CS 46A - Lecture 13

Pre-class reading

• Section 6.1

while Loops

• A while statement executes a block of code repeatedly
• A condition controls how often the loop is executed

  while (condition)
     statement

• Most commonly, the statement is a block statement (set of statements delimited by { })

Calculating the Growth of an Investment

• Want to know when bank account has reached a particular balance:

  while (balance < targetBalance)
  {
     years++;
     double interest = balance * rate / 100;
     balance = balance + interest;
  }

ch06/invest1/Investment.java

/**
   A class to monitor the growth of an investment that 
   accumulates interest at a fixed annual rate.
*/
public class Investment
{
   private double balance;
   private double rate;
   private int years;

   /**
      Constructs an Investment object from a starting balance and
      interest rate.
      @param aBalance the starting balance
      @param aRate the interest rate in percent
   */
   public Investment(double aBalance, double aRate)
   {
      balance = aBalance;
      rate = aRate;
      years = 0;
   }

   /**
      Keeps accumulating interest until a target balance has
      been reached.
      @param targetBalance the desired balance
   */
   public void waitForBalance(double targetBalance)
   {
      while (balance < targetBalance)
      {
         years++;   
         double interest = balance * rate / 100;
         balance = balance + interest;
      }
   }

   /**
      Gets the current investment balance.
      @return the current balance
   */
   public double getBalance()
   {
      return balance;
   }

   /**
      Gets the number of years this investment has accumulated
      interest.
      @return the number of years since the start of the investment
   */
   public int getYears()
   {
      return years;
   }
}

ch06/invest1/InvestmentRunner.java

/**
   This program computes how long it takes for an investment
   to double.
*/
public class InvestmentRunner
{
   public static void main(String[] args)
   {
      final double INITIAL_BALANCE = 10000;
      final double RATE = 5;
      Investment invest = new Investment(INITIAL_BALANCE, RATE);
      invest.waitForBalance(2 * INITIAL_BALANCE);
      int years = invest.getYears();
      System.out.println("The investment doubled after "
            + years + " years");
   }   
}

Program Run:

• The investment doubled after 15 years

while Loop Flowchart

while Loop Examples

Syntax 6.1 The while Statement

Lecture 13 Clicker Question 1

while (false) print("Hello");
println(" World");

1. Hello World
2. HelloHelloHello... (and never stops)
3. World
4. Nothing—the code segment has a syntax error

Lecture 13 Clicker Question 2

1. The waitForBalance method would return immediately, without entering the loop
2. The waitForBalance method would enter the loop once and then return
3. The waitForBalance method would never return due to an infinite loop.
4. The waitForBalance method would throw an exception

Common Error: Infinite Loops

• Example:

  int years = 0;
  while (years < 20)
  {
     double interest = balance * rate / 100;
     balance = balance + interest;
  }

• Example:

  int years = 20;
  while (years > 0)
  {
     years++; // Oops, should have been years--
     double interest = balance * rate / 100;
     balance = balance + interest;
  }

• Loops run forever – must kill program

Common Error: Off-by-One Errors

• Off-by-one error: a loop executes one too few, or one too many, times
• Example:

  int years = 0;
  while (balance < 2 * initialBalance)
  {
     years++;
     double interest = balance * rate / 100;
     balance = balance + interest;
  }
  System.out.println("The investment reached the target after " +
     years + " years.");

• Should years start at 0 or 1?
• Should the test be < or <=?

Avoiding Off-by-One Error

• Look at a scenario with simple values:
  initial balance: $100
  interest rate: 50%
  after year 1, the balance is $150
  after year 2 it is $225, or over $200
  so the investment doubled after 2 years
  the loop executed two times, incrementing years each time
  Therefore: years must start at 0, not at 1.
• interest rate: 100%
  after one year: balance is 2 * initialBalance
  loop should stop
  Therefore: must use <
• Think, don't compile and try at random

Tracing a Loop

• Use the familiar tracing form

  year	balance	targetBalance
  0	10000	20000
  1	11000
  2	12100
  ...

• Cross out values and write the new values below
• Also keep a marker at the code to mark the current line

Loop Tracing Example

• int n = 1729;
  int sum = 0;
  while (n > 0)
  {
     int digit = n % 10;
     sum = sum + digit;
     n = n / 10;
  }
  System.out.println(sum);

• n	sum	digit
  1729	0

Lecture 13 Clicker Question 3

What does this loop print?

int n = 1729;
String result = "";
while (n > 0)
{
   int digit = n % 10;
   result = result + digit;
   n = n / 10;
}
System.out.println(result);

1. 19
2. 1729
3. 9271
4. Nothing--the code has a syntax error

Lecture 13 Clicker Question 4

What does this loop print?

String greeting = "Hello";
String result = "";
int i = greeting.length() - 1;
while (i > 0)
{
   String letter = greeting.substring(i, i + 1);
   result = result + letter;
   i--;
}
System.out.println(result);

1. olleH
2. olle
3. lleH
4. Nothing—an exception is thrown

Loop Practice

• CodingBat

  DoubleChar

• CodeCheck (experimental; follow the “Code Completion” link)

  Count Sevens