TOPIC 4: REPETITION CONTROL STRUCTURE...2. The loop condition is evaluated. If the loop condition...

LOGO

REPETITION CONTROL

STRUCTURE

CSC 128: FUNDAMENTALS OF COMPUTER PROBLEM SOLVING

1

LOGOContents

Introduction 1

for loop2

while loop3

do … while loop4

2

LOGOIntroduction

It is used when a statement or a block of

statements need to be executed several

times.

Programmers use the repetition structures,

referred to more simply as a loop, when they

need the computer to repeatedly process one

or more program instructions until some

condition is met, at which time the repetition

structures end.

Repetition is also known as iteration or loop.

3

LOGOIntroduction

Two types of loop:

Pretest loop• Evaluation occurs before the instructions within the

loop are processed

• Instruction may never be processed

•while statement, for statement

Posttest loop• Evaluation occurs after the instructions within the loop

are processed

• Instructions will be processed at least once

•do..while statement

4

LOGOIntroduction

Repeat sequence of instruction many

times (as long as condition is TRUE )

Repetition = iteration = loop

Similar function, different names

Example

You have to create a simple program that will

receive 5 integer inputs from user. Sum those

5 inputs and calculate the average.

Check out the C++ code with and without

looping

5

LOGOIntroduction

A program that will receive 5 integer inputs

from user and sum those 5 inputs and

calculate the average./*

Problem: Sum 5 numbers, calculate the average,

display the results

*/

int num1, num2, num3, num4, num5;

int sum;

float average;

cin>>num1>>num2>>num3>>num4>>num5;

sum = num1 + num2 + num3 + num4 + num5;

average = sum / 5;

cout<<"Sum: "<<sum<<"\tAverage: "<<average;

Is this what

you are

thinking???

6

LOGOIntroduction

The correct way by using repetition

int num, sum = 0, count = 0;

float average;

while(count < 5)

{

cout<<"Num "<<(count + 1)<<": ";

cin>>num;

sum = sum + num;

count = count + 1;

}

average = sum / 5;

cout<<"Sum: "<<sum<<"\tAverage: "<<average;

Solution

using

looping!!!

7

LOGOTypes of Repetition Statement

Types of repetition

for

while

do…while

8

LOGOTypes of Repetition Structure

Types of repetition structure

Counter-controlled repetition• Number of repetition is fixed (known in advance)

• e.g. repeat a process 10 times

Sentinel-controlled repetition• Stop looping whenever a special value is entered

• e.g. enter -1 to end the loop

Flag-controlled repetition• When a defined value is matched, it stops looping, else loop

will continue

• e.g. looping will continue 1000 times, however it will stop immediately if the value entered by user matches with ID = 102

9

LOGORequirements of Repetition

/* example using while loop */

int i = 0; //initialize

while(i < 10)

{

statements;

i++;

}Loop Body

Loop Control

Variable

(LCV)

Loop

Condition

10


/* example using for loop */

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

{

statements;

}

Loop Body

Loop Control

Variable

(LCV)

Loop

Condition

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12

Loop Control Variable (LCV)

A variable whose value determines whether

the loop body will be executed or not

It has initial value and increment/decrement

value

Loop Condition

If the condition is true, the loop body is

executed; if condition is false the loop exits

Loop Body

A block of statements to be repeated


Execution of loop body is controlled by 3

operations:

Initialization of the LCV

Evaluation of LCV in the loop condition

Update of the LCV by incrementing (e.g. x++) or decrementing (e.g. x--)

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Increment operator

14

Common

statement

Equivalent to

x++ x = x + 1 x += 1

- x = x + 3 ?

- ? x += 5

- x = x + i ?

Table 4.1: Increment operator


Decrement operator

15

Common

statement

Equivalent to

x-- x = x - 1 x -= 1

- x = x - 9 ?

- ? x -= 2

- ? x -= sum

Table 4.2: Decrement operator

LOGOIntroduction – example

Let say, you want to display “ I love C++”5 times.

I love C++

I love C++

I love C++

I love C++

I love C++

void main()

{

cout << “ I love c++!\n”;





}

16

LOGO

The idea of using a loop…

Introduction – example

Pseudocode

Begin

Repeat

output “I love C++”End

Flow Chart

I love C++

QUESTION:HOW DO WE STOP THE LOOP???

