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Thinking in Java
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Multidimensional arrays

Java allows you to easily create multidimensional arrays:

//: c04:MultiDimArray.java
// Creating multidimensional arrays.
import com.bruceeckel.simpletest.*;
import java.util.*;

public class MultiDimArray {
  static Test monitor = new Test();
  static Random rand = new Random();
  public static void main(String[] args) {
    int[][] a1 = {
      { 1, 2, 3, },
      { 4, 5, 6, },
    };
    for(int i = 0; i < a1.length; i++)
      for(int j = 0; j < a1[i].length; j++)
        System.out.println(
          "a1[" + i + "][" + j + "] = " + a1[i][j]);
    // 3-D array with fixed length:
    int[][][] a2 = new int[2][2][4];
    for(int i = 0; i < a2.length; i++)
      for(int j = 0; j < a2[i].length; j++)
        for(int k = 0; k < a2[i][j].length; k++)
          System.out.println("a2[" + i + "][" + j + "][" +
            k + "] = " + a2[i][j][k]);
    // 3-D array with varied-length vectors:
    int[][][] a3 = new int[rand.nextInt(7)][][];
    for(int i = 0; i < a3.length; i++) {
      a3[i] = new int[rand.nextInt(5)][];
      for(int j = 0; j < a3[i].length; j++)
        a3[i][j] = new int[rand.nextInt(5)];
    }
    for(int i = 0; i < a3.length; i++)
      for(int j = 0; j < a3[i].length; j++)
        for(int k = 0; k < a3[i][j].length; k++)
          System.out.println("a3[" + i + "][" + j + "][" +
            k + "] = " + a3[i][j][k]);
    // Array of nonprimitive objects:
    Integer[][] a4 = {
      { new Integer(1), new Integer(2)},
      { new Integer(3), new Integer(4)},
      { new Integer(5), new Integer(6)},
    };
    for(int i = 0; i < a4.length; i++)
      for(int j = 0; j < a4[i].length; j++)
        System.out.println("a4[" + i + "][" + j +
            "] = " + a4[i][j]);
    Integer[][] a5;
    a5 = new Integer[3][];
    for(int i = 0; i < a5.length; i++) {
      a5[i] = new Integer[3];
      for(int j = 0; j < a5[i].length; j++)
        a5[i][j] = new Integer(i * j);
    }
    for(int i = 0; i < a5.length; i++)
      for(int j = 0; j < a5[i].length; j++)
        System.out.println("a5[" + i + "][" + j +
            "] = " + a5[i][j]);
    // Output test
    int ln = 0;
    for(int i = 0; i < a3.length; i++)
      for(int j = 0; j < a3[i].length; j++)
        for(int k = 0; k < a3[i][j].length; k++)
          ln++;
    monitor.expect(new Object[] {
      "a1[0][0] = 1",
      "a1[0][1] = 2",
      "a1[0][2] = 3",
      "a1[1][0] = 4",
      "a1[1][1] = 5",
      "a1[1][2] = 6",
      new TestExpression(
        "%% a2\\[\\d\\]\\[\\d\\]\\[\\d\\] = 0", 16),
      new TestExpression(
        "%% a3\\[\\d\\]\\[\\d\\]\\[\\d\\] = 0", ln),
      "a4[0][0] = 1",
      "a4[0][1] = 2",
      "a4[1][0] = 3",
      "a4[1][1] = 4",
      "a4[2][0] = 5",
      "a4[2][1] = 6",
      "a5[0][0] = 0",
      "a5[0][1] = 0",
      "a5[0][2] = 0",
      "a5[1][0] = 0",
      "a5[1][1] = 1",
      "a5[1][2] = 2",
      "a5[2][0] = 0",
      "a5[2][1] = 2",
      "a5[2][2] = 4"
    });
  }
} ///:~


The code used for printing uses length so that it doesn’t depend on fixed array sizes.

The first example shows a multidimensional array of primitives. You delimit each vector in the array by using curly braces:

    int[][] a1 = {
      { 1, 2, 3, },
      { 4, 5, 6, },
    };


Each set of square brackets moves you into the next level of the array.

The second example shows a three-dimensional array allocated with new. Here, the whole array is allocated at once:

int[][][] a2 = new int[2][2][4];


But the third example shows that each vector in the arrays that make up the matrix can be of any length:

    int[][][] a3 = new int[rand.nextInt(7)][][];
    for(int i = 0; i < a3.length; i++) {
      a3[i] = new int[rand.nextInt(5)][];
      for(int j = 0; j < a3[i].length; j++)
        a3[i][j] = new int[rand.nextInt(5)];
    }


The first new creates an array with a random-length first element and the rest undetermined. The second new inside the for loop fills out the elements but leaves the third index undetermined until you hit the third new.

You will see from the output that array values are automatically initialized to zero if you don’t give them an explicit initialization value.

You can deal with arrays of nonprimitive objects in a similar fashion, which is shown in the fourth example, demonstrating the ability to collect many new expressions with curly braces:

    Integer[][] a4 = {
      { new Integer(1), new Integer(2)},
      { new Integer(3), new Integer(4)},
      { new Integer(5), new Integer(6)},
    };


The fifth example shows how an array of nonprimitive objects can be built up piece by piece:

    Integer[][] a5;
    a5 = new Integer[3][];
    for(int i = 0; i < a5.length; i++) {
      a5[i] = new Integer[3];
      for(int j = 0; j < a5[i].length; j++)
        a5[i][j] = new Integer(i*j);
    }


The i*j is just to put an interesting value into the Integer.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire