loops and iteration for statements, while statements and do-while statements

Loops and Iteration

for Statements,while Statements anddo-while Statements

6/28/2004Copyright 2004, by the authors of these slides, and Ateneo

de Manila University. All rights reserved L9: Loops

Slide 2

Loops Sometimes we want to execute a

statement or a group of statements repeatedly

Java structures for loops: for statement while statement do-while statement



Slide 3

Example applyInterest() method in BankAccount

(takes an intRate parameter)

public class BankAccount{ double balance; // make balance double so it can handle centavos ... public void applyInterest( double intRate ) { double interest = balance*intRate/100; balance += interest; System.out.println( “Interest:” + interest +”, balance is now” +

balance ); } ...}



Slide 4

Compounding Interest Suppose interest is computed monthly but

compounded quarterlypublic void applyQtrlyInterest( double intRate ){ double interest; interest = balance*intRate/100; balance += interest; System.out.println( “Month 1 - interest:” + interest +”, balance:” + balance ); interest = balance*intRate/100; balance += interest; System.out.println( “Month 2 - interest:” + interest +”, balance:” + balance ); interest = balance*intRate/100; balance += interest; System.out.println( “Month 3 - interest:” + interest +”, balance:” + balance );}



Slide 5

Repeated Execution We want a way to indicate that the

following should be executed three times: interest = balance*intRate/100; balance += interest; System.out.println( “Monthly-interest:” + interest +”, balance:” +

balance );

Better still, we want the following executed where i takes the value 1, 2, 3:

interest = balance*intRate/100; balance += interest; System.out.print( “Month ” + i + “- interest: “+ interest ); System.out.println( “, balance:” + balance );



Slide 6

The for Statement Syntax

for ( expr1; expr2; expr3 ) statement

Notes expr1: initialization or setup (e.g., i = 1 ) expr2: condition (e.g., i <= 3 ) expr3: increment (e.g., i++) statement means any valid statement in

Java (including blocks)



Slide 7

Compounding Interestpublic void applyQtrlyInterest( double intRate ){ double interest; int i; for( i = 1; i <= 3; i++) { interest = balance*intRate/100; balance += interest; System.out.print( “Month ” + i + “- interest: “+ interest ); System.out.println( “, balance:” + balance );

}}



Slide 8

Another Example: Factorial

Given an integer n, compute n! We want: result = 1*2*3*…*n; Repeated operation(s)

multiply a number i to result increment the number i

Do n times starting with i = 1, result = 1:

result = result * i;i = i + 1;

Looping code aka “Iterative” code one round of the loop is called an “iteration”

properly pronounced “ih-teration” not “eye-teration”(think of how you say “reiterate”)



Slide 9

The while Statement

public int factorial( int n ){ int result = 1; int i = 1; while ( i <= n ) { result = result * i; i = i + 1; }

return result;}

Condition

Loop Body

Setup

Increment / Go to next step

Syntax: while ( condition )statement



Slide 10

The do-while Statement

public int factorial( int n )

{

int result = 1;

int i = 1;

do

{

result = result * i;

i = i + 1;

} while ( i <= n );

return result;

}

Condition(at the end of loop)

Loop Body

Setup


Syntax: dostatement

while ( condition );



Slide 11

Components of a Loop Setup/Initialization Terminating/Continuing condition Incrementing step

not necessarily an increment, but something that moves the program further on – i.e., to a state possibly closer to a terminating condition

e.g., decrementing, dividing, multiplitying, getting another input, etc.

Loop body



Slide 12

Using a for loop for factorial

public int factorial( int n ){ int result = 1; int i; for ( i = 1; i <= n; i++ ) { result = result * i; }

return result;}

Condition

Loop Body

Setup




Slide 13

for loop (version 2)public int factorial( int n ){ int result;

result = 1;

for ( int i = 1; i <= n; i++ ) { result = result * i; }

return result;}

You can declare the “counter” variable inside the for scope is within the loop’s block

good when i is not used outside of the loop



Slide 14

for loop (version 3)

public int result( int n ){ int i, result;

for ( result = 1, i = 1; i <= n; i++ ) { result = result * i; }

return result;}

You can have multiple statements in “setup” part of for separated by commas

need to declare the variables before the for, since we can’t have declarations here

also works for increment part

NOT RECOMMENDED! generally bad style to put several statements on one line

if possible, choose a single counter variable, and just use that



Slide 15

for loop (version 3b)public int factorial( int n ){ int i, result;

for ( result = 1, i = 1; i <= n; result *= i, i++ ) { }

return result;}

This is legal, but BAD! cryptic

the for loop has no body!