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LOGO

Adding a loop control variable

Introduction – example

Pseudocode

Begin counter = 1

Repeat (if counter <= 5)

output “I love C++”counter ++

End

Variable counter is LCV

Flow Chart

I love C++

counter = 1

counter <=

5

counter ++

T

F

18

LOGOIntroduction – loop control variable

A LCV controls the number of times the

statement or the block of statements is

being executed.

Pseudocode

Begin

counter = 1

Repeat (if counter <= 5)

output “I love C++”counter ++

End

Flow Chart

I love C++

Counter = 1

Counter

<= 5

Counter ++

T

F

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LOGOIntroduction

Requirement of a repetition structure

Flow Chart

I love C++

Counter = 1

Counter

<= 5

Counter ++

T

F

Initialize

LCV

Evaluate

LCV (loop condition)

Loop body

Update

LCV20

LOGOIntroduction

Task associated with loop:

Counter – to determine number of items.

• Is done by adding a constant, such as 1 or 2, to

the value of a variable.

Accumulator – to find totals.

• Is done by adding a variable to another variable.

counter = counter + 1

sum = sum + variable

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LOGOIntroduction

Indicator – value use to end the loop

Counter

control

Indicators

Sentinel

control

While(counter<=5)

Counter control

sentinel=999;

While(number!=sentinel)

sentinel control22

LOGO

The for loop

23

LOGOfor loop

Also called as a counted or indexed for

loop

The general form of the for statement is:

The initial statement, loop condition, and update statement are called for loop

control statements

Items in square brackets ([ ]) are optional.

for ([initial statement]; loop condition; [update statement])

statement;

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LOGOfor loop

The for loop executes as follows:

1. The initial statement executes.

2. The loop condition is evaluated. If the loop condition

evaluates to true

i. Execute the for loop statement.

ii. Execute the update statement (the third expression

in the parentheses).

3. Repeat Step 2 until the loop condition evaluates to false.

The initial statement usually initializes a variable.

In C++, for is a reserved word.

for ([initial statement]; loop condition; [update statement])

statement;

25

LOGOfor loop – Example 1

Example : Displaying the numbers 1

through 3

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

cout << count << endl;

initialization condition update

Result:

1

2

3

26


Using for loop to display ‘Welcome to

C++’.

Pseudocode:

Start

For( set i to 1; i less than or equal to 3; add 1 to i)

display “welcome to C++”Endfor

End

27


Flowchart Start

i = 0

i <= 10

Display “welcome

to C++”

i ++

End

T

F

28


29


Example: to create a program to display

backward the first 10 non negative

number.

30

LOGOfor loop – Exercises

Exercise 1: create a program that display

the first 10 positive odd integers.

31


Exercise 1 - answer

32


Exercise 2: how many time the following

loop processed?

Answer:

for (int count = 6; count < 6; count = count + 1)


33


Exercise 3: how many time the following

loop processed?

Answer:



34


Example: to calculate and display total of

3 numbers

35


Pseudocode:

Start

Initialize total = 0

For(set counter to 1; counter less than or

equal to 3; add 1 to counter)

input number

total = total + number

Endfor

Display total

End

36


Flowchart Start

counter=1, total = 0,

for

counter

<= 3

Input

number

total = total + number

counter = counter + 1

Output

total

End

F

T

37


C++ program segment

total = 0;


{

cin>>number;

total = total + number;

}

cout << “total:” <<total<<endl;

38


Exercise 4: Suppose j, sum, and num are int

variables, and the input values are 26, 34, 61, 4,

and -1. What is the output of the code below?

cout << "Enter a number : ";cin >> num;

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

sum = sum + num;cout << "Enter a number : ";

cin >> num;}cout << sum << endl;

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Exercise 4: answer

40

LOGOfor loop

A semicolon at the end of the forstatement (just before the body of the loop) is a semantic error. In this case, the action of the for loop is empty.

In the for statement, if the loop condition is omitted, it is assumed to be true.

In a for statement, you can omit all three statements—initial statement, loop condition, and update statement. The following is a legal for loop:

41

LOGOfor loop

This is an infinite loop, continuously printing the word Hello

for (;;)

cout << "Hello" << endl;

42

LOGO

The while loop

43

LOGOwhile loop

Repeat or loop as long as the condition is

true.

The general form of the while statement

is:

while is a reserved word.

