Arrays

1An array is a numbered row of one type

An array is a fixed-length row of slots that all hold the same type. Meera's three seats are String[] seats, not seat1, seat2, seat3 as unrelated names.

The length is part of the object and does not grow. If the hall adds a seat, you need a new array.

Figure. Three slots, one type, indices 0, 1 and 2. Not three loose variables.

How it works

1. An array is a numbered row of one typeOne type, a fixed length, an index from 0. That is an array.

2A one-dimensional array is one index

String[] seats = new String[3]; makes three slots, each starting as null. seats[0] = "A12" fills the first.

A for loop from 0 to seats.length - 1 walks every seat. The type can be int, Ticket, or any other type — the index rule does not change.

Figure. String[] seats = new String[3] is three slots. seats[0] is A12, seats[1] is A13, seats[2] is A14. The printed index is 1 — the second seat.

Index 1 is the second seat

String[] seats = new String[3];
seats[0] = "A12";
seats[1] = "A13";
seats[2] = "A14";
System.out.println(seats[1]);

Coding lab. Fill three seats and print the middle runs in the app, with checks on your output.

3A two-dimensional array is a row of rows

int[][] hall = new int[2][3]; is two rows, three seats each. hall[r][c] is the seat in row r, column c.

Java stores this as an array of arrays. hall[0] is the first row, itself a one-dimensional array.

Figure. Two rows of three. hall[0][1] is the top-middle seat. Not drawn to the scale of a real hall.

Row 0, column 1 holds 200

int[][] hall = new int[2][3];
hall[0][1] = 200;
System.out.println(hall[0].length);

4A three-dimensional array is a stack of grids

A 3D array is an array of 2D arrays. int[][][] multiplex = new int[2][2][3]; can mean two screens, each a 2-by-3 hall.

multiplex[s][r][c] is screen, then row, then column. You still walk with three nested loops. Most desk work stops at 2D.

Figure. A 3D array is an array of 2D arrays. multiplex[s][r][c] is screen, then row, then column. multiplex[1][0][2] holds 180.

How it works

1. A three-dimensional array is a stack of grids3D is a grid per slice. Read the indices in the order you declared them.

Screen 1, row 0, column 2

int[][][] multiplex = new int[2][2][3];
multiplex[1][0][2] = 180;

5Sort the row, then search it

A linear search walks index 0, 1, 2, … until the seat matches or the row ends. It does not need order.

A binary search jumps to the middle of a sorted row and throws away half each time. If the row is not sorted, the answer is not merely slow — it is not defined.

Figure. A linear search walks index 0, 1, 2 until the seat matches. A binary search jumps to the middle of a sorted row and throws away half each time. If the row is not sorted, the answer is not defined.

6java.util.Arrays is the helper

The Arrays class in java.util holds static helpers for one-dimensional arrays: sort, binarySearch, fill, equals, toString.

Arrays.sort(prices) rearranges the same array. Arrays.binarySearch(prices, 200) needs that sort to have happened.

Figure. Arrays.sort rearranges the same array: 220, 180, 200. Arrays.binarySearch for 200 needs that sort to have happened.

How it works

1. java.util.Arrays is the helperArrays.sort changes the array in place. binarySearch on that result needs the sort.

sort, then binarySearch, then print

import java.util.Arrays;
int[] prices = {220, 180, 200};
Arrays.sort(prices);
int at = Arrays.binarySearch(prices, 200);
System.out.println(Arrays.toString(prices));