Slide 16

for loop (version 3w)public int factorial( int n ){ int i, result;

for ( result = 1, i = 1; i <= n; result *= i++ ) { }

return result;}

(“w” for worse!) even more cryptic

Some C programmers actually like writing like this!

DON’T!



Slide 17

for and while are equivalent!

public int factorial( int n )

{

int result;

result = 1;

{

int i = 1;

while ( i <= n )

{

result = result * i;

i++;

}

}

return result;

}

public int factorial( int n ){ int result;

result = 1; for ( int i = 1; i <= n; i++ ) { result = result * i; }

return result;}

Setup

Condition

Increment

Braces here are shown becausein Java (not in C), scope of vars declared in setup of foris limited



Slide 18

Deciding which statement to use

Using a for statement most appropriate when the number of

iterations is known (e.g., factorial of n) Difference between while and do-while

loop condition is performed at the top (while) or at the bottom (do-while) of the loop

in do-while, body is executed at least once in while, we check first before executing



Slide 19

Nested Loops It is possible to have a loop within a

loop Example (What gets printed out?)

for ( int i = 0; i < 5; i++ )

{

System.out.println( i );

for ( int j = 0; j < 5; j++ )

{

System.out.println( j );

}

}



Slide 20

Nested Loops The condition can vary depending on

the outer loop Example (What gets printed out?)

for ( int i = 0; i < 5; i++ ) { System.out.println( i ); for ( int j = 0; j < i; j++ ) { System.out.print( j ); } System.out.println( “” );}



Slide 21

Loop Pitfall # 1

Infinite Loops

Both loops will not terminate because the boolean expressions will never become false.

Infinite Loops

Both loops will not terminate because the boolean expressions will never become false.

int count = 1;

while ( count != 10 ) {

count = count + 2;

}

22

int product = 0;

while ( product < 500000 ) {

product = product * 5;

}

11

examples and slide adapted from Introduction to Object Oriented Programming with Java by C. Thomas Wu, Copyright 2000, McGraw-Hill



Slide 22

Loop Pitfall # 2Using Real Numbers

Loop 2 terminates, but Loop 1 does not because a float or double is only an approximation of a real number. Operations are not necessarily exact.

Using Real Numbers

Loop 2 terminates, but Loop 1 does not because a float or double is only an approximation of a real number. Operations are not necessarily exact.

float count = 0.0f;

while ( count != 1.0f ) {

count = count + 0.33333333f;

} //eight 3s

22

float count = 0.0f;

while ( count != 1.0f ) {

count = count + 0.3333333f;

} //seven 3s

11

examples and slide adapted from Introduction to Object Oriented Programming with Java by C. Thomas Wu, Copyright 2000, McGraw-Hill



Slide 23

Loop Pitfall - 3 Goal: Execute the loop body 10 times.

count = 1;

while ( count < 10 ) {

. . .

count++;

}

11

count = 0;

while ( count <= 10 ) {

. . .

count++;

}

33

count = 1;

while ( count <= 10 ) {

. . .

count++;

}

22

count = 0;

while ( count < 10 ) {

. . .

count++;

}

44

11 33and exhibit off-by-one error (OBOE).examples and slide adapted from Introduction to Object Oriented Programming with Java by C. Thomas Wu, Copyright 2000, McGraw-Hill



Slide 24

Avoiding Pitfalls Infinite Loop

loop body or increment statement should contain a statement that eventually leads to termination

Real Numbers Avoid using == or != when using real numbers Use <= or >= depending on direction of loop

OBOE (Off-By-One-Error) aka “Fencepost error” To execute the loop body N times …

if counter starts at 0, check for counter < N if counter starts at 1, check for counter <= N

Remember that after the loop, the counter (if still in scope) will be beyond the limit of your condition

if we started from 0, counter would be N, if we started from 1, counter would be N+1



Slide 25

Some Exercises Using Loops

List all even numbers less than a given limit Approach 1: Use an if-statement inside a for-

loop Approach 2: Arrange it so that the for-loop does

skips through the odd numbers Print out all 16 pairs of numbers from the

set {0,1,2,3} What if the numbers have to be distinct? What if the the order does not matter (i.e., the

pair 1,2 is the same as the pair 2,1) Print out a block of asterisks of a given size

loops and iteration for statements, while statements and do-while statements

Documents

int result result

public int result int

double balance

balance double

bpublic int factorial

wpublic int factorial

intrate100 balance

print month i