Chapter 8 - Repetition Statements

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Chapter 8Repetition Statements
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Introduction

• Iteration - process of looping or the repetition
  of one or more statements
• Loop body - the statement, or statements, that
  will be repeated

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8.1 General Repetition Concepts

• Two different categories of loops
• Pre-test Test condition first - complete the
  body of the loop if condition is true
• Post-test Test condition after body has executed
  once - body will only be repeated if the
  condition is true

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8.1 General Repetition Concepts

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8.1 General Repetition Concepts

• Looping structures must have the following
• Variable(s) with an initial value(s) to control
  whether the body of the loop is executed (i.e.,
  control variable)
• Conditional expression involving control
  variable(s)
• Statement within the body where control variable
  is modified each time the body of the loop is
  executed

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8.1 General Repetition Concepts

• Infinite loop a loop that continuously executes
  - the program or loop has to be terminated by the
  user or programmer
• Usually an error situation
• Most common cause - failure to manipulate the
  control variable
• In Windows/Unix - pressing Ctrl C causes
  program to stop execution

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8.1 General Repetition Concepts

• Nested loop a loop embedded in another loop
• Almost any statement type can be the loop body
  including loops or conditional statements
• Once inner loop finishes, flow transfers to the
  next statement following the inner loop

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8.2 The while Loop

• Pre-test loop
• Syntax
• while ( ltconditiongt )
• ltaction/bodygt
• Action, or body, will continue to execute while
  the condition remains true
• If the body needs to include multiple statements,
  surround them (action) with curly braces

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8.2 The while Loop

• Sometimes see a loop written as while ( 1 )
  ...
• The numeric literal 1 is treated as a true
  condition and never changes causing an infinite
  loop

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8.2 The while Loop

char again N' cout ltlt "\nDo you wish to
multiply two numbers (y/n)? " cin gtgt again //
Priming read //- Read two numbers as long as
usery says so. while ( again 'y' again
'Y' ) // Notice no semicolon cout ltlt
"Enter first number " cin gtgt operand1
cout ltlt "Enter second number " cin gtgt
operand2 cout ltlt "Result " ltlt operand1 ltlt
" " ltlt operand2 ltlt " " ltlt operand1
operand2 ltlt endl // Dont forget to change
the control variable cout ltlt "\nDo you wish
to multiply two more numbers (y/n)? " cin
gtgt again // End of the loop body
(action) cout ltlt "The End" ltlt endl
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8.2 The while Loop

• Priming read - a read before loop is reached
• Initializes a variable that can be used to
  control the loop.
• Initializes a variable being used as an
  accumulator
• General Concept initialization
• To provide a correct answer when the loop does
  not execute its body
• Examples sum 0 min first input value, etc.

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8.3 The do-while Loop

• Post-test loop (the body executes at least once)
• Syntax
• do
• ltbody/actiongt
• while ( ltconditiongt )
• Note semicolon after ()!!
• If the body needs to include multiple statements,
  surround them with curly braces

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8.3 The do-while Loop

char menu_choice float number cout ltlt "Please
enter a number " cin gtgt number do cout ltlt
"\n1) Square the Number\n" ltlt "2) Cube
the Number\n" ltlt "3) Exit\n\n"
ltlt "Please enter menu choice " ltlt endl cin
gtgt menu_choice switch ( menu_choice )
case '1' cout ltlt number ltlt "
Squared " ltlt number number
ltlt endl break case '2'
cout ltlt number ltlt " Cubed "
ltlt number number number ltlt endl
break case '3' cout
ltlt "Goodbye" ltlt endl break
default cout ltlt "Invalid menu
option" ltlt endl while ( menu_choice !
'3' ) // Notice the semicolon
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8.4 The for Loop

• Generally used when
• a specific number of iterations is required
• The control of the repetition is based on a
  sequence of values (e.g., 1,2,3,,7 odd numbers
  5,4,3,2,1.
• Both while loops and for loops are pre-test
  loops - could be used interchangeably

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8.4 The for Loop

• Syntax (Red for loop specification)
• for ( ltinitial_value_assignmentgt
• ltcontinuation_conditiongt
• ltcontrol_variable_updategt )
• ltactiongt
• Example for (k4 k lt 10 k)
• cin gtgt x

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8.4 The for Loop

for ( int i 0 i lt 5 i ) cout ltlt i ltlt '
' // Output 0 1 2 3 4
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8.4 The for Loop

• for ( ltinitgt
• ltcontinue_conditiongt
• ltcontrol_var_updategt )
• ltbody/actiongt
• Four sections
• Loop specification components separated by
• ltinitgt and ltupdategt components can include
  multiple statements separated by commas
• body can be any executable statement.

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8.4 The for Loop

• Order in which the parts are executed
• ltinitgt
• ltcontinuation_conditiongt
• ltbody/actiongt (if true condition)
• ltupdategt
• ltconditiongt
• ...
• The ltinitgt section is only executed once

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8.4 The for Loop

• Variable(s) may be declared in ltinitgt section
• Scope is limited to the loop specification and
  body
• for (int k1 klt 5 k) cout ltlt k ltlt
• ? output sequence 1 2 3 4

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8.4 The for Loop

// Example 1 for ( int i 0 i lt 5 i ) // No
semicolon cout ltlt i ltlt endl // Example 2 //
Notice the multiple expressions for ( int i 0,
j 5 i lt 5 i, j-- ) cout ltlt i ltlt ' ' ltlt
j ltlt endl // Example Output // Ex. 1 // Ex. 2
0 0 5 1 1 4 2 2 3
3 3 2 4 4 1
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8.4 The for loop

int sum 0, value for ( int i 0 i lt 5 i )
// No semicolon cout ltlt "Enter value " ltlt
i 1 ltlt " " cin gtgt value sum
value cout ltlt "The sum of the five values
is " ltlt sum ltlt endl // Example Output Enter
value 1 4 Enter value 2 7 Enter value 3
3 Enter value 4 12 Enter value 5 99 The sum of
the five values is 125
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8.4.1 Nested for Loops

• for loops can be nested
• Used in many algorithms
• Important when using multi-dimensional arrays

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8.4.1 Nested for Loops

for ( int row 0 row lt 5 row ) for (
int col 0 col lt 5 col ) cout ltlt col
ltlt ' ' cout ltlt endl // Example Output 0 1
2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4
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8.4.1 Nested for Loops

for ( int row 0 row lt 5 row ) for (
int col row col lt 5 col ) cout ltlt
col ltlt ' ' cout ltlt endl // Example
Output 0 1 2 3 4 1 2 3 4 2 3 4 3 4 4