Chapter 4

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TOPIC 4: REPETITION CONTROL STRUCTURE

CSC 128: FUNDAMENTALS OF

COMPUTER PROBLEM SOLVING

Contents

Introduction 1

while loop2

for loop3

Introduction

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.

Introduction – example

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

void main(){cout << “ I love c++!\n”;cout << “ I love c++!\n”;cout << “ I love c++!\n”;cout << “ I love c++!\n”;cout << “ I love c++!\n”;

}

Repetition control structures

C++ provides a few types of loop structures: while loop for loop

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The while loop

while loop

Repeat or loop as long as the condition is true.

The general form of the while statement is:

while is a reserved word.The parentheses are part of the syntax

while (loop condition){

//loop bodystatement;

}

while loop

Loop condition acts as a decision maker and is usually a logical expression. A Boolean expression that controls the execution of the body

of the loop statement executes if the condition initially evaluates to true loop condition is then reevaluated statement continues to execute until the condition is no longer

true

while (loop condition){

//loop bodystatement;

}

while loop

The general form of while loop flowchart:

Loop condition

Statement (s)(loop body)

false

true

while loop

The following while loop prints I love C++ 5 times.

int count = 1;

while(count<= 5){

cout<< “ I love C++”;count ++;}

Loop control variable

while loop

Start

I love C++

End

count = 1

count <= 5

count++

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F

Loop control variable

A LCV controls the number of times the statement or the block of statements is being executed.

int count = 0;

while(count < 10){

cout<< “ hello ”;count ++;

}

How many times this loop will execute??

while loop

Basic operation in loop: Initialization Evaluation Update

while loop

The following while loop prints I love C++ 5 times.

int count = 1; //initialization

while(count<= 5) //evaluation{

cout<< “ I love C++”;count ++; //update statement}

while loop

Update statement:

count ++; ORcount = count + 1;

counter

Requirement of a repetition structure

Flow Chart

I love C++

counter = 1

counter <= 5

counter ++

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F

Initialize LCV

Evaluate LCV (loop condition)

Loop body

Update LCV

while loop

Example: to display the first five positive integers which increment by five.

while loop

Pseudocode:

BeginInitialize i to 0Repeat while i <= 20Display iadd 5 to i (update)End repeat while

End

while 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

while 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

while loop

Various form of while loops: Counter controlled Sentinel controlled

while 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:


Counter controlled while loop includes the following:◦ Counter

A numeric variable used for counting something

Is done by adding a constant, such as 1 or 2, to the value of a variable.

◦ Accumulator Numeric variable used to find totals

Is done by adding a variable to another variable

counter = counter + 1

sum = sum + variable


Counter controlled while loop includes the following:◦ 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.


#include <iostream>#include <conio>

int main(){ int count;

count = 1;

while (count <= 10) { cout << count << endl; count++; }

getch(); return 0;}

initializing

updating


Example: to find and display total of three numbers

Pseudocode:

BeginInitialize lcv to 0, total to 0Repeat while lcv is less than 3input numbertotal = total + numberUpdate lcvEnd repeat whiledisplay total

End


Program and outputvoid main(){

int lcv,total;

lcv = 0, total =0;

while (lcv < 3) { cout << “enter number" << endl;

cin>> number;

total = total + number;

lcv++; }

cout<<“the total is”<<total;

getch(); }


Exercise: Write a C++ statement associated to the following flowchart.

Begin

Initialize counter = 10

counter < 100Display counterMultiplied by 2

Add 10 to counter

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F

End


Program and output

int main(){ int count;

count = 10;

while (count < 100) { cout << count * 2 << endl; count += 10; }

getch(); return 0;}

while 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


General syntax :


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


Output screen


Exercise 1: to create a program to calculate and print total of a few numbers and the program will stop and display the result when a value 333 is read.

while loop – sentinel controlvoid main(){

int number, total;total= 0;

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

while (number != 333) { total = total + number;


}

cout <<"You have entered 333 to terminate” <<“the program.“<<endl;

cout <<“the total is “<<total;

getch(); }


Exercise 2: to create a flowchart and C++ program that process the loop as long as user enter an even number


Solution Flowchart

Begin

End

number % 2 == 0Get another

number

F

Prompt for a number

Get a number

T


Program

int main(){

int 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;}


Output

while 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

{ while (answer != 'Y') { cout << "Welcome to the program." << endl;

cout << "Do you want to quit (Y - Yes, N - No) : "; cin >> answer; } cout << "Bye.";}

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The for loop

for 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;

for 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;

for loop – Example 1

Example : Displaying star symbol three times.

for (int count = 1; count <= 3; count = count + 1) cout << “*” << endl;

initializationinitialization conditioncondition updateupdate

Result:

***


Using for loop to display ‘Welcome to C++’ three times.

Pseudocode:

Start for( set i to 1; i less than or equal to 3; add 1 to i)display “welcome to C++”End for

End


Flowchart Start

i = 1

i <= 3

Display “welcome to C+

+”

i ++

End

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F


C++ program

for (int count = 1; count <= 3; count = count + 1) cout << “welcome to C++” << endl;

initializationinitialization conditioncondition updateupdate

for loop – Exercise 1

To create a program to display the first 10 non negative number.


To create a program to display backward the first 10 non negative number.


Exercise 3: create a program that display the first 10 positive odd integers.


Exercise 3 - answer

for loop – Exercises

How many time the following loop processed?

Answer:

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


How many time the following loop processed?

Answer:

for (int count = 4; count <= 10; count = count + 2)cout << count << endl;


Example: to calculate and display total of 3 numbers


Pseudocode:

StartInitialize total = 0For(set counter to 1; counter less than or equal to 3; add 1 to counter)

input numbertotal = total + number

EndforDisplay total

End


Flowchart Start

counter=1, total = 0,

for counter <= 3

Input number

total = total + numbercounter = counter + 1

Output total

End

F

T


C++ program segment

total = 0;

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

cin>>number;total = total + number;

}cout << “total:” <<total<<endl;


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;


Exercise 4: answer

for loop

A semicolon at the end of the for statement (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:

for loop

This is an infinite loop, continuously printing the word Hello

for (;;)cout << "Hello" << endl;

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Nested Control StructuresNested Control Structures

Nested 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.

Nested loop

Example of C++ program segment

for (i = 0; i <= 1; i++){ cout << "\n i is now " << i << endl;

for (j = 1; j <= 4; j++) cout << " j = " << j ; }

Nested loop

How it works…

i is now 0j = 1 j = 2 j = 3 j = 4

i is now 1j = 1 j = 2 j = 3 j = 4

Outer loopInner loop

Nested 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.

***************

Nested 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.

Nested loop

The syntax

for (i = 1; i <= 5 ; i++){ for (j = 1; j <= i; j++) cout << "*"; cout << endl;}

Nested loop

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

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

Answer:

***************

Summary

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

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Chapter 4

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