4.12 2D Array Traversals

1. Exam Points

• Traverse a 2D array using loops (row-major order and column-major order):
  • for loops
  • for-each loops (enhanced for loops)
• Traverse 2D arrays and ArrayLists.
• Index out of range: ArrayIndexOutOfBoundsException.
• Analyze how many times a statement is executed.
• Note: how row indexes or column indexes change: ++, +=2, --

2. Knowledge Points

(1) Traverse 2D arrays : for Loops

• Nested iteration/loop statements are used to traverse and access elements in a 2D array.
• Since a 2D array is stored as an array of arrays, the way 2D arrays are traversed using for loops and for-each loops(enhanced for loops) is similar to 1D arrays.
• You can traverse a 2D array in:
  • row-major order: across each row, such as 1st row, then 2nd row, ...
    • Example:
      

    
    // outer loop for accessing each row
    // i represents the index of each row
    for (int i = 0; i < nums.length; i++) { 
    
        // inner loop for accessing all the columns in one row
        // j is the column index
        for (int j = 0; j < nums[0].length; j++) { 
            System.out.print(nums[i][j] + " ");
        }
    	
        // start a new line after printing all the elements in one row
        System.out.println();
    }	
    

  • column-major order: across each column, such as 1st column, then 2nd column, ...
    • Example:
      

    
    // in this example, i is the column index, j is the row index
    
    // is is the column index
    for(int i=0; i < nums[0].length; i++) {
    	
        for(int j=0; j < nums.length; j++) { // j is the row index
            System.out.print(nums[j][i] + " ");
        }
    	
        System.out.println();
    }
    

  • uniquely defined order

(2) Traverse 2D arrays : for-each Loops

• Remember: A 2D array is an array of arrays.

• Example 1: traverse an int array
  

  int[][] nums = { 
          { 1, 2, 3, 4 }, 
          { 5, 6, 7, 8 }, 
          { 9, 10, 11, 12 } 
  };
  
  // arr represents each row, each row is an 1D array 
  for (int[] arr : nums) {
  
      for (int x : arr) { // x means the current element
          System.out.print(x + " ");
      }
  
      // start a new line after printing all the elements of a row
      System.out.println();
  }
  

• Example 2: traverse a String array
  

  String[][] words = { 
          { "dog", "pig", "cat", "snake" }, 
          { "apple", "orange", "pear", "banana" },
          { "black", "white", "pink", "red" } 
  };
  
  // arr represents each row, each row is an 1D array 	
  for (String[] arr : words) {
  
      for (String x : arr) { // x means the current element
          System.out.print(x + " ");
      }
  
      System.out.println();
  }
  

• Example 3: traverse a Dog array
  

  class Dog {
  
      private String name;
      private int age;
  
      public Dog(String n, int a) {
          name = n;
          age = a;
      }
  
      public String getName() {
          return name;
      }
  
      public int getAge() {
          return age;
      }
  }
  
  public class MainClass {
  
      public static void main(String[] args) {
  
          Dog[][] dogs = new Dog[2][3];
  	
          // for an array of object references, you need to instantiate each element
          
          // create Dog objects of the first row
          dogs[0][0] = new Dog("lele",3);
          dogs[0][1] = new Dog("haha",1);
          dogs[0][2] = new Dog("hehe",4);
              
          // create Dog objects of the second row
          dogs[1][0] = new Dog("doudou",5);
          dogs[1][1] = new Dog("keke",6);
          dogs[1][2] = new Dog("koko",8);
  	
          // lele     haha   hehe
          // doudou   keke   koko
  	
          // 1. using for loops
          for(int i=0; i < dogs.length; i++) { // i: row index
  		
              for(int j=0; j < dogs[0].length; j++) { // j: column index
                  Dog d=dogs[i][j];
                  System.out.print(d.getName() + "-" + d.getAge() + "     ");
              }
  		
              System.out.println();
          }
  	
          System.out.println("----------------------------------");
  	
          // 2. using for-each loops
  	
          for(Dog[] arr : dogs) {
  		
              for(Dog d : arr) {
                  System.out.print(d.getName() + "-" + d.getAge() + "     ");
              }
  		
              System.out.println();
  	}
  }
  

(3) Traverse 2D arrays : for-each + for Loops

• You may combine a for loop and a for-each loop while traversing a 2D array.
• Example:
  

  int[][] nums = { 
      { 1, 2, 3, 4 }, 
      { 5, 6, 7, 8 }, 
      { 9, 10, 11, 12 } 
  };
  	
  // using for-each loop(outer) + for loop(inner)
  for(int[] row:nums) {
  		
      for(int j=0;j < row.length;j++) {
          System.out.print(row[j] + " ");
      }
  		
      System.out.println();
  }
  

3. Exercises

• Practice
• Solution