CS II Lesson 7

Professor Abdul-Quader

Project 1 Discussion / Intro to Classes

Project Hints

Handle the input first!

  • End the program with an error message if the input is invalid
  • Do not let the program crash / throw an exception on its own.

Hints: while loop

  • Keep track of where you currently are (currentRow, currentCol)
  • Start at the startRow, startCol
  • Use a while loop.
  • As long as you aren’t at the endpoint, keep going.
  • Loop condition?

More Hints

Finding all possible moves:

// somewhere at the top of the program, before the loop
int[][] possibleMoves = { {2, 1}, {2, -1}, {-2, 1}, {-2, -1},
                          {1, 2}, {1, -2}, {-1, 2}, {-1, -2} };

One step away?

for (int[] move : possibleMoves) {
  if (currentRow + move[0] == endRow &&
      currentCol + move[1] == endCol) {
    // ...
  }
}

Probably should be the first thing you check (inside the while loop)! If you can’t find a move this way, then do something else.

One step away

boolean moved = false;
for (int[] move : possibleMoves) {
  if (currentRow + move[0] == endRow && currentCol + move[1] == endCol) {
    // ...
    moved = true;
  }
}
if (!moved) {
  // then use another strategy
}

Random move?

// import java.util.Random; at the top
Random r = new Random(); // near the top of the program

// in the loop:
int[] move = possibleMoves[r.nextInt(possibleMoves.length)];
// but: (currentRow + move[0], currentCol + move[1]) might not be valid?
// isValid(row, col) method?

Other issues?

boolean[][] visited = new boolean[8][8];

// every time you visit a square, maybe you can mark it as visited?
// maybe: if you come to a square you previously visited, pick a random move from there?
// lots of possibilities

Presentation 1

  • Weeks of 3/3 and 3/10: demo your project.
  • Give a “big picture” explanation of the algorithm for how you solved the problem.
  • Show / explain how your code implements that algorithm.
  • Rubric / spreadsheet sign-up on BrightSpace.

Now

  • Quick poll: where are you on the project right now?
    1. Completely done? (100% optimals paths every time)
    2. Valid paths, but not always optimal?
    3. Not getting valid paths all the time?
    4. No idea how to get any paths to work?
    5. Haven’t finished validating input?
  • Let’s spend about 30 minutes working on the projects.
  • Get help from each other / me.

Resize Array

Problem?

int[] numbers = new int[5]; // hope that 5 is big enough!
Scanner sc = new Scanner(System.in);
int index = 0;
System.out.println("Input in the first number: ");
while (sc.hasNextInt()) {
    numbers[index] = sc.nextInt();
    index++;
    System.out.println("Input in the next number: ");
}

Exercise

Write a method which takes in an integer array as a parameter, creates a new array that is twice as big, copies over all the elements of the old array and returns the new array.

public static int[] resizeArray(int[] originalArray) {
    // what goes in here?

}

Starter code.

Main method

Use it with the following main method:

int[] numbers = new int[5];
Scanner sc = new Scanner(System.in);
int index = 0;
System.out.println("Input in the first number: ");
while (sc.hasNextInt()) {
    int number = sc.nextInt();
    if (index >= numbers.length) {
        numbers = resizeArray(numbers);
    }
    numbers[index] = number;
    index++;
    System.out.println("Input in the next number: ");
}

Generalize?

  • Every time we use arrays, we need to do this?
  • Separately, for each type of array? int[] vs String[] vs int[][] etc?
  • Better way? Built-in solution:

ArrayList class

import java.util.ArrayList;
// must import the above!

// in main:
ArrayList<String> stringList = new ArrayList<>();
for(int i = 0; i < 100; i++) {
    stringList.add("hello!");
}
System.out.println(stringList.size());

ArrayList<Integer> integerList = new ArrayList<>();
for(int i = 0; i < 20; i++) {
    integerList.add(i * i);
}
System.out.println(integerList.get(15));

ArrayList class

  • Built-in class that handles “resizeable arrays”
  • Can use with any type!

Upcoming

  • Using ArrayLists.
  • ArrayList is a data type created using a class
  • Define our own data types?
  • Read chapters 9 - 11.
  • Project 1 due Sunday.
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