Arrays

1-Dimensional Arrays

How to Declare an Array
It is declared in the following format:
type[] name: the type represents the type of variable ie. int, and the name is the name of the new array

int[] arr1; //Creates an array called arr1 that holds integer values
float[] arr2; //Creates an array called arr2 that holds floating-point values

How to Initialize the Size of an Array
In Java, you must declare the size of the array. If not, the size will always be zero.

  • type[] name;
    name = new type[size];
    You declare the array first and then you create the size of the array after the declaration.
int[] arr1; //This is the initial declaration of an array.
arr1 = new int[2]; //This sets the newly created array to size 2.
  • type[] name = new type[size]: You declare and initialize the array in the same line, and make an array of a type that has a certain size.
int[] arr2 = new int[10]; //This makes an array that can hold 10 integers.
String[] arr3 = new String[5]; //This makes an array that can hold 5 Strings.

How to Reference the Indices in an Array
The indices of an array start at 0. So, the first element of an array would be at the 0th position.
The highest position would then be size - 1.

int[] arr4 = new int[20];
arr4[1] = 20; //Sets the 2nd integer in the array to 20.
arr4[0] = 40; //Sets the 1st integer in the array to 40.

String[] arr5 = new String[25];
arr5[0] = "Hi"; //Sets the 1st indice to "Hi"

How to Find the Length in an Array
You can find the length of an array using the .length function. 

int[] arr4 = new int[20];
System.out.print(arr4.length); //This would print out 20

2-Dimensional Arrays

How to Declare a 2-Dimensional Array
Declaring a 2-dimensional array is similar to the way that the 1-dimensional array is declared. Instead of only having a "size" parameter, the 2-dimensional array has two size parameters, usually denoted with row and column.

It is declared in the following format:
type[][] name: the type represents the type of variable ie. int, and the name is the name of the new array

int[][] arr1; //Creates an array called arr1 that holds integer values
float[][] arr2; //Creates an array called arr2 that holds floating-point values

How to Initialize the Size of a 2-Dimensional Array

  • type[][] name;
    name = new type[row][col];
    You declare the array first and then you create the size of the array after the declaration.
    There are "row" x "col" elements in an array.
int[][] arr1; //This is the initial declaration of an array.
arr1 = new int[2][3]; //Sets the newly created array to 2 by 3. There are 6 elements in the array.
  • type[][] name = new type[row][col]: You declare and initialize the array in the same line, and make an array of a type that has a certain size.
int[][] arr2 = new int[10][2]; //This makes an array that can hold 20 integers.
String[][] arr3 = new String[5][3]; //This makes an array that can hold 15 Strings.

How to Reference the Indices in a 2D Array
The indice of a 1-Dimensional Array starts at 0. Similarly, the 2-Dimensional array would start at index 0 too, but since there are two size parameters, the array would start at arr[0][0]. The highest position would then be arr[size-1][size-1].

Think of a 2-Dimensional array like a table. The first index you place is the row number, the second number you place is the column number.

int[][] arr4 = new int[20][5];
arr4[19][4] = 20; //Sets the last element at row 19 and col 4 in the array to 20.
arr4[0][0] = 40; //Sets the first integer, at index [0][0] in the array to 40.

String[][] arr5 = new String[2][5];
arr5[0][0] = "Hello"; //Sets the element at row 0 and col 0 to "Hi"

How to Find the Length in an Array
To find the length of the row and the length of the column.

  • arr.length: Find the number of rows in the array "arr".
  • arr[i].length: Find the number of columns in the row "i" in the array "arr".
int[][] arr4 = new int[20][5];
System.out.println(arr4.length); //This would print out 20
System.out.println(arr4[0].length); //This would print out 5

Initializing Arrays with a List

How to Initialize in an 1-Dimensional Array with a List

You need to use the "{" and "}" to denote the beginning and the end of the new list.

  • Declare the array first and then initialize the array afterwards:
      int[] arrays;
      arrays = new int[]{w,x,y...};
    The order that the values are in the list corresponds to the index of the values in the array, ie. arrays[0] = w, and arrays[2] = y.
int[] arrays; //Declares a new array called arrays
arrays = new int[]{4,5,6,7,8}; //Initialized arrays with the values, 4, 5, 6, 7, and 8.
//arrays[0] = 4, arrays[4] = 8
  • Declare and initialize the array at the same time:
      int[] arrays = {w,x,y...};
    The order that the values are in the list corresponds to the index of the values in the array, ie. arrays[0] = w, and arrays[2] = y.
int[] arrays = {4,5,6,7,8}; //Initialized arrays with the values, 4, 5, 6, 7, and 8.
// arrays[0] = 4, arrays[4] = 8

How to Initialize in an 2-Dimensional Array with a List
The main difference between initializing a 2-Dimensional Array and a 1-Dimensional array is the addition of a second parameter for size.

Rather than just using the "{}", to denote the new list, you need to nest "{" and "}". The notation for this is usually {{a,b,c}, {d,e,f}, {g,h,i}, {j,k,l}}.

The first "{" and the last "}" denotes the entire list. The first pair of "{}" that encloses a,b, and c denotes the first row of index 0. The value a would be in index [0][0], the value b would be in [0][1], and the value b would be in [0][2]. The last grouping represented by {j,k,l} would be the fourth row, which has the index 0.

  • Declare the array first and then initialize the array afterwards:
      int[][] arrays;
      arrays = new int[][]{{w,x,y...}, {w2, x2, y2...}, ... {wz, xz, yz..};
    The order that the values are in the list corresponds to the index of the values in the array, ie. arrays[0][1] = x, and arrays[1][2] = y2.
int[][] arrays; //Declares a new array called arrays
arrays = new int[]{
  {4,5,6,7,8},
  {2,1,2,9,5}
}; //Initialized arrays with the values from the list.
//arrays[0][0] = 4, arrays[1][3] = 9
  • Declare and initialize the array at the same time:
      int[] arrays = {{w,x,y...}, {w2, x2, y2...}, ... {wz, xz, yz..};
    The order that the values are in the list corresponds to the index of the values in the array, ie. arrays[0][0] = w, and arrays[1][1] = x2.
int[][] array4 = {
  {4,5,6,7,8},
  {0,1,3,2,11}
}; //Initialized arrays with the list values.
// arrays[0][0] = 4, arrays[1][4] = 11