What is a Loop? Pretest Loops in C++ Types of Loop Testing. Count-controlled loops. Loops can be...

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CSC 211 Intermediate Programming

Looping

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What is a Loop?

A loop is a repetition control structure

It causes a single statement or a group of

statements to be executed repeatedly

It uses a condition to control iteration

Loop continues as long as condition is TRUE

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Types of Loop Testing

Begin loop

End loop Statements

in loop

Endless loop

false Test Condition

End loop Statements

in loop true

Pretest loop

Begin loop

Test Condition Statements

in loop

false

true

Posttest loop

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Pretest Loops in C++

One pretest loop in C++ is the while loop

A condition is used to control the loop

The condition is put between parentheses

while (condition)

statement;

while (condition)

{

statement₁

;

statement₂

;

. . .

}

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Loops can be ...

Count controlled

repeat a specified number of times

Event-controlled

some condition within the loop body changes

and this causes the repeating to stop

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Count-controlled loops

They contain:

An initialization of the loop control variable

A condition to test for continuing the loop

An update of the loop control variable to be

executed with each iteration of the body

(2)

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int count;

count = 4; // initialize loop variable while (count > 0) // test condition

{

cout << count << endl; // repeated action count--; // update loop variable }

cout << "Done" << endl;

Count-controlled Loop

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int count;

count = 4;

while (count > 0) {

cout << count << endl;

count--;

}

Output

count

Count-controlled Loop

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int count;

count = 4;

count--;

}

Output

count

4

Count-controlled Loop

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int count;

count = 4;

count--;

}

Output

count

4

Count-controlled Loop

True

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int count;

count = 4;

count--;

}

Output

count

4

Count-controlled Loop

4

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int count;

count = 4;

count--;

}

Output

count

3

Count-controlled Loop

4

(3)

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int count;

count = 4;

count--;

}

Output

count

3

Count-controlled Loop

4 True

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int count;

count = 4;

count--;

}

Output

count

3

Count-controlled Loop

4 3

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int count;

count = 4;

count--;

}

Output

count

2

Count-controlled Loop

4 3

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int count;

count = 4;

count--;

}

Output

count

2

Count-controlled Loop

4 3 True

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int count;

count = 4;

count--;

}

Output

count

2

Count-controlled Loop

4 3 2

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int count;

count = 4;

count--;

}

Output

count

1

Count-controlled Loop

4 3 2

(4)

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int count;

count = 4;

count--;

}

Output

count

1

Count-controlled Loop

4 3 2 True

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int count;

count = 4;

count--;

}

Output

count

1

Count-controlled Loop

4 3 2 1

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int count;

count = 4;

count--;

}

Output

count

Count-controlled Loop

4 3 2 1

0

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int count;

count = 4;

count--;

}

Output

count

0

Count-controlled Loop

4 3 2 1 False

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int count;

count = 4;

count--;

}

Output

count

0

Count-controlled Loop

4 3 2 1 Done

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Event-controlled Loops

Sentinel controlled

keep processing data until a special value which is

not a possible data value is entered to indicate

that processing should stop

End-of-file controlled

keep processing data as long as there is more

data in the file

Flag controlled

keep processing data until the value of a flag

changes in the loop body

(5)

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Examples of Kinds of Loops

Read blood pressures until a

dangerously high BP (200 or

more) is read.

Flag controlled loop

Read all the blood pressures from

a file no matter how many are

there.

End-of-file controlled loop

Read blood pressures until a

special value (like -1) selected by

you is read.

Sentinel controlled loop

Read exactly 100 blood pressures

from the user.

