COMP 14Introduction to Programming

Miguel A. Otaduy

May 21, 2004

Loops• Allow us to repeat statements some number of

times

• Must use a loop control variable– controls how many times to loop

• 4 Parts to Every Loop– initialization - set loop control variable before

condition– condition - when to stop– update - change to the loop control variable– body - actions to repeat

The while Loop

while (expression)

statement

Today in COMP 14

• The for loop• The do...while loop• break and continue• Nested loops

The for Loop

• Specialized form of while loop• Simplifies the writing of count-controlled loops

Basic Form:

for (initialization; condition; update){

statement(s);}

The for LoopExecution

1. initial statement executes

2. loop condition evaluated

3. If loop condition evaluates to true, execute for loop statement and execute update statement

4. Go back to step 2 and repeat until loop condition is false

The for LoopSyntax

for ( initialization; condition; update ){ loop body;}

Reservedword

The initializationis executed once

before the loop begins

The loop body isexecuted until the

condition becomes false

The update portion is executed at the end of each iterationThe condition-loop body-update cycle is executed repeatedly

for vs. while

A for loop is functionally equivalent to the following while loop structure:

initialization;while ( condition ){ loop body; update;}

The for LoopExample

final int LIMIT = 3;

int count;

for (count=0; count<LIMIT; count++)

{

System.out.println (count);

}

System.out.println (“All done!”);

booleancondition

loop body

update

initialization

Output:012All done!

Comparing while and for

While Loop

final int LIMIT=3;int i = 0;

while (i < LIMIT){

System.out.println (i);i++;

}

System.out.println (“All done!”);

For Loop

final int LIMIT=3;int i;

for (i=0; i<LIMIT; i++){

System.out.println (i);}

System.out.println("All done!");

Watch out!

• for statement ending in semicolon is empty; does not affect program

• while statement ending in semicolon results in infinite loop

for (count=0; count<LIMIT; count++);

while (count<LIMIT);

Examples

for (i=1; i<=5; i++){

System.out.println ("Hello");System.out.println ("*");

}

for (i=1; i<=5; i++)System.out.println ("Hello");System.out.println ("*");

for (i=1; i<=5; i++);System.out.println ("*");

Hello*Hello*Hello*Hello*Hello*

HelloHelloHelloHelloHello*

*

In-Class Exercisefor vs. while Loopsfinal int MAX = 20;

int i;

for (i = 0; i<MAX; i+=3)

{

System.out.println (i);

}

• Predict the output

Output:0369121518

In-Class Exercisefor vs. while Loopsfinal int MAX = 20;

int i;

for (i = 0; i<MAX; i+=3)

{

System.out.println (i);

}

• Translate this to a while loop.

final int MAX = 20;

int i = 0;

while (i < MAX)

{

System.out.println (i);

i += 3;

}

The do...while LoopSyntax

do{ loop body;}while ( condition );

do andwhile arereserved

words

The loop body is executed once initially,and then the condition is evaluated

The loop body is executed repeatedlyuntil the condition becomes false

do…while Loop(Post-test Loop)

do { statement(s);} while (expression);

The do...while Loop

• Like a while loop, but its condition is at the end of the loop

• Loop body always executes at least once

• Must also be checked for termination (not an infinite loop)

• Anything you can do with a do...while loop, you can do with a while loop

The do...while LoopExamplefinal int LIMIT = 3;

int count = 0;

do {

System.out.println (count);

count++;

} while (count < LIMIT);

System.out.println (“All done!”);

booleanconditio

n

loopbodyupdat

e

initialization

Output:012All done!

Comparing while and do...while

while Loop

final int LIMIT=3;int count = 0;

while (count < LIMIT){

System.out.println (count);count++;

}

System.out.println (“All done!”);

do...while Loop

final int LIMIT=3;int count = 0;

do{

System.out.println (count);count++;

}while (count < LIMIT);

System.out.println("All done!");

while vs. do...while

i = 11;while (i <= 10) {

System.out.print (i);i += 5;

}System.out.println();

i = 11;do {

System.out.print (i);i += 5;

} while (i <= 10);System.out.println();

[blank line]

11

In-Class ExerciseThe do...while Loop

int x = 0, y = 0;do {System.out.println (x*y);if (y < x){

y += 2; }x++;

} while (x < 5);

• Predict the output of the loop

x

y0

0

Output:

1 2

004616

23

4

45

break Statements

• Used to exit early from a loop • Used to skip remainder of switch

structure• Can be placed within if statement

of a loop– If condition is met, loop exited

immediately

break Example

double in;while (true){

in=Double.parseDouble(keyboard.readLine());

if (in == 0.0){

break;}System.out.println (1.0 / in);

}

inOutput

1.0 1.00

2.0 0.504.0

0.250.0

continue Statements• Used in while, for, and do...while

structures• When executed in a loop, the remaining

statements in the loop are skipped; proceeds with the next iteration of the loop

• When executed in a while/do…while structure, expression evaluated immediately after continue statement

• In a for structure, the update statement is executed after the continue statement; then the loop condition executes

continue Example

double in;while (true){

in=Double.parseDouble(keyboard.readLine());

if (in == 0.0){

continue;}System.out.println (1.0 / in);

}

inOutput

1.0 1.00

2.0 0.504.0

0.250.05.0

0.20infinite loop

Nested Control Structures• Provides new power, subtlety, and

complexity• if, if…else, and switch

structures can be placed within while loops

• for loops can be found within other for loops – each time through the outer loop, the

inner loop goes through its full set of iterations

Nested Control Structures Examplefor (int row = 1; row <= 5; row++)

{

for (int star = 1; star <= row; star++)

{

System.out.print(“*”);

}

System.out.println();

}

Output:***************

• Can't use the variable row outside the outer for loop.• Can't use the variable star outside the inner for loop.

ExercisePrinting Patterns

• Use nested for loops

* * *

* * *

* * *

* * *

* * *

* * *

1 2 3

4 5 6

7 8 9

To do

• Assignment 4

• Read ch. 6