1 Chapter 9 Additional Control Structures Dale/Weems/Headington.

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Chapter 9

Additional Control Structures

Dale/Weems/Headington

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Chapter 9 Topics

Switch Statement for Multi-way Branching Do-While Statement for Looping For Statement for Looping Using break and continue Statements

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switch Statement

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Switch Statement

Is a selection control structure for multi-way branching.

SYNTAXswitch ( IntegralExpression )

{

case Constant1 :

Statement(s); // optional

case Constant2 :

Statement(s); // optional . . .

default : // optional

Statement(s); // optional

}

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float weightInPounds = 165.8 ;char weightUnit ;

. . . // user enters letter for desired weightUnitswitch ( weightUnit ){

case ‘P’ :case ‘p’ :

cout << weightInPounds << “ pounds “ << endl ;break ;

case ‘O’ :case ‘o’ :

cout << 16.0 * weightInPounds << “ ounces “ << endl ;break ;

case ‘K’ :case ‘k’ :

cout << weightInPounds / 2.2 << “ kilos “ << endl ;break ;

case ‘G’ :case ‘g’ :

cout << 454.0 * weightInPounds << “ grams “ << endl ;break ;

default :cout << “That unit is not handled! “ << endl ;break ;

}

Case LabelCase Label

an integral expression operands must be literal or named constants value is compared to switch expression

case 'X' :

switch Observationsswitch Observations

break is required after statement(s)(if omitted all following statements execute)

{ } not required around multiple statements

default clause is a GPP

break after last statement is optional– See switch.cpp

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do-while Statement

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Do-While Statement

Is a looping control structure in which the loop condition is tested after each iteration of the loop.

SYNTAX

do

{

Statement

} while ( Expression ) ;

Loop body statement can be a single statement or a block.

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void GetYesOrNo ( /* out */ char& response )

// Inputs a character from the user

// Postcondition: response has been input // && response == ‘y’ or ‘n’

{do{

cin >> response ; // skips leading whitespace

if ( ( response != ‘y’ ) && ( response != ‘n’ ) ) cout << “Please type y or n : “ ;

} while ( ( response != ‘y’ ) && ( response != ‘n’ ) ) ;}

Function Using Do-While

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Do-While Loop vs. While Loop

POST-TEST loop (exit-condition)

The looping condition is tested after executing the loop body.

Loop body is always executed at least once.

PRE-TEST loop (entry-condition)

The looping condition is tested before executing the loop body.

Loop body may not be executed at all.

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Do-While Loop

When the expression is tested and found to be false, the loop is exited and control passes to the statement that follows the do-while statement.

Statement

Expression

DO

WHILE

FALSE

TRUE

while v. do-while Sentinel while v. do-while Sentinel

// skip to period

datafile >> inputChar;

while (inputChar != '.')

datafile >> inputChar;

// skip to period7

do

datafile >> inputChar;

while (inputChar != '.')

No priming readneeded in this case

(Still needed in many cases)

while v. do-while Sentinel while v. do-while Sentinel

cout << "Enter your age: ";

cin >> age;

while (age <= 0)

{

cout << "Your age must be "

<< "positive." <<endl;

cout << "Enter your age: ";

cin >> age;

}

Requires 2 tests ==>

do

{

cout << "Enter your age: ";

cin >> age;

if (age <=0)

{

cout << "Your age must"

<< " be positive."

<< endl;

}

} while (age <= 0)

while v. do-while CCL while v. do-while CCL

sum = 0;

counter = 1;

while (counter <= n)

{

sum = sum + counter;

counter++;

}

Pretest Loop

sum = 0;

counter = 1;

do

{

sum = sum + counter;

counter++;

} while (counter <= n)

// Note Sum=1 if n=0

Posttest Loop Loop always executes at

least once

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for Statement

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A Count-Controlled Loop

SYNTAX

for ( initialization ; test expression ; update )

{

0 or more statements to repeat

}

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The for loop contains

an initialization

an expression to test for continuing

an update to execute after each iteration of the body

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Example of Repetition

for ( int num = 1 ; num <= 3 ; num++ )

{

cout << num << “Potato” << endl;

}

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Example of Repetition num

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

OUTPUT

?

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Example of Repetition num

OUTPUT

1

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

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Example of Repetition num

OUTPUT

1

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

true

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Example of Repetition num

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

OUTPUT

1

1Potato

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Example of Repetition num

OUTPUT

2

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

1Potato

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Example of Repetition num

OUTPUT

2

true

1Potato

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

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Example of Repetition num

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

OUTPUT

2

1Potato

2Potato

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Example of Repetition num

OUTPUT

3

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

1Potato

2Potato

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Example of Repetition num

OUTPUT

3

true

1Potato

2Potato

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

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Example of Repetition num

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

OUTPUT

3

1Potato

2Potato

3Potato

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Example of Repetition num

OUTPUT

4

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

1Potato

2Potato

3Potato

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Example of Repetition num

OUTPUT

4

false

1Potato

2Potato

3Potato

int num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

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Example of Repetition num

When the loop control condition is evaluated and has value false, theloop is said to be “satisfied” and control passes to the statementfollowing the For statement.

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falseint num;

for ( num = 1 ; num <= 3 ; num++ )

cout << num << “Potato” << endl;

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The output was:

1Potato2Potato3Potato

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for (int count = 4 ; count > 0 ; count-- )

{

cout << count << endl;

}

cout << “Done” << endl;

Count-controlled Loop

OUTPUT: 4321Done

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What is output?

for ( int count = 0 ; count < 10 ; count++ )

{

cout << “”;

}

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OUTPUT

**********

NOTE: the 10 asterisks are all on one line. Why?

