sentinel a special value that indicates the
  end of data entry.
• For example
• -1 means end of data.
• 0 means end of data.
• END means ends of data
• Depends on the specific application you are
  building.

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Version 3.0 Code
1. Priming

• / Prime the while loop with user data /
• printf ("Enter the three digit book order number
  (-1 to end) ")
• scanf ("d", order_number)
• while (order_number ! -1)
• printf ("Enter the cover price ")
• scanf ("f", price)
• printf ("Enter number of copies you would like
  to order ")
• scanf ("d", number_books)
• / Calculate Running Total Sales /
• sales price number_books
• printf ("Running Total .2f\n\a", sales)
• / Prompt user for next book /
• printf ("Enter the three digit book order number
  (-1 to end) ")
• scanf ("d", order_number)

2. Test Condition
3. Update
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Next Homework

• HW 2 requires that you write several example
  programs that continue to prompt for user data.
• Use Amazon.com, Version 3.0 as an example.

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III. Number Ranges

• Suppose you have the following program
• main ()
• / Set x 2,147,483,646 /
• int x 2147483646
• printf ("x d\n", x)
• printf ("x1 d\n", x)
• printf ("x2 d", x)
• getchar()

x 2147483646 x1 2147483647 x2 -2147483648
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Why Number Ranges?

• When you declare a variable, each variable is
  allocated a specific amount of memory.
• For example, an integer is allocated four units
  of memory.
• Because each variable has a finite amount of
  memory it will have a finite number range.

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Integer Number Ranges

• Within Borland, integers take up four units of
  memory.
• Integers therefore have a specific range of
  -2,147,483,648 to 2,147,483,647
• When you go beyond the upper limit, it wraps
  around to the beginning.
• Similar to a car odometer that has gone past its
  limit.

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Platform Issues

• Number ranges vary from operating system to
  operating system.
• For example, an integer range on an IBM mainframe
  will be much bigger than a regular PC.
• This is an important issue as it may prevent your
  program from being portable.

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Data Type Range Table

• Available on the course home page.
• Under the Programs Section
• int, longs, shorts, floats, double, chars.

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char variables

• char variables hold a single character.
• Characters are stored as ASCII integer values
  (see Appendix D, page 891 in the text book.)
• Examples
• C (upper case) is stored as ASCII number 67
• c (lower case) is stored as ASCII number 99

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For example

• include ltstdio.hgt
• include ltconio.hgt
• main ()
• char c
• printf ("Enter a character ")
• scanf ("c", c)
• printf ("You entered c\n", c)
• printf ("You entered d\n", c)
• getch()

Enter a character c You entered c You
entered 99
Note the difference in format specifiers.
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IV. For Loops

• Indefinite Loop
• You do not know ahead of time how many times your
  loop will execute.
• For example, you do not know how many books a
  person might order.
•  Definite Loop
• You know exactly how many times you want the loop
  to execute.

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Basic For Loop Syntax

• For loops are good for creating definite loops.
• int counter
• for (counter 1counter lt 10counter)
• printf ("d\n", counter)

1. Priming Set the start value.
2. Test Condition Set the stop value.
3. Update Update the interval.
Note that each section is separated by a
semicolon.
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For Loop Variations

• Increment may be negative
• for (i100 igt1 i--)
• This counts from 100 to 1.
• Increment may be greater than 1
• for (i100 igt5 i-5)
• This counts from 100 to 5 in steps of 5

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Nested For Loops

• It is also possible to place
• a for loop inside another
• for loop.
• int rows, columns
•  
• for (rows1 rowslt5 rows)
• for (columns1 columnslt10 columns) printf
  ("")
• printf ("\n")

Output

Outer Loop
Inner Loop
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Nested For Loops, Example 2

• include ltstdio.hgt
• main ()
• int rows, columns
• for (rows1 rowslt5 rows)
• for (columns1 columnsltrows columns)
• printf ("")
• printf ("\n")
• getchar()

Output
Outer Loop
Inner Loop