Horstmann Chapter 7

Big Java 4

Chapter 7 – Arrays and Array Lists

Chapter Goals

identical train cars

  • To collect elements using arrays and array lists
  • To use the enhanced for loop for traversing arrays and array lists
  • To learn common algorithms for processing arrays and array lists
  • To work with two-dimensional arrays
  • To understand the concept of regression testing

Arrays

  • An array collects a sequence of values of the same type.
  • Create an array that can hold ten values of type double:
    new double[10]
    • The number of elements is the length of the array
    • The new operator constructs the array
  • The type of an array variable is the type of the element to be stored, followed by [].
  • To declare an array variable of type double[]
    double[] values; step one
  • To initialize the array variable with the array:
    double[] values = new double[10]; step 2
  • By default, each number in the array is 0
  • You can specify the initial values when you create the array
    double[] moreValues = { 32, 54, 67.5, 29, 35, 80, 115, 44.5, 100, 65 };

Arrays

  • To access a value in an array, specify which “slot” you want to use
    • use the [] operator
    values[4] = 35; step 3
  • The “slot number” is called an index.
  • Each slot contains an element.
  • Individual elements are accessed by an integer index i, using the notation array[i].
  • An array element can be used like any variable.
    System.out.println(values[4]);
  • Arrays

    declare, initialize, modify an array

    Figure 1 An Array of Size 10

    Syntax 7.1 Arrays

    Array syntax

    Arrays

    • The elements of arrays are numbered starting at 0.
    • The following declaration creates an array of 10 elements:
      double[] values = new double[10];
    • An index can be any integer ranging from 0 to 9.
    • The first element is values[0]
    • The last element is values[9]
    • An array index must be at least zero and less than the size of the array.
    • Like a mailbox that is identified by a box number, an array element is identified by an index.
      a row of post office boxes

    Arrays - Bounds Error

    • A bounds error occurs if you supply an invalid array index.
    • Causes your program to terminate with a run-time error.
    • Example:
      double[] values = new double[10];
      values[10] = value; // Error
    • values.length yields the length of the values array.
    • There are no parentheses following length.

    Declaring Arrays

    array delarations

    Array References

    • An array reference specifies the location of an array.
    • Copying the reference yields a second reference to the same array.
    • When you copy an array variable into another, both variables refer to the same array
      int[] scores = { 10, 9, 7, 4, 5 };
      int[] values = scores; // Copying array reference
    • You can modify the array through either of the variables:
      scores[3] = 10;
      System.out.println(values[3]); // Prints 10
    • Two refereces to the same array

      Figure 2 Two Array Variables Referencing the Same Array

    Using Arrays with Methods

    • Arrays can occur as method arguments and return values.
    • An array as a method argument
      public void addScores(int[] values)
      {
         for (int i = 0; i < values.length; i++)
         {
            totalScore = totalScore + values[i];
         }
      }
    • To call this method
      int[] scores = { 10, 9, 7, 10 };
      fred.addScores(scores);
    • A method with an array return value
      public int[] getScores()
          

    Partially Filled Arrays

    • Array length = maximum number of elements in array.
    • Usually, array is partially filled
    • Define an array larger than you will need
      final int LENGTH = 100;
      double[] values = new double[LENGTH];
    • Use companion variable to keep track of current size: call it currentSize

    Partially Filled Arrays

    • A loop to fill the array
      int currentSize = 0;
      Scanner in = new Scanner(System.in);
      while (in.hasNextDouble())
      {
         if (currentSize < values.length)
         {
            values[currentSize] = in.nextDouble();
            currentSize++;
         }
      }
    • At the end of the loop, currentSize contains the actual number of elements in the array.
    • Note: Stop accepting inputs when currentSize reaches the array length.

    Partially Filled Arrays

    partially-filled array

    Partially Filled Arrays

    • To process the gathered array elements, use the companion variable, not the array length:
      for (int i = 0; i < currentSize; i++) 
      { 
         System.out.println(values[i]); 
      }
    • With a partially filled array, you need to remember how many elements are filled.
      graduated cylinder

    Self Check 7.1

    Declare an array of integers containing the first five prime numbers
    • Answer:
      int[] primes = { 2, 3, 5, 7, 11 };

    Self Check 7.2

    Assume the array primes has been initialized as described in Self Check 1. What does it contain after executing the following loop?
    for (int i = 0; i < 2; i++)
    {
        primes[4 - i] = primes[i];
    }
    • Answer: 2, 3, 5, 3, 2

    Self Check 7.3

    Assume the array primes has been initialized as described in Self Check 1. What does it contain after executing the following loop?
    for (int i = 0; i < 5; i++)
    {
       primes[i]++;
    }
    • Answer: 3, 4, 6, 8, 12

