CIS162AB - C

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CIS162AB - C

• Flow Control if, while, do-while
• Juan Marquez(03_flow_control.ppt)

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Overview of Topics

• Pseudocode
• Control Structures
• Flowcharts
• Single and Compound Boolean Expressions
• Single and Compound Statements
• If, if-else, nested ifs
• While, do-while, nested loops

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Pseudocode

• Pseudocode is a mixture of C code and English
  like statements.
• Used when designing algorithms.
• When designing, we dont necessarily want to be
  concerned about where semi-colons go. We want to
  concentrate on the design.
• Ill use pseudocode throughout the course, so
  dont feel compelled to correct my syntax.
• However, if at anytime you are not sure about a
  command, please be sure to ask for clarification.
  ?

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Flow Control

• The order in which statements are executed.
• There are four control structures.
• Sequence Control Structure
• Selection Control Structure
• Also referred to as branching (if and if-else)
• Repetition Control Structure (loops)
• Case Control Structure (switch)

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Flowcharting

• A flowchart is a pictorial representation of an
  algorithm or logical steps.
• Each step is represented by a symbol and the
  arrows indicate the flow and order of the steps.
• The shape of the symbol indicates the type of
  operation that is to occur.
• Flowcharts may help the move visual students
  learn and understand logic.

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Flowchart Symbols
Input or Output
Begin or End
Decision
Processing
Branch or Direction of Flow
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1. Sequence Control Structure

• The order statements are placed (sequenced)
• cin gtgt price gtgt qty
• subtotal price qty
• cout ltlt subtotal
• The only way to display the subtotal, statements
  must be in this order.

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Flowchart Sequence Control
Begin
Input price, qty
subtotal price qty
Output subtotal
End
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2. Selection Control ( if )

• Simple if with single statement.
• discountRate 1.00
• if (qty gt 25)
• discountRate .98
• subtotal qty (price discountRate)

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Flowchart if statement
discountRate 1.00
If qty gt 25
True
discountRate .98
False
Subtotal qty (price discountRate)
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Selection Control (if-else)

• Simple if-else with single statements.
• DISCOUNT_RATE .98
• if (qty gt 25)
• subtotal qty (price DISCOUNT_RATE)
• else
• subtotal qty price
• cout ltlt subtotal

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Flowchart if-else statement
DISCOUNT_RATE .98
If qty gt 25
Subtotal qty (price DISCOUNT_RATE)
True
False
Subtotal qty price
Output subtotal
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Boolean Expressions

• Boolean expressions evaluate to true or false.
• Must be enclosed in parenthesis
• if (Boolean expression)
• true statements (Yes)
• else
• false statements (No)

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Compound Statements

• Use braces to create a block of statements.
• if (hours gt 40)
• regularPay 40 payRate
• overtimePay (hours 40)
  (payRate OVERTIME_RATE)
• else
• regularPay hours payRate
• overtimePay 0
• grossPay regularPay overtimePay

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Incorrect if-else

• if (hours gt 40)
• regularPay 40 payRate
• overtimePay (hours 40)
  (payRate OVERTIME_RATE)
• else
• regularPay hours payRate
• overtimePay 0
• grossPay regularPay overtimePay
• overtimePay would always be set to zero.

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Boolean Expressions

• Simple Expressions
• Have one comparison
• (hours gt 40)
• Compound Expressions
• Have more than one comparison
• Created by using And and Or operators
• And - (qty gt 0 qty lt 51)
• Or - (qty lt 0 qty gt 51)

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And Operator -

• Both conditions must be true.
• if (qty gt 0 qty lt 51)
• subtotal qty price
• else
• cout ltlt Quantity must be 1 50.
• What happens if qty 0, qty 25, or qty 60?
• A block with a single statement is valid.

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Or Operator -

• Either condition must be true.
• if (qty lt 1 qty gt 50)
• cout ltlt Quantity must be 1 -50.
• else
• subtotal qty price
• What happens if qty 0, qty 25, or qty 60?

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Pipe Character -

• Where is the pipe character on the keyboard?
• On most keyboards
• It is right above the Enter key
• Shares the key with the back slash - \
• Must hold the shift key go get it
• Instead of a solid line, it is shown as a broken
  line
• For the Or operator, 2 pipe characters must be
  entered - .
• For the And operator, 2 ampersands characters
  must be entered - .