Count Control Loop Example

Display integers and their squares from 1 through 10.

for (int i = 1; i <= 10; i++) cout << i << " " << i * i << endl;

For example

Display even integers and their squares from 1 through 10.

for (int i = 2; i <= 10; i = i+2) cout << i << " " << i * i << endl;

For example

Display integers and their squares from 10 down to 1.

for (int i = 10; i >= 1; i--) cout << i << “ “ << i * i << endl;

For example

Find square roots of 1.1, 1.2, 1.3, ..., 2.0

for (double x = 1.1; x <= 2.0; x =x+0.1)

cout << x << " " << sqrt(x) << endl;

Compute and return n! = 1 2 3 ... n.

int product = 1;

for (int i = 2; i <= n; i++)

product = product * i;

For example

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What output from this loop?

for (int count = 0; count < 10; count++) ;

{

cout << “”;

}

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no output from the for loop! Why? the ; right after the ( ) means that the body

statement is a null statement in general, the Body of the for loop is whatever

statement immediately follows the ( ) that statement can be a single statement, a

block, or a null statement actually, the code outputs one * after the loop

completes its counting to 10

OUTPUT

Display all divisors of each integer from 1

through 50

for (int num = 1; num <= 50; num++)

{

cout << num << " has divisors:\n\t'';

for (int div = 1; div <= num/2; div++)

if (num % div == 0)

cout << div << ", '';

cout << num << endl;

} // See divisors.cpp

Table of 2nTable of 2n

const int tableSize = 20;

long valueSquared = 1;

cout << "n" << " " << "2**n" << endl;

for (int n = 0; n <= tableSize; ++n) {

cout << n << " " << valueSquared << endl;valueSquared = valueSquared * 2;

}

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Several Statements in Body Block

const int MONTHS = 12 ;

float bill ;

float sum = 0.0 ;

for (int count = 1; count <= MONTHS; count++ )

{

cout << “Enter bill: “ ;

cin >> bill ;

sum = sum + bill ;

}

cout << “Your total bill is : “ << sum << endl ;

Converting while to forConverting while to for

Any while loop may be written as a for Note InitStmt and PostExpr are optional

while (inputVal != 999)

cin >> inputVal;

for ( ; inputVal != 999; )

cin >> inputVal;

Eliminating WhileExpressionEliminating WhileExpression

The while condition is also optional If omitted the value defaults to true

for ( ; ; )

cout << “Hi” << endl;

while (1)

cout << “Hi” << endl;

For FlexibilityFor Flexibility

cin >> ch;

while (ch != ‘.’)

cin >> ch;

for (cin >> ch; ch != ‘.’; cin >> ch)

;

Warning! Keep it simple. The trickier the code, the harder to follow.

Changing the values of any variables involved in the loop condition inside the body of the loop may change the number of repetitions & may result in an infinite loop

for (i = 1; i <= 10; i++)

{

cout << i << endl;

i++;

}

Monkeying with LCVs: PPPMonkeying with LCVs: PPP

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Break Statement

break statement can be used with Switch or any of the 3 looping structures

it causes an immediate exit from the Switch, While, Do-While, or For statement in which it appears

if the break is inside nested structures, control exits only the innermost structure containing it

Forever LoopForever Loop

loopCount = 1;while (1) // OR for ( ; ; ){ cin >> num1; if (!cin || num1 >= 100) break; cin >> num2; if (!cin || num2 >= 100) break; cout << sqrt(float(num1 + num2)) << endl; loopCount++; if (loopCount > 10) break;}

Use break As a Last ResortUse break As a Last Resort

It can become a crutch Think carefully about loop design for loop on right is better

i = 1;

while (1) for (i = 1; i <= 5; i++)

{ cout << i;

cout << i;

if (i == 5)

break;

i++;

}

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Continue Statement

continue is valid only within loops

terminates the current loop iteration, but not the entire loop

in a For or While, continue causes the rest of the body statement to be skipped--in a For statement, the update is done

in a Do-While, the exit condition is tested, and if true, the next loop iteration is begun

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Imagine using . . .

a character, a length, and a width to draw a box, for example,

using the values ‘&’, 4, and 6 would display

&&&&&&

&&&&&&

&&&&&&

&&&&&&

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Write prototype for void function

called DrawBox ( ) with 3 parameters. The first is type char, the other 2 are type int.

void DrawBox( char, int , int );

NOTE: Some C++ books include identifiers in prototypes. Any valid C++ identifiers, as long as each is different, can be used.

void DrawBox( char letter, int num1, int num2);

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void DrawBox(char what, int down, int across) // 3 function parameters

{

int row, col; // 2 local variables

for ( row = 0; row < down; row++ )

{

for (col = 0; col < across; col++ )

{

cout << what;

}

cout << endl;

}

return;

}

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#include <iostream>

void DrawBox (char, int, int); // prototype

int main ( ) {

char letter = ‘&’;

DrawBox(letter, 4, 2*3); // arguments DrawBox(‘V’, 9, 3); // appear in call

return 0;}

THE DRIVER PROGRAM

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Write a function using prototype

void DisplayTable ( int ) ; // prototype

The function displays a specified multiplication table. For example, the call DisplayTable(6) displays this table:

1 x 6 = 6

2 x 6 = 12

3 x 6 = 18...

12 x 6 = 72