4) Repetition Control Structure (Part 1)

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FUNDAMENTALS OF

COMPUTER PROBLEM SOLVING

Topic 4: Repetition Control Structure (Part 1)

Objectives

o Understand the requirement of a loop

o Understand the Loop Control Variable (LVC)

o Use increment (++) and decrement (‐‐) operators

o Understand Counter‐controlled, Sentinel‐controlled and Flag‐controlled structures

o Program loop with for, while statements

Repetition Structure

Many applications require certain operations to be carried out more than once. Such situations require repetition in control flow

Repetition structure is the control structure that allow a sequence of statement to be executed repeatedly until certain condition is reached.

Repetition Structure

Requirement of a repetition structure :- Loop control variable (LCV) – a variable that

determines whether the loop body will be executed.

Loop condition – if the condition is true, the loop body is executed, otherwise the loop exits

Loop body – statement block (statement to be repeated)

Execution of the loop is controlled by 3 operations on LCV:- 1) Initialization of LCV

2) Evaluation of the LCV in loop condition

3) Update LCV by incrementing or decrementing

Shortcut Assignment

C++ has a set of operators for applying an operation to a variable and then storing the result back into the variable

Shortcut assignments: *=, /=, +=, -=, %=

Examples:

int i = 3; i += 4; // i = i + 4 cout << i << endl; // i is now 7 double a = 3.2; a *= 2.0; // a = a * 2.0 cout << a << endl; // a is now 6.4 int change = 1265; change %= 100; // change = change % 100 cout << change << endl; // change is now 65

Increment and Decrement Operator

Increment and decrement operator – unary operator (operator that requires one operand)

Special operators that increment or decrement a variable by one

Two types of increment and decrement operator Preincrement or predecrement operator – operator is

placed before a variable example : ++a increment a by 1, then use

the new value of a. --b decrement b by 1, then use

the new value of b


Postincrement or postdecrement operator – operator is placed after a variable

example : a++ use the current value of a, then increment by 1

b-- use the current value of b, then decrement by 1


Examples

int a, b, c, d, k;

k = 3;

a = ++k; // k=4, a=4

b = --a; // a=3, b=3

c = b++; // c=3, b=4

d = c--; // d=3, c=2


Examples

int k = 4;

++k; // k=k+1 : k is 5

k++; // k=k+1 : k is 6

cout << k << endl;

int i = k++; // i is 6, k is 7

cout << i << " " << k << endl;

int j = ++k; // j is 8, k is 8

cout << j << " " << k << endl

Types of Repetition Structure

Three types of repetition structure:

Counter controlled repetition structure is used when

the number of repetition is fixed. – repeat a process for N times.

example : enter mark for 10 students.

This type of repetition structure requires :- The name of a control variable (loop counter)

The initial value of the control variable

The condition that tests for the final value of the control variable.

The increment (or decrement) by which the control variable modified each time through the loop.


Sentinel controlled repetition structure is used when the number of repetition is depends on certain condition. – repeat a process until user input a value to terminate the process

sentinel value = a value used to terminate the loop

process example : enter all the positive number

This repetition structure depends on given condition. example : the loop will be terminated when user key

in the value of zero or –999 (to indicate the termination)


Flag controlled repetition structure uses a bool variable to control the loop.

General form of flag‐controlled structure:

found = false; //initialize the loop control variable while (!found) //test the loop control variable { … … if (expression) found = true; //update LCV … … }

Flag Control Example

#include<iostream.h>

main()

{

int num;

bool found = false; //initialize the loop control variable

while (!found) //test the loop control variable

{

cout << "Enter num: ";

cin >> num;

if (num == -1)

found = true; //update the LCV

}

}


The variable found above, which is used to control

the execution of the while‐loop is called flag variable.

Can use bool, int or char variable.

bool variable:

bool testValue = true;

cout << “The value is true:” << testValue << endl;

testValue = false;

cout << “The value is false:” << testValue << endl;


Repetition structure in C+ comes in three form :-

For loop

While loop

Do… while loop

For loop

A counter controlled loop in which a loop variable manages the repetition through counting

Purpose : execute the initialization statement. While the condition remains true, execute the statement and the update expression.

For loop

o Syntax :-

for (initialization_statement; condition; update_expression)

statementBlock;

where :-

initialiazation_statement = initialization of loop control variable

condition = evaluation of loop condition

update_expression = updating of loop control variable

statementBlock = either or single statement or a group of statement

enclosed between braces

For loop

How it works:

Execute initialization_statement

While condition is true

Execute statementBlock

Execute update_expression

Initalize LCV

condition

LCV Statement block Update LCV

false

true

For loop Flowchart

For loop

Example 1 :- int sum =0; for (int i = 1; i <= 10; i++) sum+=i; cout << “the sum is “ <<sum<<endl; Example 2 :- to find summation of 5 numbers entered by user int num, sum = 0; for (int i = 0; i <= 4; i++) { cout < “Enter a number : “; cin >> num; sum+= num; } cout << “The sum is “ << sum << endl;

While loop

o While loop employs a loop pretest, testing the condition before each loop iteration/repetition

o Can apply counter, sentinel and flag control loop o Purpose : Execute the statement while the condition remains true. Syntax :- while (condition) statementBlock; where :- condition = evaluation of loop condition statementBlock = either or single statement or a group of

statement enclosed between braces

While loop

How it works:

If condition is true then execute statementBlock

Repeat this process until condition evaluates to false

Initalize LCV

condition

LCV Statement block Update LCV

false

true

While loop Flowchart

While loop Example

o Example 1 :-

while (x >= 10)

x = sqrt(x);

o Example 2 :- to find summation of 5 numbers entered by user

int num, sum = 0;

int count = 0; initialization of LCV

while (count <= 4) evaluate loop condition

{

cout < “Enter a number : “;

cin >> num;

sum+= num;

count++; update loop LCV

}

cout << “The sum is “ << sum << endl;

o Example 3 :- to find maximum number among 6 numbers entered by user

int x, max = 0;

int count = 0; initialization of LCV

while (count <= 5) evaluate loop condition

{

cout < “Enter a number : “;

cin >> x;

if ( x > max)

max = x;

count++; update loop LCV

}

cout << “The maximum number is “ << max << endl;

While Loop - counter-controlled repetition

While Loop – sentinel-controlled repetition

o A repetition statement in which the loop control condition changes within the body of the loop. It uses end-of-data value called sentinel value that serves as a signal for termination.

o Example 1 :-

while (mark != 999)

{ statementblock; }

o Example 2 :-

char choice;

cout << “Do you want to quit? “;

cin >> choice;

while (choice !=‘Y’)

{

cout <<“HELLO!!! \n”;

cout <<“Do you want to quit? “;

cin >> choice;

}

Test Yourself

Write a program to find summation of 5 integer numbers using:

For loop

While – counter loop

While – sentinel loop

While – flag control loop

END

Thank You

4) Repetition Control Structure (Part 1)

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