    Self Check 7.4

    Given the declaration
    int[] values = new int[10];
    write statements to put the integer 10 into the elements of the array values with the lowest and the highest valid index.
    • Answer:
      values[0] = 10;
      values[9] = 10;
           or better:
      values[values.length - 1] = 10;

    Self Check 7.5

    Declare an array called words that can hold ten elements of type String.
    • Answer: String[] words = new String[10];

    Self Check 7.6

    Declare an array containing two strings, "Yes", and "No".
    • Answer: String[] words = { "Yes", "No" };

    Self Check 7.7

    Can you produce the output on page 312 without storing the inputs in an array, by using an algorithm similar to the algorithm for finding the maximum in Section 6.7.5?
    • Answer: No. Because you don’t store the values, you need to print them when you read them. But you don’t know where to add the <= until you have seen all values.

    Self Check 7.8

    Declare a method of a class Lottery that returns a combination of n numbers. You don’t need to implement the method.
    • Answer:
      public class Lottery
      {
         public int[] getCombination(int n) { . . . }
         . . .
      }

    Make Parallel Arrays into Arrays of Objects

    • Don't do this
      int[] accountNumbers;
      double[] balances;
    • Don't use parallel arrays
      parallel arrays
    • Figure 4

    Make Parallel Arrays into Arrays of Objects

    Avoid parallel arrays by changing them into arrays of objects:
    BankAccount[] accounts;

    turn parallel arrays into objects

    Figure 5

    The Enhanced for Loop

    • You can use the enhanced for loop to visit all elements of an array.
    • Totaling the elements in an array with the enhanced for loop
      double[] values = . . .;
      double total = 0;
      for (double element : values)
      {
         total = total + element;
      }
    • The loop body is executed for each element in the array values.
    • Read the loop as for each element in values.
    • Traditional alternative:
      for (int i = 0; i < values.length; i++)
      {
         double element = values[i];
         total = total + element;
      }

    The Enhanced for Loop

    • Not suitable for all array algorithms.
    • Does not allow you to modify the contents of an array.
    • The following loop does not fill an array with zeros:
      for (double element : values)
      {
         element = 0; // ERROR: this assignment does not modify array elements
      }
    • Use a basic for loop instead:
      for (int i = 0; i < values.length; i++)
      {
         values[i] = 0; // OK
      }
    • Use the enhanced for loop if you do not need the index values in the loop body.
    • The enhanced for loop is a convenient mechanism for traversing all elements in a collection.
      conveyer belt of boxes

    Syntax 7.2 The Enhanced for Loop

    Syntax of enhanced for loop

    Self Check 7.9

    What does this enhanced for loop do?
    int counter = 0;
    for (double element : values)
    {
       if (element == 0) { counter++; }
    }
    • Answer: It counts how many elements of values are zero.

    Self Check 7.10

    Write an enhanced for loop that prints all elements in the array values.
    • Answer:
      for (double x : values)
      {
         System.out.println(x);
      }

    Self Check 7.11

    Write an enhanced for loop that multiplies all elements in a double[] array named factors, accumulating the result in a variable named product.
    • Answer:
      double product = 1;
      for (double f : factors)
      {
         product = product * f;
      }

    Self Check 7.12

    Why is the enhanced for loop not an appropriate shortcut for the following basic for loop?
    for (int i = 0; i < values.length; i++) { values[i] = i * i; }
    • Answer: The loop writes a value into values[i]. The enhanced for loop does not have the index variable i.

    Common Array Algorithm: Filling

    • Fill an array with squares (0, 1, 4, 9, 16, ...):
      for (int i = 0; i < values.length; i++) 
      { 
         values[i] = i * i; 
      }

    Common Array Algorithm: Sum and Average

    • To compute the sum of all elements in an array:
      double total = 0; 
      for (double element : values) 
      { 
         total = total + element; 
      }
    • To obtain the average:
      double average = 0;
      if (values.length > 0) { average = total / values.length; }

    Common Array Algorithm: Maximum or Minimum

    • Finding the maximum in an array
      double largest = values[0];
      for (int i = 1; i < values.length; i++)
      {
         if (values[i] > largest)
         {
            largest = values[i];
         }
      } 
              
      • The loop starts at 1 because we initialize largest with values[0]. line of children
    • Finding the minimum: reverse the comparison.
    • These algorithms require that the array contain at least one element.

