Science II Arrays Li Xiong 1 Roadmap Basics of Array Number - PowerPoint PPT Presentation

CS 171: Introduction to Computer Science II Arrays Li Xiong 1

Roadmap • Basics of Array • Number guessing and Binary Search • Hw1: Number guessing game • Arraylist • OO and inheritance

Guess-a-Number Game • I am asking you to guess a number that I am thinking of between 1 and 100 • When you make a guess, I will tell you one of three things: too large, too small, your guess is correct. • Think about a strategy that leads to the fewest number of guesses on average.

Guess-a-Number Game • Assuming the correct answer is 33:

Ordered Array • An array in which data elements are sorted in ascending values. – The smallest value at index 0 – Each element is larger that its previous element • Search – The main advantage of an ordered array is that it allows faster search using binary search .

Binary Search • Goal : given a specific key, find it’s location in an ordered array if it exists.

Binary Search - BinarySearch.java // precondition: array a[] is sorted public static int rank( int key, int [] a) { int lo = 0; int hi = a.length - 1; while (lo <= hi) { // Key is in a[lo..hi] or not present. int mid = lo + (hi - lo) / 2; if (key < a[mid]) hi = mid - 1; else if (key > a[mid]) lo = mid + 1; else return mid; } return -1; }

Binary Search Cost • What’s the number of steps for binary search in a sorted array of size N?

Binary Search Cost • What’s the number of steps for binary search in a sorted array of size N? – Best case: 1 – Worst case: log N

Hw1: Number guessing game • Guess a number I am thinking of between [1000, 9999] • When you make a guess, I will tell you the number of matching digits between your guess and my number • Think about a strategy that leads to the fewest number of guesses on average.

Hw1 Discussion • Read the instructions carefully • Think before you code • Some hints and useful classes/methods – Algorithm – ArrayList – Random Number generation

Hw1 Algorithm • Initialize a list of possible numbers • Repeat – Give a random guess • think about how to generate a random index – Update the list based on input matching digits • think about how to eliminate impossible numbers (read the hint) • Until the list is empty or the guess is correct

ArrayList • ArrayList – resizable-array implementation • Use generics - parameterized types – Type parameters have to be instantiated as reference types – Autoboxing: Automatically casting a primitive type to a wrapper type – Auto-unboxing: automatically casting a wrapper type to a primitive type ArrayList<Integer> numbers = new ArrayList<Integer>(); numbers.add(1001); int mynumber = numbers.remove(0);

ArrayList • Useful methods – add(E e): Appends the specified element to the end of this list – size(): returns the number of elements in this list – remove(int index): Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices) – get(int index): Returns the element at the specified position in this list. ArrayList<Integer> numbers = new ArrayList<Integer>(); numbers.add(1001); numbers.add(1002); int n = numbers.size(); int mynumber = numbers.get(0); mynumber = numbers.remove(0); mynumber = numbers.get(0);

Random Number Generation • Method static double Math.random() - returns a random double value between [0.0, 1.0) • Examples – Generate a random number between [0, 100) int index = (int) Math.random() * 100; – Generate a random integer between [a, b) int x = a+ (int) (Math.random() * (b-a));

Roadmap • Basics of Array • Number guessing and Binary Search • Hw1: Number guessing game • OO and inheritance