CONTROLLING PROGRAM FLOW

1
CHAPTER 4

• CONTROLLING PROGRAM FLOW

2
The if Statement

• Allow a program to make decisions.
• Example
• if (Temp lt 5)
• cout ltlt Wear a coat today. ltlt endl
• Syntax
• if (condition)
• statement

3
Boolean expression

• An expression that evaluates to either true or
  false.
• Use relational operators
• Operator Meaning
• equal
• lt less than
• lt less than or equal to
• gt greater than
• gt greater than equal to
• ! not equal to

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Examples

• ch lt Z
• Grade 12
• FruitName Banana
• Radius ! 0
• 2piRadius gt Area

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/Temperature program /include
ltiostream.hgtint main( ) double Temp cout
ltlt Enter todays temperature(Celsius) cin
gtgt Temp if (Temp lt 5) cout ltlt Wear a
coat today. ltlt endl cout ltlt Have a gret
day! ltlt endl return (0)
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4.2 if Pitfalls

• Never make equality comparisons ( or !) with
  values that may have fractional parts.
• if (480 4.8 100)
• cout ltlt These are equal!
• Compare values of only the same type
• Example 2 gt 3 (page 7.12)
• vs
• Misplaced semicolon

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4. 3 The if-else Statement

• Can include an else clause when condition is
  false
• if (condition)
• statement
• else
• statement
• Indention is good programming style.

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• if (Temp lt 5)
• cout ltlt Wear a coat today. ltlt endl
• else
• cout ltlt Dont wear a coat today. ltlt endl

9

• Modify the Circle Area program to display an
  error message when the value entered for Radius
  is negative. If Radius is a positive value or
  zero, only the resulting area in a message should
  be displayed. The error message should look
  similar to
• Radius entered was -3.4
• Negative radii are illegal.

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• include ltiostream.hgt
• int main( )
• double Radius
• cout ltlt Enter a radius
• cin gtgt Radius
• if (Radius lt 0)
• cout ltlt Radius entered was ltlt Radius
• cout ltlt Negative radii are illegal. ltlt endl
• else
• cout ltlt Radius entered was ltlt Radius
• cout ltlt Area ltlt (3.14 Radius Radius)
  ltlt endl
• return (0)

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4.5 Nested if Statements

• An if statement that contains another if
  statement
• if (Votes1 Votes2)
• cout ltlt It was a tie! ltlt endl
• else
• if (Votes1 gt Votes 2)
• cout ltlt Candidate1 ltlt is the winner
• else
• cout ltlt Candidate2 ltlt is the winner.

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4.6 The else-if Ladder

• Used to decide among three, four, or more
  actions.
• if (temp lt 0)
• cout ltlt Freezing
• else if (temp lt 13)
• cout ltlt Cool
• else if (temp lt 41)
• cout ltlt Hot
• else
• cout ltlt Very hot

Does not contain an if statement. It is executed
of all other conditions are false. DEFAULT.
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4.8 Logical Operators

• and represented by
• (true) (true) evaluates to true
• (true) (false) evaluates to false
• (false) (true) evaluates to false
• (false) (false) evaluates to false
• if ((Temp lt 5) (Wind gt 24))
• cout ltlt Wear a coat today!

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4.8 Logical Operators

• or represented by
• (true) (true) evaluates to true
• (true) (false) evaluates to true
• (false) (true) evaluates to true
• (false) (false) evaluates to false
• if ((Temp lt 5) (Wind gt 24))
• cout ltlt Wear a coat today!

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4.8 Logical Operators

• not represented by !
• !(true) evaluates to false
• !(false) evaluates to true
• if (!(Temp lt 5))
• cout ltlt DONT wear a coat today!

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4.9 Looping The do-while Statement

• iteration
• Repeat one or more statements during program
  execution.
• Referred to as looping.
• Form

One or more C statements form the body of the
loop
do statement while (condition)
A Boolean expression used to determine if the
loop is to be repeated.
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4.9 Continued

• The do-while loop is executed at least once
  because the condition is not evaluated until
  AFTER the first iteration of the loop.
• If the condition is true, statement is executed
  again and then the condition reevaluated.
• Looping process is repeated until the condition
  is evaluated and found to be false.

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• include ltiostream.gt
• int main()
• char Answer
• cout ltlt Circle Area Calculator ltlt endl
• do
• const double PI 3.14159
• double Radius
• cout ltlt Enter the radius
• cin gtgt Radius
• cout ltlt Area is ltlt (PI Radius Radius)ltlt
  endl
• // Ask if another calculation is desired
• cout ltlt Do another circle (Y/N)?
• cin gtgt Answer
• while (Answer Y)
• cout ltlt Thanks for using the Circle Calculator
  ltlt endl
• Return (0)

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4.11 Looping The while Statement

• Evaluates its condition BEFORE each iteration
• May execute zero or more times.
• Syntax (form) of a while loop body
• while (condition)
• statement

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• include ltiostream.hgt
• int main()
• int number
• cout ltlt Enter a positive number
• cin gtgt number
• while (number lt 0)
• cout ltlt Number must be positive\n
• cout ltlt Please re-enter
• cin gtgt number

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4.16 Looping The for Statement

• Used to execute a loop body a fixed number of
  times.
• Form
• for (initialization, condition, increment)
• statement

Evaluated before each iteration
Performed after each iteration - counter
Performed only once
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4.16 Looping The for Statement

• Example
• The following loop display the numbers 1 through
  10.

for (int i 1 i lt 10 i) cout ltlt I ltlt endl
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4.17 and -

• Decrement (decrease)
• Total--
• Special form of assignment operator indicates the
  value after the operator is to be subtracted from
  the value of the variable on the left of the
  operator.
• Total Total Value
• Total -

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PROGRAMS

• Review 21 and 22
• Exercises
• 14, p. 4-32
• 15, p. 4-32
• 16a, p. 4-33
• 18a, b, c, p. 4-33/34
• 21
• Quiz on Friday (for statements)