    Common Array Algorithm: Element Separators

    • When you display the elements of an array, you usually want to separate them:
      Ann | Bob | Cindy
    • Note that there is one fewer separator than there are elements
    • Print the separator before each element except the initial one (with index 0):
      for (int i = 0; i < names.size(); i++) 
      { 
         if (i > 0) 
         { 
            System.out.print(" | "); 
         } 
         System.out.print(names.value[i]); 
      }
    • To print five elements, you need four separators.
      toe separators

    Common Array Algorithm: Linear Search

    • To find the position of an element:
      • Visit all elements until you have found a match or you have come to the end of the array
    • Example: Find the first element that is equal to 100
      int searchedValue = 100;
      int pos = 0;
      boolean found = false;
      while (pos < values.length && !found)
      {
         if (values[pos] == searchedValue)
         {
            found = true;
         }
         else
         {
            pos++;
         }
      }
      if (found) { System.out.println("Found at position: " + pos); }
      else { System.out.println("Not found"); }

    Common Array Algorithm: Linear Search

    • This algorithm is called a linear search.
    • A linear search inspects elements in sequence until a match is found.
    • To search for a specific element, visit the elements and stop when you encounter the match.
      searching letters

    Common Array Algorithm: Removing an Element

    Problem: To remove the element with index pos from the array values with number of elements currentSize.
    • Unordered
      1. Overwrite the element to be removed with the last element of the array.
      2. Decrement the currentSize variable.
      values[pos] = values[currentSize - 1];
      currentSize--;
    • Removing an element from an unordered array

      Figure 6 Removing an Element in an Unordered Array


    Common Array Algorithm: Removing an Element

    • Ordered array
      1. Move all elements following the element to be removed to a lower index.
      2. Decrement the variable holding the size of the array .
      for (int i = pos + 1; i < currentSize; i++)
      {
         values[i - 1] = values[i];
      }
      currentSize--;
    • removing an element from an ordered array

      Figure 7 Removing an Element in an Ordered Array

    Common Array Algorithm: Inserting an Element

    • If order does not matter
      1. Insert the new element at the end of the array.
      2. Increment the variable tracking the size of the array.
      if (currentSize < values.length) 
      {
         currentSize++ 
         values[currentSize -1 ] = newElement; ; 
      }
    • inserting an element in an unsorted array

      Figure 8 Inserting an Element in an Unordered Array

    Common Array Algorithm: Inserting an Element

    • If order matters
      1. Increment the variable tracking the size of the array.
      2. Move all elements after the insertion location to a higher index.
      3. Insert the element .
      if (currentSize < values.length)
      {
         currentSize++;
         for (int i = currentSize - 1; i > pos; i--)
         {
            values[i] = values[i - 1];
         }
         values[pos] = newElement;
      }
    • Inserting an element into an ordered array

      Figure 9 Inserting an Element in an Ordered Array

    Common Array Algorithm: Swapping Elements

    • To swap two elements, you need a temporary variable.
      ""
    • We need to save the first value in the temporary variable before replacing it.
      double temp = values[i];
      values[i] = values[j];
    • Now we can set values[j] to the saved value.
      values[j] = temp;

    Common Array Algorithm: Swapping Elements

    swapping

    Figure 10 Swapping Array Elements

    Common Array Algorithm: Copying an Array

    • Copying an array variable yields a second reference to the same array:
      double[] values = new double[6]; 
      . . .  // Fill array 
      double[] prices = values;  ""
    • To make a true copy of an array, call the Arrays.copyOf method:
      double[] prices = Arrays.copyOf(values, values.length);  
    • Copying arrays

      Figure 11 Copying an Array Reference versus Copying an Array

    • To use the Arrays class, you need to add the following statement to the top of your program
      import java.util.Arrays;

    Common Array Algorithm: Growing an Array

    • To grow an array that has run out of space, use the Arrays.copyOf method:
    • To double the length of an array
      double[] newValues = Arrays.copyOf(values, 2 * values.length); ""
      values = newValues;); ""
      growing an array
    • Figure 12 Growing an Array

    Reading Input

    • To read a sequence of arbitrary length:
      • Add the inputs to an array until the end of the input has been reached.
      • Grow when needed.
      double[] inputs = new double[INITIAL_SIZE];
      int currentSize = 0;
      while (in.hasNextDouble())
      {
         // Grow the array if it has been completely filled
         if (currentSize >= inputs.length)
         {
            inputs = Arrays.copyOf(inputs, 2 * inputs.length); // Grow the inputs array
         }
         inputs[currentSize] = in.nextDouble();
         currentSize++;
      }
    • Discard unfilled elements.
    inputs = Arrays.copyOf(inputs, currentSize);

    section_3/LargestInArray.java

    Your browser does not support this feature Program Run:
    • Please enter values, Q to quit:
      34.5 80 115 44.5 Q
      34.5
      80
      115 <== largest value
      44.5
      
      

    Self Check 7.13

    Given these inputs, what is the output of the LargestInArray program?
    20 10 20 Q
    • Answer:
      20 <== largest value
      10
      20 <== largest value

    Self Check 7.14

    Write a loop that counts how many elements in an array are equal to zero.
    • Answer:
      int count = 0;
      for (double x : values)
      {
         if (x == 0) { count++; }
      }

    Self Check 7.15

    Consider the algorithm to find the largest element in an array. Why don’t we initialize largest and i with zero, like this?
    double largest = 0;
    for (int i = 0; i < values.length; i++)
    {
       if (values[i] > largest)
       {
          largest = values[i];
       }
    }
    • Answer: If all elements of values are negative, then the result is incorrectly computed as 0.

