1 what is a loop? a loop is a repetition control structure that causes a single statement or block...

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What is a loop?

A loop is a repetition control structure that causes a single statement or block to be executed repeatedly

Loops

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Two Types of Loops

1. Count controlled loopsRepeat a statement or block a specified number of times

2. Event-controlled loopsRepeat a statement or block until a condition within the loop body changes that causes the repetition to stop

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While StatementSYNTAX

while (Expression)

{ .

. // loop body

.

}

Loop body can be a single statement, a null statement, or a block

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When the expression is tested and found to be false, the loop is exited and control passes to the statement that follows the loop body

WHILE LOOP

FALSE

TRUE

bodystatement

Expression

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Count-controlled loops contain An initialization of the loop control

variable An expression to test if the proper

number of repetitions has been completed

An update of the loop control variable to be executed with each iteration of the body

1. Count-Controlled Loops

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Count-controlled Loop Example

int count;

count = 4;

while(count > 0)

{ cout << count << endl;

count --;}cout << “Done” << endl;

OUTPUT

count

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Count-controlled Loopint count;

count = 4;

while(count > 0)


count --;}

cout << “Done” << endl;

OUTPUT

count

4

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count = 4;

while(count > 0) TRUE{ cout << count << endl;


OUTPUT

count

4

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count = 4;

while(count > 0)



OUTPUT

4

count

4

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count = 4;

while(count > 0){ cout << count << endl;

count --;

}cout << “Done” << endl;

OUTPUT

4

count

3

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count = 4;

while(count > 0) TRUE



OUTPUT

4

count

3

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count = 4;

while(count > 0)



OUTPUT

4 3

count

3

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count = 4;

while(count > 0)



OUTPUT

4 3

count

2

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count = 4;

while(count > 0) TRUE{ cout << count << endl;


OUTPUT

4 3

count

2

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count = 4;

while(count > 0)

{

cout << count << endl;


OUTPUT

4 3 2

count

2

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count = 4;

while(count > 0)



OUTPUT

4 3 2

count

1

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count = 4;

while(count > 0) TRUE



OUTPUT

4 3 2

count

1

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count = 4;

while(count > 0)

{

cout << count << endl;


OUTPUT

4 3 2 1

count

1

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count = 4;


count --;

}cout << “Done” << endl;

OUTPUT

4 3 2 1

count

0

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count = 4;

while(count > 0) FALSE{ cout << count << endl;


OUTPUT

4 3 2 1

count

0

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count = 4;


count --;}

cout << “Done” << endl;

OUTPUT

4 3 2 1 Done

count

0

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2. Types of Event-Controlled Loops

1. Sentinel controlled

Keep processing data until a special value that is not a possible data value is entered to indicate that processing should stop

2. End-of-file controlled

Keep processing data as long as there is more data in the file

3. Flag controlled

Keep processing data until the value of a flag changes in the loop body

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A Sentinel-controlled Loop

Requires a “priming read”

A priming read is the reading of one set of data before the loop to initialize the variables in the expression

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// Sentinel controlled loop

total = 0;

cout << “Enter a blood pressure(-1 to stop) ”;

cin >> thisBP; // Priming read

while(thisBP != -1) // While not sentinel

{

total = total + thisBP;

cout << “Enter a blood pressure(-1 to stop)”;

cin >> thisBP;

}

cout << total;

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End-of-File Controlled Loop

Uses the fact that a file goes into the fail state when you try to read a data value beyond the end of the file to control the loop

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total = 0;

myInfile >> thisBP; // Priming read

while(myInfile) // While last read successful

{ total = total + thisBP; myInfile >> thisBP; // Read another}

cout << total;

// End-of-file controlled loop

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int count;char previous;char current;

count = 0;inFile.get(previous); // Priming readsinFile.get(current);

while(inFile){ if((current == ‘=‘) && (previous == ‘!’))

count++; previous = current; // Update

inFile.get(current); // Read another}

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initialize outer loop

while (outer loop condition)

{ . . .

initialize inner loop

while(inner loop condition)

{

inner loop processing and update

}

. . .}

Nested Loops

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Designing Nested Loops

Analyze the algorithm.

Begin with outer loop

Functional Decomposition When you get to where the inner loop appears, make it a separate module and we can come back to its design later

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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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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’));}

Example of 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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For Loop

SYNTAX

for (initialization; test expression; update) {

Zero or more statements to repeat

}

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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

int num;

for(num = 1; num <= 3; num++) cout << num << “Potato” << endl;

num

OUTPUT

1

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OUTPUT

1

int num;

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

cout << num << “Potato” << endl;

true

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int num;

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


OUTPUT

1

1Potato

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OUTPUT

2

int num;



1Potato

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OUTPUT

2

true

1Potato

int num;



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int num;



OUTPUT

2

1Potato

2Potato

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OUTPUT

3

int num;



1Potato

2Potato

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OUTPUT

3

true

1Potato

2Potato

int num;



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int num;



OUTPUT

3

1Potato

2Potato

3Potato

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OUTPUT

4

int num;



1Potato

2Potato

3Potato

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OUTPUT

4

false

1Potato

2Potato

3Potato

int num;



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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;



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int counter;

for (counter= 0; counter< 5; counter++) cout << “HELLO”; cout << “counter”<< counter;

OUTPUT

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1 what is a loop? a loop is a repetition control structure that causes a single statement or block...

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