CS503 Lec5

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Lecture 5:

Repetition (looping) structures

In repetition, the program repeats particular

statements a certain number of times

based on some condition(s).

There are many situations in which it is necessary to repeat a set of statements.

EX: for each student in a class, the formula for determining the course

grade is the same.

C++ has three repetition (looping) structures that let you repeat statements

over and over until certain conditions are met. These are:

1. while loop

2. do while loop

3. for loop

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1. while loop

The general form of the while statement is:

while is a reserved word.

the statement can be one or compound statement between {and}It is called body of the loop.

Note that the parentheses around the expression are part of the syntax.

The expression acts as a decision maker and is usually a logical expression

and provides an entry condition.

If it initially evaluates to true, the statement executes. Then the expression is

then reevaluated.If it again evaluatesto true, the statement executes again.

The statement continues to execute until the expression is no longer true.

A loop that continues to execute endlessly is called an infinite loop.

To avoid an infinite loop, make sure that the loop’s body contains statement(s)

that assure that the expression in the while loop will eventually be false.

Designing while Loops

The body of a while executes only when the expression evaluates to true.

Typically, the expression checks whether a variable(s) (called the loop control

variable (LCV)) satisfies certain conditions.

The LCV must be properly initialized before the while loop, and it should

eventually make the expression evaluate to false by updating or reinitializing

the LCV in the body of the while loop.

If you put a semicolon after the logical expression, the action is empty or null.

Therefore, typically, while loops are written in the following form:

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EX1:

int i = 0;

while (i

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5 minute question

What is the output of this program?

#include

using namespace std;

int main()

{

int i = 20; //wrong initialization

while (i < 20) //initially the loop entry condition evaluates to false

{ //the body of the while loop will never execute

cout

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EX:a program to add SOME entered numbers& find their average (you don't know how many integers)

#include


int main() {

int limit; //store the number of the entered items

int number; //variable to store the entered number

double sum = 0.0; //variable to store the sum

int counter = 0; //loop control variable MUST BE INITIALIZED BEFORE THE LOOP

cout > limit;

cout

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//A program to read some positive integers and average them, but you do not have a preset number of them.

//Suppose the number -999 marks the end of the entered integers.

#include


const int SENTINEL = -999;

int main() {

int number=0; //variable to store the number

double sum = 0.0; //variable to store the sum

int count = 0; //variable to store the totalnumbers read

cout > number;

}

cout

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EX: The game of Number Guessing

The following program randomly generates an integer greater than or equal to 0

and less than 100.

The program then prompts the user to guess the number. If the user guesses the

number correctly, the program outputs an appropriate message. Otherwise,the program checks whether the guessed number is less than the random

number. If the guessed number is less than the random number generated by

the program, the program outputs the message “Your guess is lower than the

number. Guess again!”; otherwise, the program outputs the message “Your

guess is higher than the number. Guess again!”. The program then prompts

the user to enter another number. The user is prompted to guess the random

number until he enters the correct number.

To generate a random number, use the function rand() of the header file cstdlib.

EX: The expression rand() returns an int value between 0 and 32767.

nc u e < ostream>

#include

using namespace std;int main()

{

int num = rand() % 100; //variable to store the random number

int guess; //variable to store the number guessed by the user

bool isGuessed = false; //boolean variable to control the loop

while (!isGuessed)

{

cout > guess;

if (guess == num)

{

cout

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Sample Run:

Enter an integer greater than or equal to 0 and less than 100: 45

Your guess is higher than the number.

Guess again!


Your guess is lower than the number.

Guess again!


Your guess is higher than the number.

Guess again!


Your guess is lower than the number.

Guess again!


You guessed the correct number.

In the previous examples, the expression in the while loop is quite simpleas it is controlled by a single variable.

However, there are situations where the expression in the while may bemore complex.

EX:

The previous program gives as many tries as the user needs to guess thenumber. Suppose you want to give the user no more than five triesto guess the number. If the user does not guess the number correctlywithin five tries, then the program outputs the random numbergenerated by the program as well as a message that you have lost the

game.

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#include

#include


int main() {

int num= rand() % 100; //variable to store the random number

int guess; //variable to store the number guessed by the user bool isGuessed = false; //boolean variable to control the loop

int noOfGuesses = 0;

while ( (noOfGuesses < 5) && (!isGuessed) ) //2 conditions with 2 loop control variables

{

cout > guess;

noOfGuesses++;

if (guess == num)

{

cout

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The for loop executes as follows:

1. The initial statement executes.

2. The loop condition is evaluated. If the for loop conditionevaluates 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 for loop condition evaluates to false.

The initial statement is the first statement to execute and it is executed onl

once.

EX1:

A program to print the first 10 nonnegative integers

#include


int main()

{

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

cout

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A for loop can have compound statement.

EX2:

#include


int main()

{

for (int i = 1; i

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Some comments on for loops:

- If the loop condition is initially false, the loop body does not execute.

- The update expression changes the value of the loop control variable which eventually

sets the value of the loop condition to false. The for loop body executes indefinitelyif the loop condition is always true.

- 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 loop, you can omit all three statements (initial statement, loop condition, and

update statement). Such as:

for (;;)

cout

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EX5:

You can increment (or decrement) the loop control variable by any fixed number.

In the following example, the variable is initialized to 1; at the end of the for loop, i is

incremented by 2. to outputs the first 10 positive odd integers.

#include


int main()

{

for (int i = 1; i

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EX7:

The loop condition evaluates to true, so the output statement executes, which outputs 10.

#include


int main()

{

for (int i = 10; i

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EX9:

In this example, a for loop reads five numbers and finds their sum and average.

#include


int main(){

int i, newNum, sum = 0;

double average;

cout

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do...while Looping (Repetition) Structure

The general form of a do. . .while statement is as follows:

do is a reserved word.

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 is true, the statement executes.

To avoid an infinite loop:

you must make sure that the loop body contains a statement

that ultimately makes the expression false and assures that it

exits properly.

EX:

#include using namespace std;

int main()

{

int i = 0;

do

{

cout

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The difference between the 3 loop structures

In a while and for loop, the loop condition is evaluated before executing the

body of the loop. Therefore, while and for loops are called pretest loops.

On the other hand, the loop condition in a do. . .while loop is evaluated after

executing the body of the loop. Therefore, do. . .while loops are called

posttest loops.

Because the while and for loops both have entry conditions, these loops

may never activate.

The do...while loop, on the other hand, has an exit condition and therefore

always executes the statement at least once.

Compare between the following two codes:

a. #include


int main()

{

int i = 11;

while (i

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Choosing the Right Looping Structure

All three loops have their place in C++.

If you know, or the program can determine in advance, the number of

repetitions needed, the for loop is the correct choice.

If you do not know, and the program cannot determine in advance the

number of repetitions needed, and it could be zero, the while loop is

the right choice.

If you do not know, and the program cannot determine in advance the

number of repetitions needed, and it is at least one, the do...while

loop is the right choice.

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