    Self Check 7.16

    When printing separators, we skipped the separator before the initial element. Rewrite the loop so that the separator is printed after each element, except for the last element.
    • Answer:
      for (int i = 0; i < values.length; i++)
      {
         System.out.print(values[i]);
         if (i < values.length - 1)
         {
            System.out.print(" | ");
         }
      }
      Now you know why we set up the loop the other way.

    Self Check 7.17

    What is wrong with these statements for printing an array with separators?
    System.out.print(values[0]);
    for (int i = 1; i < values.length; i++)
    {
       System.out.print(", " + values[i]);
    }
    • Answer: If the array has no elements, then the program terminates with an exception.

    Self Check 7.18

    When finding the position of a match, we used a while loop, not a for loop. What is wrong with using this loop instead?
    for (pos = 0; pos < values.length && !found; pos++)
    {
       if (values[pos] > 100)
       {
          found = true;
       }
    }
    • Answer: If there is a match, then pos is incremented before the loop exits.

    Self Check 7.19

    When inserting an element into an array, we moved the elements with larger index values, starting at the end of the array. Why is it wrong to start at the insertion location, like this?
    for (int i = pos; i < currentSize - 1; i++)
    {
       values[i + 1] = values[i];
    }
    • Answer: This loop sets all elements to values[pos].

    Problem Solving: Adapting Algorithms

    • By combining fundamental algorithms, you can solve complex programming tasks.
    • Problem: Compute the final quiz score by dropping the lowest and finding the sum of all the remaining scores.
    • Related algorithms:
      • Calculating the sum
      • Finding the minimum value
      • Removing an element
    • A plan of attack
      Find the minimum.
      Remove it from the array.
      Calculate the sum.
    • Try it out. The minimum is 4
      Scores array
    • We need to use a linear search to find the position of the minimum.

    Problem Solving: Adapting Algorithms - continued

    • Revise the plan of attack
      Find the minimum value.
      Find its position.
      Remove that position from the array.
      Calculate the sum.
    • Try it out
      • Find the minimum value of 4. Linear search tells us that the value 4 occurs at position 5.
        scores array
      • Remove it
        scores array
    • Compute the sum: 8 + 7 + 8.5 + 9.5 + 7 + 10 = 50.
    • This walk-through demonstrates that our strategy works.

    Problem Solving: Adapting Algorithms - continued

    • Inefficient to find the minimum and then make another pass through the array to obtain its position.
    • Modify minimum algorithm to remember the position of the smallest element
      int smallestPosition = 0;
      for (int i = 1; i < values.length; i++)
      {
         if (values[i] < values[smallestPosition])
         {
            smallestPosition = i;
         }
      }
    • Final Strategy
      Find the position of the minimum.
      Remove it from the array.
      Calculate the sum.

    Self Check 7.20

    Section 7.3.6 has two algorithms for removing an element. Which of the two should be used to solve the task described in this section?
    • Answer: Use the first algorithm. The order of elements does not matter when computing the sum.

    Self Check 7.21

    It isn’t actually necessary to remove the minimum in order to compute the total score. Describe an alternative.
    • Answer:
      Find the minimum value.
      Calculate the sum.
      Subtract the minimum value.

    Self Check 7.22

    How can you print the number of positive and negative values in a given array, using one or more of the algorithms in Section 6.7?
    • Answer: Use the algorithm for counting matches (Section 6.7.2) twice, once for counting the positive values and once for counting the negative values.

    Self Check 7.23

    How can you print all positive values in an array, separated by commas?
    • Answer: You need to modify the algorithm in Section 7.3.4.
      boolean first = true;
      for (int i = 0; i < values.length; i++)
      {
         if (values[i] > 0))
         {
            if (first) { first = false; }
            else { System.out.print(", "); }
         }
         System.out.print(values[i]);
      }
      Note that you can no longer use i > 0 as the criterion for printing a separator.

    Self Check 7.24

    Consider the following algorithm for collecting all matches in an array: int matchesSize = 0;
    for (int i = 0; i < values.length; i++)
    {
       if (values[i] fulfills the condition)
       {
          matches[matchesSize] = values[i];
          matchesSize++;
       }
    }
    How can this algorithm help you with Self Check 23?
    • Answer: Use the algorithm to collect all positive elements in an array, then use the algorithm in Section 7.3.4 to print the array of matches.