Count controlled loop

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A Sentinel-controlled Loop

Sentinel value – A special value of input

that causes the program to stop a loop

Must be a value that would not occur in

the normal course of operation

Requires a “priming read” if done with a

while loop

“priming read” means you read one set of

data before the while

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Example

int bloodPressure, total = 0;

cout << "Enter a blood pressure (-1 to stop ): ";

cin >> bloodPressure;

while (bloodPressure != -1) // while not sentinel {

total = total + bloodPressure;

cout << "Enter a blood pressure (-1 to stop): ";

}

cout << total << endl;

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End-of-File Controlled Loop

Depends on fact that a file goes into fail

state when you try to read a data value

beyond the end of the file

Several ways to accomplish

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Using the ifstream variable

Uses a priming read

The variable alone is the loop control variable

If the stream has entered a fail state, it has a false, otherwise it has a true

ifstream inFile(“data.txt”);

int x;

inFile >> x;

while(inFile) { //Statements

inFile >> x;

}

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Combining the steps

The priming read can be done as part of the

condition

The logic is still the same as the previous slide

ifstream inFile(“data.txt”);

int x;

while(inFile >> x)

{

//statements

}

(6)

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eof Function

eof function returns a true if the program has read

past the end of the file; otherwise it returns a false

Requires priming read

ifstream inFile(“data.txt”);

int x;

inFile >> x;

while(!inFile.eof())

{ //Statements

inFile >> x;

}

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Assignment

With a partner write two loops that

would read from the same file,

money.dat, and print the contents to

the screen.

The program should loop until the end

of the file is reached.

Each of the two loops should use a

different method

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Flag-controlled Loops

Use a bool type flag variable and

initialize it (to true or false)

Use meaningful name for the flag

A condition in the loop body changes

the value of the flag

Test for the flag in the loop test

condition

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Example

int bloodPressure, countGoodReadings = 0;

bool isSafe = true; // initialize Boolean flag

while (isSafe) {

if (bloodPressure >= 200)

isSafe = false; // change flag value else

countGoodReadings++;

}

cout << countGoodReadings << endl;

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Posttest Loops in C++

Posttest done with the do while statement.

Also uses a condition placed in parentheses.

do

statement;

while (condition);

do

{

statement₁

;

statement₂

;

. . .

} while (condition);

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Example

// display the numbers from 4 to 0 // using the posttest do while loop

int count;

count = 4; // initialize loop variable do

{

cout << count << endl; // repeated action count--; // update loop variable } while (count > 0); // test condition

(7)

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Sentinel-controlled loops

with do while

Does not require a “priming read”

int bloodPressure = 0, total = 0;

do {

total = total + bloodPressure;

cout << "Enter a blood pressure (-1 to stop): ";

} while (bloodPressure != -1); // while not sentinel cout << total << endl;

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while vs. do while

Loop body is always

executed at least once.

Loop body may not be

executed at all.

The loop condition is

tested after executing the

loop body.

The loop condition is

tested before executing

the loop body.

It is a Posttest loop.

It is a Pretest loop.

do while

while

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Accumulating

Loops often used to

Count data values

Sum data values

Keep track of previous and current values

This requires accumulating and counting

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Accumulating (Cont ..)

Accumulating is keeping a running result

Take the value of a variable, modify the value,

and store it back in the original variable

total = total + new;

Any arithmetic operator (+ − * / %) can be part

of an accumulation

**total = total * new;**

Counting is a simple form of accumulation

count = count + 1;

Make sure you INITIALIZE accumulation or

counting variables

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Accumulating (Cont ..)

Accumulation operators

Shorthand for performing accumulation

Same precedence & associativity as assignment

Example Equivalent statement

total += new; total = total + new;

total -= new; total = total - new;

**total = new; total = total new;**

total /= new; total = total / new;

total %= new; total = total % new;

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Example

// calculate the sum of the integers from 1 to 10 // Initialize the counting and accumulating variables int number = 1, sum = 0;

while ( number <= 10) {

sum += number; // accumulating the sum // sum = sum + number;

number += 1; // counting, number = number + 1;

}

cout << "The sum is " << sum << endl;

(8)

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Assignment

Write a C++ program to calculate the

factorial of 10.

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The for loop

A specially designed count-controlled loop

for (initialization; test; updatecounter)

statement;

for (initialization; test; updatecounter)

{

statement₁

;

statement₂

;

. . .

