CS 46A - Lecture 14

Pre-class reading

• Section 6.2

for Loops

• Example:

  for (int i = 1; i <= n; i++)
  {
     double interest = balance * rate / 100;
     balance = balance + interest;
  }

• Use a for loop when a variable runs from a starting value to an ending value with a constant increment or decrement

Syntax 6.2 The for Statement

for Loop Flowchart

Execution of a for Loop

ch06/invest2/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;
      }
   }

   /**
      Keeps accumulating interest for a given number of years.
      @param numberOfYears the number of years to wait
   */
   public void waitYears(int numberOfYears)
   {
      for (int i = 1; i <= numberOfYears; i++)
      {
         double interest = balance * rate / 100;
         balance = balance + interest;
      }
      years = years + n;
   }

   /**
      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/invest2/InvestmentRunner.java

/**
   This program computes how much an investment grows in
   a given number of years.
*/
public class InvestmentRunner
{
   public static void main(String[] args)
   {
      final double INITIAL_BALANCE = 10000;
      final double RATE = 5;
      final int YEARS = 20;
      Investment invest = new Investment(INITIAL_BALANCE, RATE);
      invest.waitYears(YEARS);
      double balance = invest.getBalance();
      System.out.printf("The balance after %d years is %.2f\n", 
            YEARS, balance);
   }   
}

Program Run:

• The balance after 20 years is 26532.98

Lecture 14 Clicker Question 1

Which of the following is true?

1. Every for loop can be rewritten as a while loop, and every while loop can be rewritten as a for loop
2. Some for loops can be rewritten as while loops, and all while loops can be rewritten as for loops
3. All for loops can be rewritten as while loops, and some while loops can be rewritten as for loops
4. There are for loops that can't be rewritten as while loops, and there are while loops that can't be rewritten as for loops

Lecture 14 Clicker Question 2

int sum = 0;
for (i = 0; i <= 10; i++)
   sum = sum + i;

1. 9 times
2. 10 times
3. 11 times
4. 55 times

for Loop Examples

Symmetric Bounds

• for (i = 1; i <= n; i++)

  1 ≤ i ≤ n

  Executed n times

• for (i = 10; i <= 20; i++)

  10 ≤ i ≤ 20

  Executed 11 times: 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20

• Common source of error: “fence post error”

  A fence with ten segments has eleven posts

• for (i = a; i <= b; i++) is executed b - a + 1 times

Asymmetric Bounds

• for (i = 10; i < 20; i++)

  Executed 10 times

• for (i = a; i < b; i++) is executed b - a times
• Particularly common when a is zero
  • for (i = 0; i < n; i++) is executed n times

  for (i = 0; i < str.length(); i++)
  {
     String letter = str.substring(i, i + 1);
     ...
  }

Traversing Strings

• Traverse all positions:

  for (i = 0; i < str.length(); i++)

• Or traverse them backwards

  for (i = str.length() - 1; i >= 0; i--)

• Extract each letter

  String letter = str.substring(i, i + 1);

• Or use char:

  char ch = str.charAt(i);

• I prefer working with strings. char is only useful

Common Errors: Semicolons

• A missing semicolon:

  for (years = 1;
     (balance = balance + balance * rate / 100) < targetBalance;
     years++)
        System.out.println(years);

• A semicolon that shouldn't be there:

  sum = 0;
  for (i = 1; i <= 10; i++);
     sum = sum + i;
  System.out.println(sum);

Lecture 14 Clicker Question 3

How many times is the condition of this for loop evaluated?

for (int i = 1; i <= 6; i = i + 2)

• 3 times
• 4 times
• 6 times
• 7 times

Lecture 14 Clicker Question 4

What does this loop do?

int result = 0;
for (int i = 0; i <= str.length(); i++)
{
   char ch = str.charAt(i);
   if (Character.isDigit(ch))
      result = result + ch;
}

1. It extracts all digits from a string, e.g. "He110!" -> "110"
2. It sums all digits from a string, e.g. "He110!" -> 3
3. It extracts the integer inside a string, e.g. "He110!" -> 110
4. Something else

Loop Practice

• CodingBat

  bobThere

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

PhoneNumbers