    Problem Solving: Discovering Algorithms by Manipulating Physical Objects

    • Manipulating physical objects can give you ideas for discovering algorithms.
      boy with toy soldiers
    • The Problem: You are given an array whose size is an even number, and you are to switch the first and the second half.
    • Example
      • This array
        first array
      • will become
        switched array

    Problem Solving: Discovering Algorithms by Manipulating Physical Objects

    • Use a sequence of coins, playing cards, or toys to visualize an array of values.
    • Original line of coins
      original coins
    • Removal of an array element
      removing coin
    • Insertion of an array element
      insert coin

    Problem Solving: Discovering Algorithms by Manipulating Physical Objects

    • Swapping array elements
      swap
    • Swap the coins in positions 0 and 4:
      swap 0 and 4
    • Swap the coins in positions 1 and 5:
      swap position 1 and 5

    Problem Solving: Discovering Algorithms by Manipulating Physical Objects

    • Two more swaps
      last two swaps
    • The pseudocode
      i = 0
      j = size / 2
      While (i < size / 2)
         Swap elements at positions i and j
         i++
         j++

    Self Check 7.25

    Walk through the algorithm that we developed in this section, using two paper clips to indicate the positions for i and j. Explain why there are no bounds errors in the pseudocode.
    • Answer: The paperclip for i assumes positions 0, 1, 2, 3. When i is incremented to 4, the condition i < size / 2 becomes false, and the loop ends. Similarly, the paperclip for j assumes positions 4, 5, 6, 7, which are the valid positions for the second half of the array.

      coins

    Self Check 7.26

    Take out some coins and simulate the following pseudocode, using two paper clips to indicate the positions for i and j.
    i = 0
    j = size - 1
    While (i < j)
       Swap elements at positions i and j
       i++
       j--
    What does the algorithm do?
    • Answer: It reverses the elements in the array.

    Self Check 7.27

    Consider the task of rearranging all elements in an array so that the even numbers come first. Otherwise, the order doesn’t matter. For example, the array

    1 4 14 2 1 3 5 6 23

    could be rearranged to

    4 2 14 6 1 5 3 23 1

    Using coins and paperclips, discover an algorithm that solves this task by swapping elements, then describe it in pseudocode.
    • Answer: Here is one solution. The basic idea is to move all odd elements to the end. Put one paper clip at the beginning of the array and one at the end. If the element at the first paper clip is odd, swap it with the one at the other paper clip and move that paper clip to the left. Otherwise, move the first paper clip to the right. Stop when the two paper clips meet. Here is the pseudocode:
      i = 0
      j = size - 1
      While (i < j)
         If (a[i] is odd)
            Swap elements at positions i and j.
            j--
         Else
            i++

    Self Check 7.28

    Discover an algorithm for the task of Self Check 27 that uses removal and insertion of elements instead of swapping.
    • Answer: Here is one solution. The idea is to remove all odd elements and move them to the end. The trick is to know when to stop. Nothing is gained by moving odd elements into the area that already contains moved elements, so we want to mark that area with another paper clip.
      i = 0
      moved = size
      While (i < moved)
         If (a[i] is odd)
            Remove the element at position i and add it at the end.
         moved--

    Self Check 7.29

    deck of cards

    Consider the algorithm in Section 6.7.5 that finds the largest element in a sequence of inputs—not the largest element in an array. Why is this algorithm better visualized by picking playing cards from a deck rather than arranging toy soldiers in a sequence?

     

    • Answer: When you read inputs, you get to see values one at a time, and you can’t peek ahead. Picking cards one at a time from a deck of cards simulates this process better than looking at a sequence of items, all of which are revealed.

    Two-Dimensional Arrays

    • An arrangement consisting of rows and columns of values
      • Also called a matrix.
    • Example: medal counts of the figure skating competitions at the 2010 Winter Olympics.
    • figure skating medal counts

      Figure 13 Figure Skating Medal counts

    • Use a two-dimensional array to store tabular data.
    • When constructing a two-dimensional array, specify how many rows and columns are needed:
      final int COUNTRIES = 7;
      final int MEDALS = 3;
      int[][] counts = new int[COUNTRIES][MEDALS];

    Two-Dimensional Arrays

    • You can declare and initialize the array by grouping each row:
      int[][] counts =
      {
         { 1, 0, 1 },
         { 1, 1, 0 },
         { 0, 0, 1 },
         { 1, 0, 0 },
         { 0, 1, 1 },
         { 0, 1, 1 },
         { 1, 1, 0 }
      };
    • You cannot change the size of a two-dimensional array once it has been declared.