Statement can be simple or compound

while (expression)

{

statement;

}

44

LOGOwhile loop

Expression acts as a decision maker and

is usually a logical expression

Statement is called the body of the loop

The parentheses are part of the syntax

while (expression)

{

statement;

}

45

LOGOwhile loop

Expression provides an entry condition

statement executes if the expression initially

evaluates to true

loop condition is then reevaluated

statement continues to execute until the

expression is no longer true

while (expression)

{

statement;

}

46

LOGOwhile loop

Infinite loop

continues to execute endlessly

can be avoided by including statements in the

loop body that assure exit condition will eventually be false

47

LOGOwhile loop

The general form of while loop flowchart:

48

LOGOwhile loop

Example: to display the first five positive

integers which increment by five.

49

LOGOwhile loop

Pseudocode:

Begin

Initialize i to 0

While i is less than or equal to 20

Display i

add 5 to I (update)

End while

End

50

LOGOwhile loop

Enter the while statement

i <= 20

Expression evaluates to zero

False condition

Expression evaluates to a nonzero number

True condition

Go back and reevaluate the expression

LoopDisplay

i

i = i + 5

i = 0

Start

End

51

LOGOwhile loop

Example C++ program segment:

i = 0;

while ( i <= 20)

{

cout << i << “ “;i = i + 5;

}

1) Loop Control

Variable (LCV)

2) A starting point / Initialization of the LCV

3) Testing the loop repetition

condition

4) Updating the LCV

52

LOGOwhile loop

Various form of while loops:

Counter controlled

Sentinel controlled

Flag controlled

53

LOGOwhile loop – counter control

If you know exactly how many pieces of data need to be read, the while loop

becomes a counter-controlled loop.

General syntax:

54


Counter controlled while loop includes the following:

◦ Counter

A numeric variable used for counting something

◦ Accumulator

Numeric variable used for accumulating something

◦ Initializing

Assign a beginning value to the counter or

accumulator; typically 0

◦ Updating

Also called incrementing, means adding a number to

the values stored in the counter or accumulator

55


Example

#include <iostream>

#include <conio>

int main()

{

int count;

count = 1;

while (count <= 10)

{


count++;

}

getch();

return 0;

}

initializing

updating

56


Problem: Create a program that displays

the word ‘Hello’ on the screen 10 times.

Solution:

Psedocode

Begin

Initialize lcv to 0

While lcv is less than 10

Display “Hello”Update lcv

End while

End

57


Flowchart

Begin

Initialize

counter = 0

counter < 10 Helloupdate

counter T

F

End

58


Program and output

int main(){

int count;

count = 0;

while (count < 10){

cout << "Hello" << endl;count++;

}

getch();return 0;

}

59


Exercise: Write a C++ statement

associated to the following flowchart.

Begin

Initialize

counter = 10

counter < 100 Display counter

Multiplied by 2

Add 10 to

counter

T

F

End

60


Program and output

int main()

{

int count;

count = 10;

while (count < 100)

{

cout << count * 2 << endl;

count += 10;

}

getch();

return 0;

}

61

LOGOwhile loop – sentinel control

A sentinel-controlled while loop uses a

special value called sentinel to control

the loop.

Sentinel value is a special value that indicates

the end of a set of data or of a process

Sentinel variable is tested in the condition

and loop ends when sentinel is

encountered

62


General syntax :

63


Example #include <iostream>#include <conio>

int main(){

char answer;

cout << "Do you want to quit (Y - yes, N - no) : ";cin >> answer;

while (answer != 'Y'){

cout << "Welcome to the program." << endl;cout << "Do you want to quit (Y - Yes, N - No) : ";cin >> answer;

}cout << "Bye.";

getch();return 0;

}

Sentinel value

64


Output screen

65


Exercise: to create a program that process

the loop as long as user enter an even

number

66


Solution

Flowchart

Begin

End

number % 2 == 0Get another

number

F

Prompt for

a number

Get

a number

T

67


Program

int main(){

int number;

cout << "Enter a number : ";cin >> number;

while (number % 2 == 0){cout << "Enter the next number : ";cin >> number;

}

cout <<"You have entered an odd number to terminate”<<“the program.";

getch();return 0;

}

68


Output

69

LOGOwhile loop – flag control

A flag-controlled while loop uses a

bool variable to control the loop

The flag-controlled while loop takes the

form:

70

LOGOwhile loop – flag control

Example

void main()

{

bool found = false;

char continues;

while (!found)

{

cout << " Program continued..still want to continue"

<< " the loop? Press Y for yes, N for No"<< endl;

cin>>continues;

if(continues == ‘Y’)found = false;

else

found = true;

}

cout << "Program terminated";

getch();

}

71

LOGO

The do … while loop

72

LOGOdo … while loop

The general form of a do...while

statement is:

The statement executes first, and then the

expression is evaluated.

