1. Loops and Iteration CS 21a: Introduction to Computing I
Department of Information Systems and Computer Science
Ateneo de Manila University
(Chapter 6, Horstmann text)

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
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

3. Example
applyInterest() method in BankAccount (takes an intRate parameter)
public class BankAccount {
double balance;
...
public void applyInterest( double intRate )
double interest = balance*intRate/100;
balance += interest;
System.out.println( “Interest:” + interest +”, balance is now” + balance ); }
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

4. Example
applyInterest() method in BankAccount (takes an intRate parameter)
public class BankAccount {
double balance;
...
public void applyInterest( double intRate )
double interest = balance*intRate/100;
balance += interest;
System.out.println( “Interest:” + interest +”, balance is now” + balance ); }
Shorthand for:
balance = balance + interest;
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

5. Example improved Let’s add attributes for interest rate and years
public class BankAccount {
double balance;
double intRate; // interest rate on balance
int years; // age of bank account
public void applyInterest( )
double interest = balance*intRate/100;
balance += interest;
System.out.println( “Interest:” + interest +”, balance is now” + balance ); }
...
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

6. Example – yearly interest
yearlyInterest() method in BankAccount
public class BankAccount {
...
public void yearlyInterest()
double interest = balance*intRate/100;
balance += interest;
years++;
System.out.println( “Interest:” + interest +”, balance is now” + balance ); }
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

7. Repeatedly applying interest
Suppose you want to apply interest on the balance for three years
public void applyThreeYearInterest( ) {
double interest;
interest = balance*intRate/100;
balance += interest;
years++;
System.out.println( “Year 1 - interest:” + interest +”, balance:” + balance );
System.out.println( “Year 2 - interest:” + interest +”, balance:” + balance );
System.out.println( “Year 3 - interest:” + interest +”, balance:” + balance ); }
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

8. Repeated execution
We want a way to indicate that the following should be executed three times:
interest = balance*intRate/100;
balance += interest;
years++;
System.out.println( “Yearly-interest:” + interest +”, balance:” + balance );
Better still, we want the following executed where i takes the value 1, 2, 3:
System.out.print( “Year ” + i + “- interest: “+ interest );
System.out.println( “, balance:” + balance );
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

9. The for statement Syntax Notes for ( expr1; expr2; expr3 ) statement
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)
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

10. Applying yearly interest
public void applyYearlyInterest( int numYears ) {
double interest;
int i;
for( i = 1; i <= numYears; i++)
interest = balance*intRate/100;
balance += interest;
System.out.print( “Year ” + i + “- interest: “+ interest );
System.out.println( “, balance:” + balance );
years++; }
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

11. 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”
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

12. The while statement Setup Condition Loop Body Increment /
Syntax: while ( condition )
statement
Setup
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
Increment /
Go to next step
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

13. The do-while Statement
Syntax: do
statement
while ( condition );
public int factorial( int n ) {
int result = 1;
int i = 1; do
result = result * i;
i = i + 1;
} while ( i <= n );
return result; }
Setup
Loop Body
Increment /
Go to next step
Condition
(at the end of loop)
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

14. 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
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

15. 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;
Setup
Condition
Loop Body
Increment /
Go to next step
Increment /
Go to next step
Increment /
Go to next step
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

16. for loop (version 2)
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
public int factorial( int n ) {
int result;
result = 1;
for ( int i = 1; i <= n; i++ )
result = result * i; }
return result;
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

17. for loop (version 3)
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
public int result( int n ) {
int i, result;
for ( result = 1, i = 1;
i <= n;
i++ )
result = result * i; }
return result;
NOT RECOMMENDED!
generally bad style to put several statements on one line
if possible, choose a single counter variable, and just use that
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

18. 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!
Shorthand for: result = result*i;
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

19. for loop (version 3w) (“w” for worse!) public int factorial( int n ) {
even more cryptic
Some C programmers actually like writing like this!
DON’T!
public int factorial( int n ) {
int i, result;
for ( result = 1, i = 1;
i <= n;
result *= i++ ) }
return result;
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