    Syntax 7.3 Two-Dimensional Array Declaration

    syntax of 2-d array

    Accessing Elements

    • Access by using two index values, array[i][j]
      int medalCount = counts[3][1];
    • Use nested loops to access all elements in a two-dimensional array.
    • Example: print all the elements of the counts array
      for (int i = 0; i < COUNTRIES; i++)
      {
         // Process the ith row
         for (int j = 0; j < MEDALS; j++)
         {
            // Process the jth column in the ith row
            System.out.printf("%8d", counts[i][j]);
         }
         System.out.println(); // Start a new line at the end of the row
      }
    • Accessing an element in a 2-d array

      Figure 14 Accessing an Element in a Two-Dimensional Array

    Accessing Elements

    • Number of rows: counts.length
    • Number of columns: counts[0].length
    • Example: print all the elements of the counts array
      for (int i = 0; i < counts.length; i++)
      {
         for (int j = 0; j < counts[0].length; j++)
         {
            System.out.printf("%8d", counts[i][j]);
         }
         System.out.println();
      }

    Locating Neighboring Elements

      locating neighboring elemenrts

      Figure 15 Neighboring Locations in a Two-Dimensional Array

    • Watch out for elements at the boundary array
      • counts[0][1] does not have a neighbor to the top

    Accessing Rows and Columns

    • Problem: To find the number of medals won by a country
      • Find the sum of the elements in a row
    • To find the sum of the ith row
      • compute the sum of counts[i][j], where j ranges from 0 to MEDALS - 1.

        ith_row
    • Loop to compute the sum of the ith row
      int total = 0;
      for (int j = 0; j < MEDALS; j++)
      {
         total = total + counts[i][j];
      }

    Accessing Rows and Columns

    • To find the sum of the jth column
      • Form the sum of counts[i][j], where i ranges from 0 to COUNTRIES - 1

      int total = 0;
      for (int i = 0; i < COUNTRIES; i++)
      {
         total = total + counts[i][j];
      }
  • sum of the column
  • section_6/Medals.java

    Your browser does not support this feature Program Run:
    •         Country    Gold  Silver  Bronze   Total
               Canada       1       0       1       2
                China       1       1       0       2
              Germany       0       0       1       1
                Korea       1       0       0       1
                Japan       0       1       1       2
               Russia       0       1       1       2
        United States       1       1       0       2

    Self Check 7.30

    What results do you get if you total the columns in our sample data?
    • Answer: You get the total number of gold, silver, and bronze medals in the competition. In our example, there are four of each.

    Self Check 7.31

    Consider an 8 × 8 array for a board game:

    int[][] board = new int[8][8];

    Using two nested loops, initialize the board so that zeros and ones alternate, as
    on a checkerboard:

    0 1 0 1 0 1 0 1
    1 0 1 0 1 0 1 0
    0 1 0 1 0 1 0 1
    . . .
    1 0 1 0 1 0 1 0

    Hint: Check whether i + j is even.
    • Answer:
      for (int i = 0; i < 8; i++)
      {
         for (int j = 0; j < 8; j++)
         {
            board[i][j] = (i + j) % 2;
         }
      }

    Self Check 7.32

    Declare a two-dimensional array for representing a tic-tac-toe board. The board has three rows and columns and contains strings "x", "o", and " ".
    • Answer: String[][] board = new String[3][3];

    Self Check 7.33

    Write an assignment statement to place an "x" in the upper-right corner of the tic-tac-toe board in Self Check 32.
    • Answer: board[0][2] = "x";

    Self Check 7.34

    Which elements are on the diagonal joining the upper-left and the lower-right corners of the tic-tac-toe board in Self Check 32?
    • Answer: board[0][0], board[1][1], board[2][2]

    Array Lists

    • An array list stores a sequence of values whose size can change.
    • An array list can grow and shrink as needed.
    • ArrayList class supplies methods for many common tasks, such as inserting and removing elements.
    • An array list expands to hold as many elements as needed.
      expansion ladder

    Syntax 7.4 Array Lists

    syntax of array list

    Declaring and Using Array Lists

    • To declare an array list of strings
      ArrayList<String> names = new ArrayList<String>();
          
    • To use an array list
      import java.util.ArrayList;
    • ArrayList is a generic class
    • Angle brackets denote a type parameter
      • Replace String with any other class to get a different array list type

    Declaring and Using Array Lists

    • ArrayList<String> is first constructed, it has size 0
    • Use the add method to add an object to the end of the array list:
      names.add("Emily"); // Now names has size 1 and element "Emily"
      names.add("Bob"); // Now names has size 2 and elements "Emily", "Bob"
      names.add("Cindy"); // names has size 3 and elements "Emily", "Bob", and "Cindy"
    • The size method gives the current size of the array list.
      • Size is now 3
    • Adding an element to an array list