}

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The for loop

Actions of the for spread out around

the loop

initialization occurs only ONCE at the start

testing is the first repeated action of the

loop (it gets done before the body each

iteration)

updatecounter occurs at the end of the loop

body

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int count;

for (count = 1; count <= 3; count++) {

}

cout << "Count = " << count << endl;

Example

Initialization Test Updatecounter

End Body

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int count;

}

Output

count

Example (Cont ..)

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int count;

}

Output

count

Example (Cont ..)

1

(9)

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int count;

}

Output

count

Example (Cont ..)

1 True

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int count;

}

Output

count

Example (Cont ..)

1

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int count;

}

Output

count

Example (Cont ..)

2

1

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int count;

}

Output

count

Example (Cont ..)

2 True

1

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int count;

}

Output

count

Example (Cont ..)

2

1 2

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int count;

}

Output

count

Example (Cont ..)

3

1 2

(10)

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int count;

}

Output

count

Example (Cont ..)

3 True

1 2

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int count;

}

Output

count

Example (Cont ..)

3

1 2 3

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int count;

}

Output

count

Example (Cont ..)

4

1 2 3

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int count;

}

Output

count

Example (Cont ..)

4 False

1 2 3 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 structure.

int count;

}

int count;

}

int count;

}

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int count;

}

Example (Cont ..)

Output

count

4

1 2 3 Count = 4

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Assignment

Write a program to ask the user to

enter 5 integers and then find the

maximum value.

(11)

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What output do you expect from this loop?

int count;

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

**cout << "*";**

What about this one?

int count;

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

**cout << "*";**

Programming Error I

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Programming Error I (Cont ..)

The second loop in the previous slide does

not produce any output. 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 second code segment in the

**previous slide outputs one * after the loop**

completes its counting to 10

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Programming Error II

C++ will accept any simple data type as a

counter, but floating-point counters can

lead to unexpected results

Floating-point values are stored in E-

notation and approximated to decimal

values

So the value 1.0 might be approximated to

1.000000000001 or 0.999999999999

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Programming Error II (Cont ..)

float count;

cout << "Values from 0 to 1 in steps of 0.1\n" << endl;

cout << fixed << setprecision(2);

for (count = 0; count <= 1; count += 0.1) cout << count << " ";

cout << endl << "Final count: " << count << endl;

Possible Output

Values from 0 to 1 in steps of 0.1

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 Final count: 1.00

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Programming Error II (Cont ..)

int count;

cout << "Values from 0 to 1 in steps of 0.1\n" << endl;

cout << fixed << setprecision(2);

for (count = 0; count <= 10; count++) cout << count/10.0 << " ";

cout << endl << "Final count: " << count/10.0 << endl;

Output

Values from 0 to 1 in steps of 0.1

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 Final count: 1.10

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Nested Loops

A loop within another is a nested loop

Nest as deep as you want, but …

An inner loop must be entirely contained within an

outer one

If the loops are counter controlled, each loop must

have a different loop counter variable

Nested loops used often for rows and

columns

Outer loop controls the rows

Inner loop controls the columns

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Example of Nested Loops

Output:

int column, row;

for (row = 1; row <= 4; row++) {

for (column = 1; column <= 5; column++) cout << setw(2) << row * column << " ";

cout << endl;

}

1 2 3 4 5 2 4 6 8 10 3 6 9 12 15 4 8 12 16 20

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Assignment

Write a program to display the following output:

1 1 2

1 2 3

1 2 3 4

1 2 3 4 5

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Validation Loops

Sometimes you may need to check for

validity of input data

The user can enter negative value where it

is supposed to be positive ( year )

The user can enter a value that is outside

the range of possible values (entering 110

for a test score)

This can be done with validation loops

Continue prompting the user for valid value

as long as invalid value is entered

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Example

int score;

cout << "Enter the test score: ";

cin >> score;

// test score should be between 0 and 100 while ( score > 100 || score < 0) {

cout << "Invalid input, enter again: ";

cin >> score;

}

cout << endl << "The test score is " << score << endl;

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Example (Cont ..)

Output

Enter the test score: -18 Invalid input, enter again: 110 Invalid input, enter again: 89