additional control structures

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Additional Control Structures

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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 To be able to choose the most appropriate

looping statement for a given problem

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Switch Statement

Is a selection control structure for multi-way branching.SYNTAX

switch ( IntegralExpression ){

case Constant1 :Statement 1; // optional

case Constant2 :Statement 2; // optional

. . .

default : // optionalStatement n; // 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 ;

}

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Switch Statement

the value of IntegralExpression (of char, short, int, long or enum type ) determines which branch is executed

case labels are constant ( possibly named ) integral expressions. Several case labels can precede a same statement

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Switch(letter)

{case ‘X’ : Statement1;

break;

case ‘L’ :

case ‘M’: Statement2;

break;

case ‘S’ : Statement3;

break;

default : Statement4;

}

Statement5;

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In this example, letter is the switch expression.

The statement means

If letter is ‘X’,execute Statement1 and continue with Statement5.If letter is ‘L’ or ‘M’, execute Statement2 and continue with Statement5.If letter is ‘S’, execute Statement3 and continue with Statement5.If letter is none of the characters mentioned, execute Statement4 and continue with Statement5.

The Break statement causes an immediate exit from the Switch statement.

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Control in Switch Statement control branches to the statement following the case

label that matches the value of IntegralExpression. Control proceeds through all remaining statements, including the default, unless redirected with break

if no case label matches the value of IntegralExpression, control branches to the default label(if present)——otherwise control passes to the statement following the entire switch statement

forgetting to use break can cause logical errors

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Switch (grade) // Wrong Version

{

case ‘A’ :

case ‘B’ : cout <<“Good Work”;

case ‘C’ : cout <<“Average Work”;

case ‘D’ :

case ‘F’: cout <<“Poor Work”;

numberInTrouble++;

default : cout << grade <<“is not a valid letter grade.”

}

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If grade is ‘A’, the resulting output is this:

Good WorkAverage WorkPoor WorkA is not a valid letter grade.

Remember:

After a branch is taken to a specific case label, control proceeds sequentially until either a Break statement or the end of the Switch statement occurs.

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Do-While StatementIs a looping control structure in which the loop

condition is tested at the end(bottom) of the loop.

SYNTAX

do{

Statement

} while ( Expression ) ;

Loop body statement can be a single statement or a block.Note that the Do-While statement ends with a semicolon.

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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 whilespace

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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Example

While Solution

sum=0;

counter=1;

while(counter<=n)

{

sum=sum+counter;

counter++;

}

Do-While Solution

sum=0;

counter=1;

do

{

sum=sum+counter;

counter++;

} while(counter<=n);

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If n is a positive number, both of these versions

are equivalent.

But if n is 0 or negative, the two loops give

different results.In the While version, the final value of sum is 0 because the

loop body is never entered.

In the Do-While version, the final value of sum is 1 because

the body executes once and then the loop test is made.

Example (Cont.)

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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

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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

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++ )


OUTPUT

?

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OUTPUT

1

int num;

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


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OUTPUT

1

int num;

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


true

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int num;

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


OUTPUT

1

1Potato

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OUTPUT

2

int num;

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


1Potato

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OUTPUT

2

true

1Potato

int num;

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


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int num;

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


OUTPUT

2

1Potato

2Potato

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OUTPUT

3

int num;

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


1Potato

2Potato

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OUTPUT

3

true

1Potato

2Potato

int num;

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


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int num;

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


OUTPUT

3

1Potato

2Potato

3Potato

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OUTPUT

4

int num;

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


1Potato

2Potato

3Potato

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OUTPUT

4

false

1Potato

2Potato

3Potato

int num;

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


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

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falseint num;

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


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The output was:

1Potato2Potato3Potato

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int count ;

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

{

cout << count << endl;

}

cout << “Done” << endl;

Count-controlled Loop

OUTPUT: 4321Done

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What is output?

int count;

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

cout << “”;}

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OUTPUT

**********

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

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What output from this loop?

int count;

for (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

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Several Statements in Body Block

const int MONTHS = 12 ;

int count ;

float bill ;

float sum = 0.0 ;

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

{

cout << “Enter bill: ” ;

cin >> bill ;

sum = sum + bill ;

}

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

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Nested For Loops

For( lastNum=1; lastNum<=7; lastNum++)

{

for(numToPrint=1; numToPrint<=lastNum; numToPrint++)

cout << numToPrint;

cout << endl;

}

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Output

It prints the following triangle of numbers.

1

12

123

1234

12345

123456

1234567

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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

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Continue Statement 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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Be sure to note

the difference between continue and break.

The Continue statement means “Abandon the current

iteration of the loop, and go on to the next iteration.”

The Break statement means “Exit the entire loop

immediately.”

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Guidelines for Choosing a Looping Statement

If the loop is a simple count-controlled loop, the For statement

is natural.

If the loop is an event-controlled loop whose body should

execute at least once, a Do-While statement is appropriate.

If the loop is an event-controlled loop and nothing is

known about the first execution, use a While statement.

When in doubt, use a While statement.

Using Break statement and Continue statement only after

careful consideration.

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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

additional control structures

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