      Figure 17 Adding an Array List Element with add

    Declaring and Using Array Lists

    • To obtain an array list element, use the get method
      • Index starts at 0
    • To retrieve the name with index 2:
      String name = names.get(2); // Gets the third element of the array list
    • The last valid index is names.size() - 1
      • A common bounds error:
      int i = names.size();
      name = names.get(i); // Error
  • To set an array list element to a new value, use the set method:
    names.set(2, "Carolyn");
  • Declaring and Using Array Lists

    • An array list has methods for adding and removing elements in the middle.
      inserting and removing
    • This statement adds a new element at position 1 and moves all elements with index 1 or larger by one position.
      names.add(1, "Ann")
    • The remove method,
      • removes the element at a given position
      • moves all elements after the removed element down by one position
      • and reduces the size of the array list by 1.
      names.remove(1);
  • To print an array list:
    System.out.println(names); // Prints [Emily, Bob, Carolyn]
  • Declaring and Using Array Lists

    inserting elements into an array list

    Figure 18 Adding and Removing Elements in the Middle of an Array List

    Using the Enhanced for Loop with Array Lists

    • You can use the enhanced for loop to visit all the elements of an array list
      ArrayList<String> names = . . . ;
      for (String name : names)
      {
         System.out.println(name);
      }
    • This is equivalent to:
      for (int i = 0; i < names.size(); i++)
      {
         String name = names.get(i);
         System.out.println(name);
      }

    Copying Array Lists

    • Copying an array list reference yields two references to the same array list.
    • After the code below is executed
      • Both names and friends reference the same array list to which the string "Harry" was added.
      ArrayList<String> friends = names;
      friends.add("Harry");
    coping array list references

    Figure 19 Copying an Array List Reference


  • To make a copy of an array list, construct the copy and pass the original list into the constructor:
    ArrayList<String> newNames = new ArrayList<String>(names);
  • Working with Array Lists

    ArrayList<String> names =
       new ArrayList<String>();
    Constructs an empty array list that can hold strings.
    names.add("Ann");
    names.add("Cindy");
    
    Adds elements to the end.
    System.out.println(names);
    Prints [Ann, Cindy].
    names.add(1, "Bob");
    Inserts an element at index 1. names is now [Ann, Bob, Cindy].
    names.remove(0);
    Removes the element at index 0. names is now [Bob, Cindy].
    names.set(0, "Bill");
    Replaces an element with a different value. names is now [Bill, Cindy].
    String name = names.get(i);
    Gets an element.
    String last =
       names.get(names.size() - 1);
    Gets the last element.
    ArrayList<Integer> squares =
       new ArrayList<Integer>();
    for (int i = 0; i < 10; i++)
    {
       squares.add(i * i);
    }
    Constructs an array list holding the first ten squares.

    Wrapper Classes

    • You cannot directly insert primitive type values into array lists.
    • Like truffles that must be in a wrapper to be sold, a number must be placed in a wrapper to be stored in an array list.
      truffles in wrappers
    • Use the matching wrapper class.
      wrapper classes

    Wrapper Classes

    • To collect double values in an array list, you use an ArrayList<Double>.
    • if you assign a double value to a Double variable, the number is automatically “put into a box”
    • Called auto-boxing:
      • Automatic conversion between primitive types and the corresponding wrapper classes:
      Double wrapper = 29.95; 
    • Wrapper values are automatically “unboxed” to primitive types
    double x = wrapper;
       

    wrapper class variable

    Figure 20 A Wrapper Class Variable


     

    Using Array Algorithms with Array Lists

    • The array algorithms can be converted to array lists simply by using the array list methods instead of the array syntax.
    • Code to find the largest element in an array:
      double largest = values[0];
      for (int i = 1; i < values.length; i++)
      {
         if (values[i] > largest)
         {
            largest = values[i];
         }
      }
    • Code to find the largest element in an array list
      double largest = values.get(0);
      for (int i = 1; i < values.size(); i++)
      {
         if (values.get(i) > largest)
         {
             largest = values.get(i);
         }
      }

    Storing Input Values in an Array List

    • To collect an unknown number of inputs, array lists are much easier to use than arrays.
    • Simply read the inputs and add them to an array list:
      ArrayList<Double> inputs = new ArrayList<Double>();
      while (in.hasNextDouble())
      {
         inputs.add(in.nextDouble());
      }

    Removing Matches

    • To remove elements from an array list, call the remove method.
    • Error: skips the element after the moved element
      ArrayList<String> words = ...;
      for (int i = 0; i < words.size(); i++)
      {
         String word = words.get(i);
         if (word.length() < 4)
         {
            Remove the element at index i.
         }
      }
    • Concrete example
      short words
    • Should not increment i when an element is removed

    Removing Matches

    • Pseudocode
      If the element at index i matches the condition
      Remove the element.
      Else
      Increment i.
    • Use a while loop, not a for loop
      int i = 0;
      while (i < words.size())
      {
         String word = words.get(i);
         if (word.length() < 4)
         {
            words.remove(i);
         }
         else
         {
            i++;
         }
      }

    Choosing Between Array Lists and Arrays

    • For most programming tasks, array lists are easier to use than arrays
      • Array lists can grow and shrink.
      • Arrays have a nicer syntax.
    • Recommendations
      • If the size of a collection never changes, use an array.
      • If you collect a long sequence of primitive type values and you are concerned about efficiency, use an array.
      • Otherwise, use an array list.