If the expression evaluates to true, the

statement executes again

As long as the expression in a do...while statement is true, the

statement executes

do

statement

while (expression);

73


General form of flowchart:

74


To avoid an infinite loop, the loop body

must contain a statement that makes the expression false

The statement can be simple or

compound. If compound, it must be in

braces

do...while loop has an exit condition

and always iterates at least once (unlike for and while)

75


Example: to display the first five positive

integers which increment by five.

76


Pseudocode:

Begin

Initialize i to 0

do

Display i

add 5 to I (update)

While i is less than or equal to 20

End

77


Flowchart

Enter do statement

i <= 20

Expression evaluates to zero

False condition

Expression evaluates to a nonzero number

True condition

Go back and reevaluate the expression

Loop

Display

i

i = i + 5

i = 0

Start

End

78


C++ program segment

i = 0;

do

{

cout << i << “ “;i = i + 5;

}

while ( i <= 20);

79


Exercise 1

a. The while loop produces nothing.b. The do..while loop outputs the number 11 and also

changes the value of i to 16.

80


Exercise 2 – determine the output of the

following program

int x = 20;

do

{

cout << x << endl;

x = x – 4;

}

while (x > 10)

81



following program

int x = 1;

do

{

cout << x << endl;

x = x + 1;

}

while (x < 5)

82



following program

int total = 1;

do

{

cout << total << endl;

total = total + 2;

}

while (total >=3)

83


Answer: INFINITE LOOP!

84

LOGO

Nested Control Structures

85

LOGONested loop

In many situations, it is very convenient to

have a loop contained within another loop.

Such loops are called nested loops.

For each single trip, through the outer

loop, the inner loop runs through its entire

sequence.

Each time counter i increases by 1, the inner

loop executes completely.

86

LOGONested loop

Example of C++ program segment

for (i = 0; i <= 5; i++)

{

cout << "\n i is now " << i << endl;

for (j = 1; j <= 4; j++)

cout << " j = " << j ;

}

87

LOGONested loop

How it works…

i is now 0

j = 1 j = 2 j = 3 j = 4

i is now 1

j = 1 j = 2 j = 3 j = 4

i is now 2

j = 1 j = 2 j = 3 j = 4

i is now 3

j = 1 j = 2 j = 3 j = 4

i is now 4

j = 1 j = 2 j = 3 j = 4

i is now 5

j = 1 j = 2 j = 3 j = 4

Outer loopInner loop

88

LOGONested loop

Suppose we want to create the following

pattern.

In the first line, we want to print one star,

in the second line two stars and so on.

*

**

***

****

*****

89

LOGONested loop

Since five lines are to be printed, we start

with the following for statement.

for (i = 1; i <= 5 ; i++)

The value of i in the first iteration is 1, in

the second iteration it is 2, and so on

Can use the value of i as limit condition in

another for loop nested within this loop to

control the number of starts in a line.

90

LOGONested loop

The syntax

for (i = 1; i <= 5 ; i++)

{

for (j = 1; j <= i; j++)

cout << "*";

cout << endl;

}

91

LOGONested loop

What pattern does the code produce if we replace the first for statement with the following?

for (i = 5; i >= 1; i--)

Answer:

*****

****

***

**

*92

LOGO

The jump statements

93

LOGObreak

Causes an exit from loop or switch

statementvoid main()

{

int x;

for (x = 1; x<=10; x++)

{

if (x == 5)

break;

cout<< x <<“”;

}

cout<< “ loop terminated at x:”<<x<<endl;

}

1 2 3 4 loop terminated at x: 5

Press any key to continue 94

LOGOcontinue

Skips the remaining statements in the loop

and proceed with the next loopvoid main()

{

int x;

for (x = 1; x<=10; x++)

{

if (x == 5)

{

a=x;

continue;

}

cout<< x <<“”; }

cout<< “ the number”<<a<<endl;cout<< “ is not printed”<<endl;

}

1 2 3 4 6 7 8 9 10 the number 5 is not printed

Press any key to continue 95

LOGO

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96

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TOPIC 4: REPETITION CONTROL STRUCTURE...2. The loop condition is evaluated. If the loop condition...

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