COMP 110COMP 110More loopsMore loops

Luv KohliSeptember 15, 2008

MWF 2-2:50 pmSitterson 014

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AnnouncementsAnnouncementsFacebook Tech Talk tonight, 6pm, SN011

Google info session, 6pm, SN014, tomorrow night

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Questions?Questions?

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Today in COMP 110Today in COMP 110do-while loops

for loops

Review: loopsReview: loopsOften you need to repeat an action in a

programStart

Enough sandwiches?

Distribute sandwiches

NoYesMake

sandwich

Review: loopsReview: loops Loop: part of a program that

repeats

Body: statements being repeated

Iteration: each repetition of body

Stopping condition

Start

Enough sandwiches?

Distribute sandwiches

NoYesMake

sandwich

Review: Review: Types of LoopsTypes of Loopswhile◦Safest choice◦Not always most elegant

do-while◦Loop iterates AT LEAST once

for◦Similar to while, but often more convenient

syntax

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Review: Review: whilewhile loop loop

start;while (not enough

sandwiches){ make a sandwich;}distribute sandwiches;

Start

Enough sandwiches?

Distribute sandwiches

NoYesMake

sandwich

Review: Review: whilewhile loop syntax loop syntax

while (boolean expression){ statements;}

Review: Using a Review: Using a whilewhile loop loop

int n = 1;while (n <= 10){ System.out.println(n); n = n + 1;}

n=1

n <= 10?

End

Output n

No

n=n+1

Yes

whilewhile loop loop

EvaluateBoolean expression

Execute Body

End loop

truefalse

n=1

n <= 10?

End

Output n

No

n=n+1

Yes

whilewhile loop loop

First, evaluate Boolean expression

If true, go into the body

If false, do not go into the body

n=20

n <= 10?

End

Output n

No

n=n+1

Yes

Types of LoopsTypes of Loopswhile◦Safest choice◦Not always most elegant

do-while◦Loop iterates AT LEAST once

for◦Similar to while, but often more convenient

syntax

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dodo--whilewhile loop loop Similar to while loop

At least one iteration of the loop◦First execute the loop body◦Then evaluate the Boolean expression◦ If true, execute the body again◦ If false, quit the body

dodo--whilewhile loop loop

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EvaluateBoolean expression

Execute Body End loop

true false

Execute Body

From outside the loop

EvaluateBoolean expression

End loop

false

Execute Body

From outside the loop

true

Equivalent

dodo--whilewhile loop syntax loop syntax

do{ statements; // loop body} while (boolean expression);

If Boolean expression is true, repeat loop bodyOtherwise, exit loop

Using a Using a dodo--whilewhile loop loop

int n = 1;do{ System.out.println(n); n = n + 1;} while (n <= 10);

Don’t forgetthe semicolon!

Using a Using a dodo--whilewhile loop loopWhat if n = 20?

int n = 20;do{ System.out.println(n); n = n + 1;} while (n <= 10);

Infinite loopInfinite loop

int n = 1;do{ System.out.println(n); // n = n + 1;} while (n <= 10);

Types of LoopsTypes of Loopswhile◦Safest choice◦Not always most elegant

do-while◦Loop iterates AT LEAST once

for◦Similar to while, but often more convenient

syntax

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forfor loop loop Components of a for

loop:◦ Initializing actions

◦ Boolean expression

◦ Loop body

◦ Update actions

n=1

n <= 10?

End

Output n

No

n=n+1

Yes

forfor loop loop

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EvaluateBoolean

expression

Execute BodyEnd loop

true false

Execute Initializing

actions

Execute Update actions

forfor loop syntax loop syntax

for (initializing actions; Boolean expression; update actions){ statements; // loop body}

Using a Using a forfor loop loop

int n;

for (n = 1; n <= 10; n++){ System.out.println(n);}

whilewhile loops and loops and forfor loops loops

int n;

for (n = 1; n <= 10; n++){ System.out.println(n);}

int n = 1;while (n <= 10){ System.out.println(n); n = n + 1;}

forfor loop: initializing actions loop: initializing actionsOnly done ONCE at the beginning of the

loop, before the Boolean expression is tested

n=1

n <= 10?

End

Output n

No

n=n+1

Yes

int n;

for (n = 1; n <= 10; n++){ System.out.println(n);}

forfor loop: update actions loop: update actionsExecuted at the end of every iteration◦ After the loop body, but before the next Boolean

expression evaluation

n=1

n <= 10?

End

Output n

No

n=n+1

Yes

int n;

for (n = 1; n <= 10; n++){ System.out.println(n);}

Tracing a Tracing a forfor loop loopint n;

for (n = 1; n <= 3; n++){ System.out.println(n);}

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n = ?n = 1n = 2n = 3n = 4

Execute initializing actionsEvaluate Boolean expressionExecute bodyExecute update actions

Infinite loopInfinite loop

int n;

for (n = 1; n <= 10; n = 0){ System.out.println(n);}

Types of LoopsTypes of Loopswhile◦Safest choice◦Not always most elegant

do-while◦Loop iterates AT LEAST once

for◦Similar to while, but often more convenient

syntax

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WednesdayWednesdaySpend some more time with loops

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