    Choosing Between Array Lists and Arrays

    comparing arrays and arraylists

    section_7/LargestInArrayList.java

    Your browser does not support this feature Program Run:
    • Please enter values, Q to quit:
      35 80 115 44.5 Q
      35
      80
      115 <== largest value
      44.5
      

    Self Check 7.35

    Declare an array list primes of integers that contains the first five prime numbers (2, 3, 5, 7, and 11).
    • Answer:
      ArrayList<Integer> primes = new ArrayList<Integer>();
      primes.add(2);
      primes.add(3);
      primes.add(5);
      primes.add(7);
      primes.add(11);

    Self Check 7.36

    Given the array list primes declared in Self Check 35, write a loop to print its elements in reverse order, starting with the last element.
    • Answer:
      for (int i = primes.size() - 1; i >= 0; i--)
      {
         System.out.println(primes.get(i));
      }

    Self Check 7.37

    What does the array list names contain after the following statements?
    ArrayList<String> names = new ArrayList<String>;
    names.add("Bob");
    names.add(0, "Ann");
    names.remove(1);
    names.add("Cal");
    • Answer: "Ann", "Cal"

    Self Check 7.38

    What is wrong with this code snippet?
    ArrayList<String> names;
    names.add(Bob);
    • Answer: The names variable has not been initialized.

    Self Check 7.39

    Consider this method that appends the elements of one array list to another:
    public void append(ArrayList<String> target, ArrayList<String> source)
    {
       for (int i = 0; i < source.size(); i++)
       {
          target.add(source.get(i));
       }
    }
    What are the contents of names1 and names2 after these statements?
    ArrayList<String> names1 = new ArrayList<String>();
    names1.add("Emily");
    names1.add("Bob");
    names1.add("Cindy");
    ArrayList<String> names2 = new ArrayList<String>();
    names2.add("Dave");
    append(names1, names2);
    • Answer: names1 contains "Emily", "Bob", "Cindy", "Dave"; names2 contains "Dave"

    Self Check 7.40

    Suppose you want to store the names of the weekdays. Should you use an array list or an array of seven strings?
    • Answer: Because the number of weekdays doesn’t change, there is no disadvantage to using an array, and it is easier to initialize:
      String[] weekdayNames = { "Monday", "Tuesday", "Wednesday", "Thursday", “Friday”,
          "Saturday", "Sunday" };

    Self Check 7.41

    The ch07/section_7 directory of your source code contains an alternate implementation of the problem solution in How To 7.1 on page 334. Compare the array and array list implementations. What is the primary advantage of the latter?
    • Answer: Reading inputs into an array list is much easier.

    Regression Testing

    • Test suite: a set of tests for repeated testing
    • Cycling: bug that is fixed but reappears in later versions
    • Regression testing: involves repeating previously run tests to ensure that known failures of prior versions do not appear in new versions

    Regression Testing - Two Approaches

    • Organize a suite of test with multiple tester classes: ScoreTester1, ScoreTester2, ...
      public class ScoreTester1
      {
         public static void main(String[] args)
         {
            Scanner in = new Scanner(System.in);
            Student fred = new Student(100);
            fred.addScore(10);
            fred.addScore(20);
            fred.addScore(5);
            System.out.println("Final score: " + fred.finalScore());
            System.out.println("Expected: 30");
         }
      }
          
    • Provide a generic tester, and feed it inputs from multiple files.

    section_8/ScoreTester.java

    Generic tester:

    Your browser does not support this feature

    Input and Output Redirection

      Section_8/input1.txt

    30
    10
    20
    5
  • Type the following command into a shell window
    • Input redirection
    java ScoreTester < input1.txt
  • Program Run:
    Final score: 30
    Expected: 30
  • Output redirection:
    java ScoreTester < input1.txt > output1.txt
  • Self Check 7.42

    Suppose you modified the code for a method. Why do you want to repeat tests that already passed with the previous version of the code?
    • Answer: It is possible to introduce errors when modifying code.

    Self Check 7.43

    Suppose a customer of your program finds an error. What action should you take beyond fixing the error?
    • Answer: Add a test case to the test suite that verifies that the error is fixed.

    Self Check 7.44

    Why doesn't the ScoreTester program contain prompts for the inputs?
    • Answer: There is no human user who would see the prompts because input is provided from a file.