20. for and while are equivalent!
public int factorial( int n ) {
int result;
result = 1;
for ( int i = 1; i <= n; i++ )
result = result * i; }
return result;
public int factorial( int n ) {
int result;
result = 1;
int i = 1;
while ( i <= n )
result = result * i;
i++; }
return result;
Setup
Braces here are shown because
in Java (not in C), scope of vars declared in setup of for
is limited
Condition
Increment
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

21. 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
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

22. 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 ); }
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

23. 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( );
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

24. Loop Pitfall # 1 Infinite Loops 1
int years = 0;
while ( years < 20 )
{ double interest = balance * rate / 100;
balance = balance + interest; } 1
Infinite Loops
Both loops will not terminate because the boolean expressions will never become false.
int years = 20;
while ( years > 0 )
{ years++;
double interest = balance * rate / 100;
balance = balance + interest; } 2
Note: In theory, this while statement is an infinite loop, but in programming languages other than Java, this loop will eventually terminate because of an overflow error. An overflow error will occur if you attempt to assign a value larger than the maximum value the variable can hold. When an overflow error occurs, the execution of the program is terminated in almost all programming languages. With Java, however, an overflow will not cause the program termination. When an overflow occurs in Java, a value that represents infinity (IEEE 754 infinity, to be precise) is assigned to a variable and no abnormal termination of a program will happen. Also, in Java an overflow occurs only with float and double variables; no overflow will happen with int variables. When you try to assign a value larger than the maximum possible integer an int variable can hold, the value “wraps around” and becomes a negative value.
Whether the loop terminates or not because of an overflow error, the logic of the loop is still an infinite loop, and we must watch out for it. When you write a loop, you must make sure that the boolean expression of the loop will eventually become false.
examples adapted from Chapter 6 of Java Concepts by C. Horstmann
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

25. Loop Pitfall # 2 Using Real Numbers 1 2
float count = 0.0f;
while ( count != 1.0f ) {
count = count f;
} //seven 3s 1
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 f;
} //eight 3s 2
Although 1/3 + 1/3 + 1/3 == 1 is mathematically true, the expression
1.0/ / /3.0
in computer language may or may not get evaluated to 1.0 depending on how precise the approximation is. In general, avoid using real numbers as counter variables because of this imprecision.
examples and slide adapted from Introduction to Object Oriented Programming with Java by C. Thomas Wu, Copyright 2000, McGraw-Hill
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

26. Loop Pitfall – 3 Goal: Execute the loop body 10 times. 1 2 3 4 1 3 and
count = 1;
while ( count < 10 ) {
. . .
count++; } 1
count = 1;
while ( count <= 10 ) {
. . .
count++; } 2
count = 0;
while ( count <= 10 ) {
. . .
count++; } 3
count = 0;
while ( count < 10 ) {
. . .
count++; } 4
Yes, you can write the desired loop as
count = 1;
while (count != 10 ) {
...
count++; }
but this condition for stopping the count-controlled loop is dangerous. We already mentioned about the potential trap of an infinite loop. 1 3
and
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
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

27. Avoiding pitfalls Infinite Loop Real Numbers
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
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

28. 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)
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

29. Printing a shape Create a triangle pattern * ** *** Loop through rows
for (int i = 1; i <= n; i++) {
// make triangle row }
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

30. Printing a shape – cont. The following loop creates a row of i stars
for (int j = 1; j <= i; j++) r = r + "*";
r = r + "\n";
Put loops together → Nested loops
String r = "" ;
for (int i = 1; i <= n; i++) {
// make triangle row }
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

31. Triangle class public class Triangle { private int width;
public Triangle(int aWidth)
width = aWidth; }
public String toString()
String r = "";
for (int i = 1; i <= width; i++)
for (int j = 1; j <= i; j++)
r = r + "*";
r = r + "\n";
return r;
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

32. TriangleRunner class public class TriangleRunner {
public static void main(String[] args)
Triangle small = new Triangle(3);
System.out.println(small.toString());
Triangle large = new Triangle(15);
System.out.println(large.toString()); }
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

33. Exercises for nested loops
Create similar classes that print out these shapes as well: * * * *
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved