AP Computer Science A Unit 7 ArrayLists

University:
Amherst College
Course:
COSC 111 | Introduction to Computer Science I
Academic year:

2022
Views:

469

Pages:

6
Author:

Prince Sampson

Unit 7: ArrayLists Introduction to ArrayList: Arraylist is a class that houses an array. An ArrayList can store any type. All ArrayLists store the first reference at spot / index position 0. An arrayList is different from an array because its size is not set at creation and can change. To create an ArrayList, call its default constructor: ArrayList house= new ArrayList(); More examples: ArrayList words; words = new ArrayList(); //can only store string references List decNums; decNums = new ArrayList(); //can only store double references ArrayList Methods: Name: Use: add(item) Adds item to the end of the list add(spot, item) Adds item at spot- shifts items up-> set(spot, item) Put item at spot z[spot] = item get(spot) Returns the item at spot return z[spot] size() Returns the # of items in the list remove( int spot) Remove item at spot from the list These are the most important and commonly used methods when using ArrayLists. Traversing ArrayLists: Below there is an example of a standard for loop. Note that it says .size() . This returns the number of elements. Ex: for (int i=0; i ray; ray = new ArrayList(); ray.add(23); ray.add(11); for( int i = 0; i < ray.size(); i++){ out.println( ray.get( i ) ); } The output: 23 11 An example below is of a standard for each loop: for ( int item : bunchOfNums ) { out.println( item ); } The for each loop will access each reference in the list, starting with the first and ending with the last. Another example of a for each loop: List ray; ray = new ArrayList(); ray.add(23); ray.add(11); ray.add(53); for(int num : ray){ out.println(num); } This will output: 23 11 53 Some extra examples using ArrayList Methods: The add method with Strings: ArrayList vals; vals = new ArrayList(); vals.add("aplus"); vals.add("rocks"); vals.add(0, "comp"); vals.add(1, "sci"); out.println(vals); Output: [comp, sci, aplus, rocks] The set method with integers: ArrayList ray; ray = new ArrayList(); ray.add(23); ray.add(11); ray.set(1,5); ray.add(4); ray.set(0,7); ray.add(53); out.println(ray); Output: [7, 5, 4, 53] List ray; ray = new ArrayList(); ray.add(23); ray.add(11); ray.add(12); ray.add(65); out.println(ray.get(3)); out.println(ray.get(4)); Output: 65 Runtime Exception Error (Because there is no 4th index position) ArrayList ray; ray = new ArrayList(); ray.add("a"); ray.add("b"); ray.remove(0); ray.add("c"); ray.add("d"); ray.remove(0); out.println(ray); Output: [c, d] ALL ABOVE EXAMPLES FROM WESTLAKE POWER POINTS!! Developing Algorithms Using ArrayLists: Note when developing Algorithms: • The length of an ArrayList is obtained from its size() method. • Accessing the element at position i uses the .get(i) method call. • Since the size of an ArrayList can change by adding and removing elements, the algorithms previously discussed can be extended to perform these actions. • The size of an ArrayList cannot be changed within the body of an enhanced for loop; this will result in an error. Searching and Sorting: Selection sort is pretty effective for small lists, but not so effective when used on large lists. Selection sort consists of two loops: The outer loops run based on the number of items in the list. The inner loop runs to find the items that need to be moved. The inner loop either locates the spot with the smallest value or the spot with the largest value. After the inner loop completes, a swap may occur if needed. At most, selection sort will make one swap per pass. A pass is one complete execution of the inner loop. Selection sort is a “search and swap” algorithm. Example 1 of Sort Algorithm: void selectionSort( ArrayList stuff ) { for(int i=0; i< stuff.size()-1; i++){ int min = i; for(int j = i+1; j< stuff.size(); j++) { if( stuff.get(j) < stuff.get(min) ) min = j; //find location of smallest } if( min != i) { int temp = stuff.get(min); stuff.set( min, stuff.get(i) ); stuff.set( i, temp); //put smallest in pos i } } } Example 2 of Sort Algorithm: void insertionSort( ArrayList stuff) { for (int i=1; i< stuff.size(); ++i) { int val = stuff.get(i); int j=i; while( j>0&&val ray; ray = new ArrayList(); ray.add(23); ray.add(11); ray.add(66); ray.add(53); Collections.sort(ray); out.println(ray); out.println(Collections.binarySearch(ray,677)); out.println(Collections.binarySearch(ray,66)); The Output: [11, 23, 53, 66] -5 3 Example using Collections.rotate(): ArrayList ray; ray = ArrayList(); ray.add(23); ray.add(11); ray.add(53); out.println(ray); rotate(ray,2); out.println(ray); rotate(ray,2); reverse(ray); out.println(ray); The Output: [23, 11, 53] [11, 53, 23] [11, 23, 53] Example using Collections.fill(): ArrayList ray; ray = new ArrayList(); ray.add(0); ray.add(0); ray.add(0); out.println(ray); Collections.fill(ray,33); out.println(ray); The Output: [0, 0, 0] [33, 33, 33] ALL EXAMPLES FROM WESTLAKE NOTES!!

AP Computer Science A Unit 7 ArrayLists

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