Array&Sparse Array

 

Array

1. How to create an array?

   // method 1
   //dataType[] name = value
   int[] nums;// declare an array
   // method 2
   //dataType name[] = value
   int nums[];

   1. Static initialization

      int[] a = {1,2,3}

   2. Dynamic initialization

      int[] a;
      a = new int[3];
      a[0] = 1;

2. How to use an array?

• Array in for loop:

int[] nums;//1. declare an array
nums = new int[3];//2. create an array
nums[0] = 1; //3. assign a value
nums[1] = 2;
nums[2] = 3;
System.output.println(nums[0]);//4.print a value
// 1. print all of elements 
for(int i = 0; i<nums.length;i++){
  System.output.println(nums[i]);
}
// 2. calculate the sum of an array
int sum = 0
for(int i = 0; i<nums.length;i++){
  sum+=nums[i];
}
//3. fint the max element of array
int max = nums[0];
for(int i = 0; i<nums.length;i++){
  if(array[i]>max){
    max = array[i];
  }
}

• Array in for each loop:

for(int num: nums){
  System.output.println(num)
}

• Invert an array:

public static int[] invert(int[] array){
  int[] invert = new int[array.length];
  for (int i = 0,j = invert.length-1; i array.length;i++,j--) {
            invert[i] = array[j];
        }
        return invert;
    }

3. Memory analysis

   1. Java memory

      1. heap to store new object and array

         nums = new int[3];//2. create an array
         nums[0] = 1; //3. assign a value
         nums[1] = 2;
         nums[2] = 3;

      2. stack to store variable type

         int[] nums;

4. Sparse Array - to save memory

   

Sparse array

1. To save memory

The following code illustrate how program transfer an original 2D array to a sparse array, and convert sparse array to original array by sparse array we generated as well

package Array;
​
import com.sun.xml.internal.ws.api.model.wsdl.WSDLOutput;
​
import java.lang.reflect.Array;
import java.util.Arrays;
​
public class SparseArray {
    public static void main(String[] args) {
        int[][] checkerboard = new int[11][11];
        checkerboard[1][2]=1;
        checkerboard[10][10]=2;
        System.out.println("Original checkerboard");
        Print2DArray(checkerboard);
        //get amount of value that is not 0
        int sum = 0;
        for (int i = 0; i < checkerboard.length; i++) {
            for (int j = 0; j < checkerboard.length; j++) {
                if (checkerboard[i][j]!=0){
                    sum+=1;
                }
            }
        }
        System.out.println("the number of none 0 element:"+sum);
        // create a sparse array
        int[][] SpaseArray = new int[sum+1][3];
        SpaseArray[0][0] = checkerboard.length;
        SpaseArray[0][1] = checkerboard.length;
        SpaseArray[0][2] = sum;
//        System.out.println(Arrays.toString(SpaseArray));
        int count = 0;
        for (int i = 0; i < checkerboard.length; i++) {
            for (int j = 0; j < checkerboard[i].length; j++) {
                if (checkerboard[i][j]!=0){
                    count++;
                    SpaseArray[count][0] = i;
                    SpaseArray[count][1] = j;
                    SpaseArray[count][2] = checkerboard[i][j];
                }
            }
        }
        System.out.println("Sparse Array");
        Print2DArray(SpaseArray);
        System.out.println("===================================");
        System.out.println("From Sparse Array to original Array");
        int[][] array = new int[SpaseArray[0][0]][SpaseArray[0][1]];
        for (int i = 1; i < SpaseArray.length; i++) {
            array[SpaseArray[i][0]][SpaseArray[i][1]] = SpaseArray[i][2];
        }
        
        Print2DArray(array);
    }
    //function Print2DArray
    public static void Print2DArray(int[][] array){
        for (int[] ints : array) {
            for (int anInt : ints) {
                System.out.print(anInt+"\t");
            }
            System.out.println();
        }
    }
}
​