Sentinel Loops

8/13/2019 Sentinel Loops

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Sentinel Controlled Loops

Say that a program is to add up a list ofnumbers entered from the keyboard. Will aloop be used in this program?

Answer..yes: A counting loop could dothis if the user first said how many integerswere in the list. But often users don't know

this in advance so, what do we do?


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The idea of a sentinel controlled loopis thatthere is a special value (the sentinel) that is usedto say when the loop is done. In this example,

the user enters a zero when the sum iscomplete. Zero is the sentinel. Here is how theprogram should work:

C:\Enter first integer (enter 0 to quit): 1Enter an integer (or 0 to quit): -3

Enter an integer (or 0 to quit): 4

Enter an integer (or 0 to quit): 0

Sum of the integers: 2


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The flow chart


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Partially completed programimport java.util.Scanner;

class AddUpNumbers

{

public static void main (String[] args )

{

Scanner scan = new Scanner( System.in );

int value; // data entered by the user

int sum = 0; // initialize the sum// get the first value

System.out.print( "Enter first integer (enter 0 to quit): " );

value = scan.nextInt();

while ( value != 0 )

{

------------------------//add value to sum

;

//get the ext value from the user

-------------------- ;

}

System.out.println( "Sum of the integers: " + sum );} }


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System.out.print( "Enter first integer (enter 0 to quit): " );


while ( value != 0 )

{

//add value to sum

sum = sum + value;

//get the next value from the user

System.out.println( "Enter next integer (enter 0 to quit):" );


}

System.out.println( "Sum of the integers: " + sum );

}

} Questions: What would happen if the user entered 0 first time?


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What if we wanted to modify the program so that it promptsthe user with the following:

Enter the 2nd integer (enter 0 to quit):

Enter the 3rd integer (enter 0 to quit):

Enter the 4th integer (enter 0 to quit):

Enter the 5th integer (enter 0 to quit):

Enter the 6th integer (enter 0 to quit): .... and so on ....

Enter the 20th integer (enter 0 to quit): Enter the 21thinteger (enter 0 to quit): The prompt shows poorgrammar for integers 21, 22, 23, 31, 32, 33 and so on.Let us regard this as acceptable.


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Just to remind you.


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So .what do we need to do

Change the output line

System.out.println( "Enter the " + (count+1)+ suffix+ " integer (enter 0 to quit):" );


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We have three choices

To make a three-way choice, a nested if isused. This is where an if-else statement ispart of a the true-branch (or false-branch)

of another if-else statement. So the nestedif-else will execute only when the outer if-else has already made a choice between

two branches. Here is a program fragmentthat makes a choice of one of the threesuffixes.


10/22

Program fragment

String suffix;

int count=0; // count is the number of integers added so far.

// count+1 is the next integer to read

// While goes here

. . . .

if ( count+1 __________ )

suffix = "nd";

else

if ( count+1 ___________) // false-branch of first if

suffix = "rd"; // false-branch of first if

else // false-branch of first if

suffix = "th"; // false-branch of first if

System.out.println( "Enter the " +

(count+1) + suffix + " integer (enter 0 to quit):" );


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Matching if's and else's

Start with the first ifand work downward.Each elsematches the closest previous

unmatched if. An if matches only one elseand an elsematches only one if.


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Here is another example. It is not indentedproperly. Use the rule to figure out which

ifs and ifs match.

if ( a == b )

if ( d == e )

total = 0;

else

total = total + a;

else

total = total + b;


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Make Matches clear with Braces

In this case, the braces are not reallyneeded, but are probably a good idea, forclarity:

if ( a == b )

{

if ( d == e )

total = 0;

else

total = total + b;

}


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New Example

Sometimes the user is asked explicitly if theloop should continue. The user enters

"yes" or "no" (or maybe "y" or "n"). Nowthe sentinelis of type String or char. Thenext example illustrates this: Say that you

are interested in the value of thepolynomial:


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7x3- 3x2+ 4x - 12

or various values of x. The value x is adouble precision value. For example,when x == 2.0 the polynomial is equal to:

7*23- 3*22+ 4*2 - 12 = 7*8 - 3*4 + 8 - 12 = 40

The program lets the user enter various values forx and see the result for each one.


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Testing the User's Responseclass EvalPoly

{public static void main (String[] args )

{

double x; // a value to use with the polynomial

String response = "y"; // "y" or "n

while ( response.equals("y") ){

// Get a value for x.

// Evaluate the polynomial.

// Print out the result.

// Ask the user if the program should continue.// The user's answer is "response".

} } }


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The condition part of the while statement,response.equals("y") evaluates to trueor false. Here ishow this happens:

response is a reference to a String object.

A String object has both data and methods (as do all objects).

The data part of response is the characters the user types.

response has an equals() method that tests if another string isequal to it.

response.equals("y") tests if the String response is equal to theString "y".

The result of the test is trueor false.


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So, what all do I need

As with all loops, there were three thingsto get right:

Initializing the loop. Testing if the loop should continue.

Setting up for the next test.


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import java.io.*;

class EvalPoly

{

public static void main (String[] args )

{

Scanner scan = new Scanner ( System.in );double x; // a value to use with the polynomial

double result; // result of evaluation

String response = "y"; // "y" or "n"

while ( response.equals( "y" ) )

{

// Get a value for x.

System.out.println("Enter a value for x:") ;

x = scan _______________;

// Evaluate the polynomial.

result = (whats the formula)__________;

// Print out the result.

System.out.println _______________________________;

// Ask the user if the program should continue.

System.out.println("continue (y or n)?");

response = scan.nextLine();

}

}}


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Questions: match the ifs.If ( val == 34 )

{

sum = sum + val;

val = 0;

next= 5;

}

else{

sum = sum - val;

if ( sum < limit )

flag = true;

else

flag = false;

}


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IF ( val == 34 )

{

if ( sum < limit )

sum = sum + val;

if ( val >= limit )

val = sum - limit;

else

val = 0;

}

elseif ( val = -12 )

sum = 0;

else

limit = 88;


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A mail order company charges $3.00 for handling, freeshipping for orders 10 pounds or less, plus $0.25 foreach pound over 10. Write a program that repeatedly

asks the user for the weight of an order, then writes outthe shipping charge. The program stops when a weightof zero or less is entered.

Weight of Order: 5Shipping Cost: $3.00

Weight of Order 20

Shipping Cost: $5.50

Weight of Order 0

bye

Sentinel Loops

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