Lecture 05 Loops 2010

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Introduction to Programming

Environments (C++/UNIX)

IPE 115, Semester 1, 2010

Lecture 05


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Repetition Structure


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INTRODUCTION TO PROGRAMMING

Types of Repetition Structures in C++

while

for

do-while


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Pre-test and Post-test loops

Pre-test loops are entrance controlled

loops. You execute the loop body after evaluating the

test.

Loop body can be executed zero or more times.

Post-test loops are exit controlled loops. You test the loop after executing the entire loop

body.

Loop body can be executed one or more times.


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The While Loop

Syntax of While Loop

while (boolean expr)

statement;

next statement;

while (boolean expr)

{

statement 1;statement 2;

}

next statement;

boolean

expr

next statementstatements

false

true


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Notes on semantics of While Loop

All variables in the boolean expression must be initializedprior to the loop.

At least one of the variables in the boolean expressionmust be assigned a new value inside the loop. In other

words the boolean expression must change its valueinside the loop.

The boolean expression is tested prior to entering the loopand before each repetition of the loop body.

The entire loop body is executed if the boolean expressionis true.

It is possible not to execute the loop body, this occurswhen the boolean expression is initially false.


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Example- Print Numbers from 1 to 10

int count =1;

while (count


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Infinite Loops

An infinite loop is one in which the condition isinitially satisfied, so the loop is entered, but thecondition for exiting the loop is never met.

Generally an infinite loop is caused by failing tomodify a variable involved in the condition withinthe loop body.

To break out of a malfunctioning program press

ctrl-C on Linux or ctrl-break, on an DOS orWindows machine.


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What Does This Loop Print?

int I =1;

while(I < 6 )

cout


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Interactive I/O and Looping

The sentinel or trailer technique uses aspecial end-of data value to indicate theend of meaningful data.

Using the while loop, we place an inputstatement before the loop to read the firstvalue and an input statement at thebottom of the loop to read the next value.

The loop condition tests that the value isnot equal to the sentinel value.


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An Example of Sentinel Input

Input a list of positive numbers from the

keyboard and find the average. The list is

terminated with the value -99.


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#include

using namespace std;

int main()

{

int number, sum, cnt; //Initialization

cout > number; //first number

sum = 0;

cnt = 0;

while (number != -99) //Loop to sum and count values{

sum += number;

++cnt;

cin >> number; //Read next number

} //while//Calculate and print average

cout


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Counter Controlled Loops

A counter controlled loop is a looping control structure

in which a loop variable manages the repetition by

counting.

The syntax for a counter controlled loop in C and C++is:

for(inti expr; boolean expr; increment expr)

statement;

A loop body of more than one statement, must be

enclosed in curly braces.


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Comparison of For and While Loops

for(i=1; i


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Example

Print the numbers from 100 to 10 as

follows;

100 90 80 70 --------- 10

for ( int i=100 ; i>=10 ; I = i-10 )

{cout


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Comments on the for loop

All three expressions that are part of thefor loop are optional. The semicolons arenot optional.

If the boolean expression is omitted youhave an infinite loop.

Use the for loop when you know exactly

how many times the loop body is to beexecuted, either as a specific value or asan expression.


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All parts of the for loop are optional

i = 0;

for(; i


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break and continue Statements with

Loops

The break statement causes the immediate

termination of the execution of the loop body and

the continuation of execution with the first

statement after the loop.

The continue statement causes the immediate

termination of the execution of the loop body butnot the exiting of the loop.


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Nesting of Loops

The statements in the loop body may be any C+

+ statement including another looping statement.

When a forloop is entered from the top, the

initialization occurs and then the booleanexpressions are executed.

When it is entered as a result of completing

execution of the loop body the increment and

then the boolean expressions are executed.


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Nested For Loops

for (I=1; I


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Loop with Post test Syntax to Do-WhileLoop

do

statement;

while (bool expr);

do

{statement 1;

statement 2;

}while (bool expr);

statement

bool

exprNext statement

true false


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An example of the Do While Loop

int i=1;

do

{

cout


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Notes on semantics of

Do While Loop

At least one of the variables in the booleanexpression must be assigned a new value insidethe loop. In other words the boolean expressionmust change value inside the loop.

The boolean expression is tested at the end ofthe loop body after each execution of the loopbody.

The entire loop body is executed if the booleanexpression is true.

The loop body is always executed at least once.


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Determining The Loop To Use

If the statements of the loop may not be

executed at all, use a while loop.

If the statements of the loop must be

executed at least once either loop may be

used but the do-while loop is preferable.


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Example involving While Loop

Write a program to calculate the value of

nth Fibonacci number. The first and

second Fibonacci numbers are 1; the third

Fibonacci number is 2; and so forth wherethe next Fibonacci number is the sum of

the two previous Fibonacci numbers.

Lecture 05 Loops 2010

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