Programming principles: Loops

8/12/2019 Programming principles: Loops

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Loops


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#i ncl ude usi ng namespace st d;i nt mai n( ){

i nt gr ade;

cout gr ade;

swi t ch( gr ade/ 10){case 10:case 9: cout


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i nt grade; cout gr ade;

/* can grade be less than zero and greater than 100 simultaneously?*/

i f( gr ade100) {cout grade;

}

swi t ch( gr ade/ 10) {case 10:

case 9: cout


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i f( gr ade100) {cout grade; }

swi t ch( gr ade/ 10) {case 10:case 9: cout


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i nt grade;

cout gr ade;

i f( gr ade100) {cout grade;

}

i f( gr ade100)

{cout grade; }

i f( gr ade100) {cout grade; }

/ * af t er N i f st at ement , t he swi t ch wi l l st i l l be eval uat ed */

swi t ch( gr ade/ 10) {. . .

}


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Loops (Iterative Constructs) Loops allow a code segment to be

executed many times.

Three constructs

whilestatement

forstatement

do-whilestatement


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The whi l eStatementwhile(condition){

Action;

} How it works:

if conditionis truethen executeAction

repeat this process untilconditionevaluates to false

Actionis either a singlestatement or a group ofstatements within braces.

condition

Action

true

false


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While Loop Syntax


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The whi l eLoop

while(it 's raining)

{

}


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11/43Slide 11

Expression provides an entry condition

Statement executes if the expression initially evaluates

to true

Loop condition is then reevaluated

Statement continues to execute until the expression isno longer true

If statement will be executed only once, whilestatement will continue repeating while the condition

is true.

The whi l eLoop (continued)


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whi l e ( gr ade100) {cout grade; }



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whi l e ( grade100) {cout


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The whi l eLoop (continued)

Infinite loop: continues to execute endlessly

Can be avoided by including statements in theloop body that assure exit condition will

eventually be false


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Initial Value

In previous code, the initial value is set by the user

using cin>>grade;

While (condition)

(grade < 0 || grade > 100)

Update Value

Infinite loops may generate when you forget to update

the variable used in the condition

Three Things to remember


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whi l e ( grade100) {cout grade; }



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Flowchart whi l eLoop


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Two ways to use while loop Sentinel while:

If you dont know how many times you have toloop

Use Sentinel value Indicates end of data entry

Variable is tested in the condition and loop ends whensentinel is encountered

Sentinel value chosen so it cannot be confused with aregular input (such as -1)

The whi l eLoop


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Two ways to use while loop counter-controlled while:

If you know exactly how many times you need torepeat

Loop repeated until counter reaches a certainvalue

The whi l eLoop


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counter-controlled while: If you need to loop specific times

Loop repeated until counter reaches a certainvalue

counter-controlled while: Example: write a program that calculates the

average of five grades

counter-controlled while:

Example


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Counter while loop FlowChartStart

Sum=0Count = 1

Input

Grade

Count


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i nt mai n( ) {

i nt grade;

cout


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i nt mai n( ) {

i nt gr ade, Sum=0;

cout


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i nt mai n( ) {

i nt gr ade, Sum=0, Aver age;

whi l e( ) {cout


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i nt mai n( ) {

i nt gr ade, count =1, Sum=0, Aver age;

whi l e( count


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i nt mai n( ) {


whi l e( count


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i nt mai n( ) {


whi l e( count


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Sentinel while:

- Variable is tested in the condition and loop

ends when sentinel is encountered (we dontknow how many times we have to loop)

Example:

write a program that calculates average ofthe grade that the user will enter, the userwill enter negative number to finish

Sentinel while loop: Example


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Sentinel while loop FlowChart

Start

Sum=0Count = 0

InputGrade

Grades>=0

Sum = Sum + GradeCount = Count + 1

Average = Sum/Count

Stop

No

Yes


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i nt mai n( ) {

i nt gr ade;

cout


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i nt mai n( ) {

i nt gr ade, Sum=0;

cout >grade;

return 0; }

What will happen if variable

Sum is not initialized ?!


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i nt mai n( ) {

i nt gr ade, Sum=0;

cout >grade; }

return 0; }

How many times we will

loop?!


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i nt mai n( ) {

i nt gr ade, Sum=0, Aver age;

cout >grade; }

Aver age=Sum/ ; cout


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i nt mai n( )

{ i nt gr ade, count =0, Sum=0, Average;

cout >grade;

}Aver age=Sum/ count ; cout


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/ / Anot her way of wr i t i ng ( sum = sum + grade) and/ / ( count = count + 1)

i nt mai n( )

{i nt gr ade, count =0, Sum=0, Average;

cout >grade; }Aver age=Sum/ count ; cout


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Compound Assignments Used to update the values of a variable by

performing an operation on the value that is currentlystored in that variable

(+=, -=, *=, /=, %=) Note that the assignment comes after the arithmetic

operators

Example


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Compound Assignments Find the output of the following program


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/ / A t hi r d way of wr i t i ng ( count = count + 1)

i nt mai n( )

{ i nt gr ade, count =0, Sum=0, Average;

cout >grade;

}Aver age=Sum/ count ; cout


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Write a C++ Program that checks if thepassword is correct or not, with

controlled invalid password entryattempts.

i t i ( )


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i nt mai n( ){

i nt pass, count =1; cout pass;

whi l e( pass ! = 1234 && count < 3) {

cout


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Write a C++ program that convertsinteger grade to letter grade, with

controlled number of invalid grade entryattempts.

/ / how many updat es you need f or t hi s whi l e?


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/ / o a y upda es you eed o s e

i nt grade, count =1; cout gr ade;

whi l e( ( grade100) && count


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/ i f t he condi t i on i s count

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Programming principles: Loops