An introduction to arrays

Problem: a hotel reservation system Say we have a hotel of 100 rooms and we need to know if the rooms are occupied or not. What data type should we use to indicate whether or not a particular room is occupied?

One solution: Let’s declare 100 boolean variables (one for each room). //false will indicate that the room is not // occupied boolean room1 = false; boolean room2 = false; … boolean room100 = false;

How can we determine if a particular room is occupied?

How do we determine if a particular room is occupied (w/ an if)? int which = in.nextInt(); if (which==1) { if (room1) System.out.println( “ occupied” ); else System.out.println( “ not occupied” ); } else if (which==2) { if (room2) System.out.println( “ occupied” ); } else if (which==3) { … } else if (which==100) { if (room100) System.out.println( “ occupied” ); }

Is a switch any better?

How do we determine if a particular room is occupied (w/ a switch)? int which = in.nextInt(); switch (which) { case 1: if (room1) System.out.println( “ occupied” ); else System.out.println( “ not occupied” ); break; case 2: if (room2) System.out.println( “ occupied” ); case 3: … case 100: if (room100) System.out.println( “ occupied” ); }

What if we want to determine the # of rooms that are occupied?

What if we want to determine the # of rooms that are occupied? int count = 0; if (room1) ++count; if (room2) ++count; … if (room100) ++count; System.out.println( count + " rooms are occupied." );

More hotel problems: What if we add more rooms to our hotel? What if we want to see if we have a block (say 3) of adjacent rooms available? Our current approach quickly becomes unwieldy! So let’s introduce something new (arrays) to help us.

Recall scalars and vectors from math. Scalar – one number Vectors – list of numbers Vector X = < 1, -2, 52 > Or more generally, X = < x1, x2, x3 > subscript

Arrays List of the same type of things. Subscript (numbering) starts with 0. How do we declare an array? int x[] = new int [ 100 ]; How do we refer to a particular int? x[0] = 1; //first one x[1] = 7; … x[99] = 52; //last one

Hint: Remember that an array of 100 elements is subscripted by 0..99.

What happens if I try the following? int ray[] = new int [100]; // ok System.out.println( ray[-1] ); // ? int k = ray[100]; // ?

What happens if I try the following? int ray[] = new int [100]; System.out.println( ray[-1] ); int k = ray[100]; Boom!

How long (how many elements) is an array? Ask it! int howLongIsMyArray = a.length; (Reminds one of .length() and Strings.)

Back to the room reservation problem (now using arrays). It becomes much easier! How can we use arrays?

Back to the room reservation problem (now using arrays). It becomes much easier! final int N = 100; //# of rooms boolean rooms[] = new boolean[ N ]; //init all rooms to unoccupied How?

Back to the room reservation problem (now using arrays). It becomes much easier! final int N = 100; //# of rooms boolean rooms[] = new boolean[ N ]; //init all rooms to unoccupied for (int i=0; i<N; i++) { rooms[i] = false; }

Back to the room reservation problem (now using arrays). It becomes much easier! final int N = 100; //# of rooms boolean rooms[] = new boolean[ N ]; //init all rooms to unoccupied for (int i=0; i<N; i++) { rooms[i] = false; } What happens is we use <= instead of <?

Back to the room reservation problem (now using arrays). It becomes much easier! final int N = 100; //# of rooms boolean rooms[] = new boolean[ N ]; //init all rooms to unoccupied for (int i=0; i<N; i++) { rooms[i] = false; } I keep forgetting if false means occupied or not. Can you suggest a change to make it clearer?

Back to the room reservation problem (now using arrays). It becomes much easier! final int N = 100; //# of rooms boolean rooms[] = new boolean[ N ]; final boolean occupied = true; //init all rooms to unoccupied for (int i=0; i<N; i++) { rooms[i] = false; } I keep forgetting if false means occupied or not. Can you suggest a change to make it clearer? Here’s a hint.

Back to the room reservation problem (now using arrays). It becomes much easier! final int N = 100; //# of rooms boolean rooms[] = new boolean[ N ]; final boolean occupied = true; //init all rooms to unoccupied for (int i=0; i<N; i++) { rooms[i] = !occupied; } I keep forgetting if false means occupied or not. Can you suggest a change to make it clearer? Here’s a hint.

Interesting code to write: Write a piece of code to find a free room. It should determine the free room #. (It should yield -1 if no rooms are free.) Can you write this code?

Interesting code to write: Write a piece of code to randomly find a free room. It should determine the free room #. It should yield -1 if no rooms are free. Why is this a better approach in practice (in the “real world”)? Can you write this code?

Interesting code to write: Write a piece of code to count the total number of free rooms. Or conversely, write a piece of code to count the number of rooms in use. Can you write this code?

Interesting code to write: Write a piece of code to find a block of 3 free rooms. It should determine the lowest room number of the 3 free rooms (otherwise, -1). Can you write this code?

Interesting code to write: Use another array for wakeup calls. Use another array to indicate whether or not a room allows smoking or not. Can you write this code?