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Notes on Boolean Expressions

• Short-circuit evaluation if the evaluation of
  the entire expression can be determined by the
  result of the first expression, the second
  expression is not evaluated.int y 1, x 0if
  (y gt 0 x gt 0) // T or F
• Not operator (!)( y ! 0) //This statement
  is OK, but the not( !(y 0)) //outside the
  parentheses can be confusing.

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Booleans Only True or False

• C does the math to get to a True or False.
• true 1, false 0
• Non-zeros converted to true, which is a one.
•   if ( (5 7) (!6) )
• T T (!T)
• T F
• 1 0 1 T

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Nested if Statements - Indented

• if (qty gt 0)
• if (qty lt 51)
• subtotal qty price
• else
• cout ltlt Quantity must be lt 51
• else
• cout ltlt Quantity must be gt 0

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Matching else to if

• How are else statements matched with an if?
• Compiler works its way back. When an else is
  encounter, it looks back to find an if that has
  not been matched to an else.
• Why do we indent each level?
• We indent to make programs easier to read.
  Indenting has no effect on how compiler matches
  an else to an if.

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Multiway BranchingIndenting not always practical

• if (qty lt 26)
• discount 0
• else
• if (qty lt 51)
• discount .01
• else
• if (qty lt 76)
• discount .02
• else
• if (qty lt 101)
• discount .03 more

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Multiway Branching

• if (qty lt 26)
• discount 0
• else if (qty lt 51)
• discount .01
• else if (qty lt 76)
• discount .02
• else if (qty lt 101)
• discount .03
• else
• discount .04

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Conditional Operator Expression

• if (qty gt 25)
• discountRate 0.98
• else
• discountRate 1.00
• discountRate (qty gt 25) ? .98 1.00

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Assignment () vs Comparison ()

• if (x 12) // always true
• if (x 12) // comparison
• To prevent accidental assignment, state constant
  first.
• if (12 x)
• Compiler will report this as a syntax error.

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3. Repetition Control (loops)

• loop a group of statements that are repeated
  until a certain conditions occurs to stop it.
• while loop
• do-while loop
• for-loop (covered later)

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While Loop Example

• count 3 //initialize controlling variable
• while (count gt 0)
• cout ltlt count
• count--
• Output
• 3 2 1 

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While Loop

• Controlling Boolean expression evaluated before
  executing loop body.
• Controlling variable must be initialized.
• Boolean expression must be true to enter loop
  body.
• No semicolon after Boolean expression.
• It is possible that body is not executed at all.
• Boolean expression must be false to get out of
  loop.
• Controlling variable must be modified inside
  loop.
• Execution continues with next statement after
  loop.

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Flowchart While Loop
Initialization important for While Loop
count 3
while count gt 0
False
True
Output count
Skip or Exit Loop
Represents Loop
count--
Next statement
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Do-while Loop Example

• count 3
• do
• cout ltlt count
• count--
• while (count gt 0)
• Output
• 3 2 1

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Do-while Loop

• Controlling Boolean expression evaluated after
  executing body.
• So, body is always executed at least one time.
• Initialization of controlling variable not
  necessarily required if it will be initialized
  inside the loop.
• Boolean expression must be false to get out of
  loop.
• Controlling variable must be modified inside
  loop.
• Semicolon required after Boolean expression.
• Execution continues with next statement after
  loop.

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Flowchart Do-while Loop
count 3
Output count
Loop will be executed at least one time, because
the condition is at the bottom.
count--
Represents Loop
while count gt 0
True
False - Exit Loop
Next statement
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Loop Summary

• Incrementor count
• Decrementor count--
• Infinite loop expression that always evaluates
  to true
• Avoid using as the operator.
• Controlling variable must be altered within the
  loop.
• Use Control-C to terminate an infinite loop.
• When to use a while or do-while will become
  evident as we continue to use and learn each
  loop.

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Nested Loops

• count 3
• while (count gt 0)
• cout ltlt endl ltlt count
• count--
• count2 1
• do
• cout ltlt count2
• count2
• while (count2 lt 4)

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1 2 3
2
1 2 3
1
1 2 3
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While Loop Error

• Semi-colon after expression creates an infinite
  loop with a single statement that does nothing.
• DO NOT DO THIS.
• while (count gt 0)
• cout ltlt count
• count--

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4. Case Control Structure

• Switch statement covered later.

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Summary

• Control Structures
• Flowcharts
• Single and Compound Boolean Expressions
• Single and Compound Statements
• If, if-else, and nested ifs
• While, do-while, and nested loops