while Loops While Loop Purpose: - PowerPoint Presentation

Slide1

while LoopsSlide2

While Loop

Purpose:

to repeat a set of 0 or more statements

0 or more times

as long as some condition is true.Slide3

While Loop

Syntax:

w

hile (

conditionalExpression

)

oneStatement

w

here

oneStatement

is exactly one statement with a semicolon, or a blockSlide4

While Loop

Semantics:

At the beginning:

conditionalExpression

is evaluated and

- If true:

oneStatement

is executed, then control

returns to the beginning.

- If false:

oneStatement

skipped.Slide5

While Loop

Style #1: one statement

w

hile (

conditional

)

Exactly one statement

; // indent the 1 statement

Style #2:

block

while (

conditional

)

{

// line up with

while

statements

//

indent statements

}

//

line up with

whileSlide6

While Loop

Quick Vocabulary

Body (of the loop):

the one statement or block

Iteration:

one execution of the loop

(one time through, one “lap”)

Slide7

While Loop

Example 1: only syntax counts!

int

a = 1;

w

hile (a <=

3

)

Prints:

a++;

4

cout

<< a <<

endl

;Slide8

While Loop

Example 2: careful of where begins/ends

int

a = 1;

w

hile (a <=

3

)

{

Prints:

a++;

2

cout

<< a <<

endl

;

3

}

4Slide9

While Loop

Example 3: 0 iterations possible

int

a = 12;

w

hile (a <=

3

) {

Prints:

a++;

Done!

cout

<< a <<

endl

;

}

c

out

<< ”Done!\n”;

Slide10

While Loop Strategies

Counter-Controlled Loop

int

i

=

start

;

w

hile (

i

<=

stop

) {

// do something some number of times

i

+=

increment

;

//

i

=

i

+

increment

;

}

Slide11

While Loop Strategies

Counter-Controlled Loop

Example: start at 1, stop at 20, count by 2's

int

i

=

1

;

w

hile (

i

<=

20

) {

cout

<<

i

<< ” ”;

i

+=

2

;

}

// prints: 1 3 5 7 9 11 13 15 17 19

Slide12

While Loop Strategies

User Input Validation

// answer could be float/string/etc.

int

answer =

wrong_answer

;

w

hile (

answer is wrong

) {

cout

<< ”Enter …”;

cin

>> answer;

}

Slide13

While Loop Strategies

User Input Validation

int

answer = 0;

w

hile (answer < 1 || answer > 10) {

cout

<< ”Enter integer 1 to 10: ”;

cin

>> answer;

}

Slide14

While Loop Strategies

User Input Validation

string answer = ””;

w

hile (answer !=”left” && answer!=”right”) {

cout

<< ”Enter left or right: ”;

cin

>> answer;

}

Slide15

While Loop Strategies

User Input Validation with error message

// ask first time before the loop

string answer;

cout

<< ”Enter left or right: ”;

cin

>> answer

;

w

hile (answer !=”left” && answer!=”right”) {

cout

<< “Error! Type left or right!!\n”;

cout

<< ”Enter left or right: ”;

cin

>> answer;

}

Slide16

While Loop Strategies

User Input Validation with error message

// use

if

to see if we need an error message

string answer = ””;

w

hile (answer !=”left” && answer!=”right”) {

cout

<< ”Enter left or right: ”;

cin

>> answer;

if (answer

!=”left” && answer!=”right

”)

cout

<< ”Invalid answer!\n”;

}

Slide17

While Loop Strategies

Summation of entered Values

i

nt

num

,

sum = 0;

w

hile (

whatever_control

) { // any type of loop

cout

<< ”Enter number: ”;

cin

>>

num

;

sum +=

num

;

// sum = sum +

num

}

Slide18

While Loop Strategies

Counting Strategy

i

nt

num

,

count = 0;

w

hile (

whatever_control

) {

cout

<< ”Enter number: ”;

cin

>>

num

;

count++;

// count = count + 1

}

cout

<< count << " numbers entered";

Slide19

While Loop Strategies

Average of Inputs: Count + Sum

float

num

,

sum = 0

, average;

i

nt

count = 0

;

while (

whatever_control

) {

cout

<< ”Enter number: ”;

cin

>>

num

;

count++;

// count = count + 1

sum +=

num

;

// sum = sum +

num

}

average = sum / count;Slide20

While Loop Strategies

Mini - find the lowest value

int

min

=

ridiculously

High

number

;

while

(

whatever_control

) {

cout

<< ”Enter number: ”;

cin

>>

num

;

if (

num

< min)

min =

num

;

} Slide21

While Loop Strategies

Max - find the highest value

int

max

=

ridiculously

Low

number

;

while

(

whatever_control

) {

cout

<< ”Enter number: ”;

cin

>>

num

;

if (

num

>

max)

max =

num

;

} Slide22

Sentinel Controlled Loop

Motivation/Purpose:

- programs must often input data

serially

(one at a time).

- the

Number of Inputs

may not be known:

when the program is written

when the program is executing and is ready to input the first data value

How many data items will there be???Slide23

Sentinel Controlled Loop

Definition of a Sentinel Value:

a value entered as data,

but

is not processed as data;

instead

, it

signals the end

of data

.

Characteristics:

choosing a Sentinel Value

- must be the

same Data Type

as the data

- should not "make sense" as dataSlide24

Sentinel Controlled Loop

General Pattern:

Input data

while (data != sentinel) {

process data

Input data

}

Note the check for the sentinel is done

immediately

after a value is inputted.Slide25

Sentinel Controlled Loop

Example:

Enter an unknown number of test scores and calculate the sum. Use -1 as the Sentinel.

Note: the value

-1

- is the same type (

int

) as a test score

- "doesn't make sense" as a test scoreSlide26

Sentinel Controlled Loop

int

score, sum=0;

c

out

<< "Enter score, -1 to quit: ";

cin

>> score;

while (score != -1) {

sum += score;

cout

<< "Enter score, -1 to quit: ";

cin

>> score;

}

cout

<< "The sum is " << sum <<

endl

;Slide27

Sentinel Controlled Loop

Origin of the word

Sentinel:

- a

guard

on the "lookout" for something

(usually the "bad guys" or other mischief)

The while() is "on the lookout" for a

particular value, and reacts (stops) when

it sees the value. The value is not treated

as "ordinary".Slide28

Vocabulary

Term

Definition

Body

the one statement or block of statements repeated by a loop.

Iteration

one execution of a loop (lap).

Sentinel

a value entered as data, but is not processed

as data; instead, it signals the end of data.