AP Computer Science A Unit 6 Arrays

Unit 6: Arrays Array Creation and Access: An array is a collection of boxes / spots / items that all store the same type of value. Each spot in the array stores a value of the same type. Ex: int[] array = new int[10]; array can store 10 integers. This array is basically a collection of 10 integer variables. Spot 0 stores the 1st integer, spot 1 stores the 2nd integer, and so on. 0 1 2 3 4 5 6 7 8 9 0 0 0 0 0 0 0 0 0 0 Index positions in green Above is a visual representation of the array that is created by int[] array = new int[10]; Note that: Once an array object has been instantiated, the size may never change. To increase or decrease the size, a new array would need to be instantiated and all old values copied. Also note: The size of an array object can never change. Arrays do not have methods that allow for the removal or addition of items. In order to add or remove items, a new array would be instantiated and all old values copied to the new array. To initialize an array with values here is an example: int[] aplus = {2,7,8,234,745}; // index positions are in green And a visual representation of the array: 0 1 2 7 2 3 4 8 234 745 Indexes for arrays will always be integers and will always start with 0. Setting array values: int[] nums = new int[10]; nums[0] = 231; nums[4] = 756; nums[2] = 123; New Array: 231 0 123 0 756 0 0 0 0 0 System.out.println(nums[0]); //prints value at index position 0 System.out.println(nums[1]); //prints value at index position 1 System.out.println(nums[4]); //prints value at index position 4 System.out.println(nums[4/2]); //prints value at index position 2 Accessing Array Values- If statements can be used to access and check specific values in the array. Sometimes it is necessary to check a value in a specific location against another value in the array at a different location. The if statement below is comparing the value at spot 1 to the value at spot 2. Ex: int[] nums = {1,2,3,4,5,6,7}; if ( nums[1] == nums[2] ) out.println( "aplus" ); else out.println( "comp" ); The if statement below is attempting to check the value at spot length to see if it is 7. int[] nums = {1,2,3,4,5,6,2}; int last = nums.length; if( nums[last] == 7 ) out.println( "aplus" ); else out.println( "comp" ); The if statement is out of bounds because the last value in the array is stored at position length-1 not length. An ArrayIndexOutOfBoundsException is thrown. Traversing Arrays + Enhanced for loops (for- each loops): Using loops to print all spots in an array is a necessary approach. Array lengths could change with different input values, it is good to use a for loop based on length. If length changes, the loop will change accordingly. How the loop works: The loop variable will start at 0 and go up to the array length. The loop variable will be used to access each [spot] in the array. Ex: int[] nums = {3,2,5,1,0,6}; for(int spot=0; spot< nums.length; spot++) { System.out.println(nums[spot]); } The output would be: 3 2 5 1 0 6 Another way: The for each loop is a great thing to use when accessing array values if a spot/index variable is not needed. The for each loop below accesses all values in nums and prints each one. Each time the loop iterates, the next value from nums is pasted into the item. The for each loop will iterate as long as the structure it is connected to has values. int[] nums = {1,2,3,4,5,6,7}; for(int item : nums) { out.print(item + " "); } Output is 1 2 3 4 5 6 7 You can also use for loops to change values in the array as shown below: int[] nums = new int[6]; for(int spot=0; spot< nums.length; spot++) { nums[spot] = spot*4; } This would output: ( Index positions are in green) 0 1 2 0 4 8 3 4 5 12 16 20 You can also traverse String arrays as shown below: String[] wrds = {"cat","pig","dog"}; for(String item : wrds) { out.println(item); } This would output: ( Index positions are in green) cat pig dog Developing Algorithms Using Arrays: There are several standard algorithms that traverse arrays and process the elements within: • Find a maximum or minimum in an array • Compute the sum or mean of the values in an array • Determine whether some or all of the elements in an array have a certain property • Count the number of elements in an array satisfying a certain condition • Access all pairs of consecutive elements 39 • Shift or rotate all elements in an array to the right or left Be familiar with these !!!