1 chapter 9 additional control structures dale/weems/headington
TRANSCRIPT
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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