DT249-Information Systems Research Practice Programming Revision Lecture 2 Lecturer: Patrick Browne

One-Dimensional arrays An array is an ordered collection, or numbered list, of values. All of the values in an array must be of the same type. The type of the array is the type of the values it holds, followed by the characters []. An index is used to refer to individual values in the array. If we have N values, we think of them as being numbered from 0 to N-1. Each cell of the array called num contains a value.

1 dimensional arrays class ArrayApp1 { public static void main (String[] args) { // 8 elements indexed 0 to 7 int a[] = {17,3,60,128,11,11,2,8}; for (int i=0; i<8; i++) System.out.print(a[i] + " ");}} Make the above program omit the last element. Make the above program omit the first element. If we have N values, we think of them as being numbered from 0 to N-1.

Two dimensional array Each cell of the array contains a value

Two dimensional array import javax.swing.JOptionPane; public class ArrayApp2 { //... Define named constants to centralize definitions static final int ROWS = 2; static final int COLS = 4; public static void main(String[] args) { int[][] a2 = new int[ROWS][COLS]; String output = ""; // Accumulate text here //Print array in rectangular form using nested for loops. for (int row = 0; row < ROWS; row++) { for (int col = 0; col < COLS; col++) { output += " " + a2[row][col];} output += "\n";} JOptionPane.showMessageDialog(null, output);}} Assign the row value to each element Assign the column value to each element.

Functions class FactApp { static public void main (String [ ] args) { int a = Integer.parseInt(args[0]); int f = factorial (a); System.out.println("Factorial of " + a + " is " + f); } static public int factorial (int n ) { int c = n - 1; int r; if (c > 0) r = n * factorial(c); else r = 1; return r; }} The factorial of a number N, denoted by N !, is the product of all positive numbers less than or equal to N. Takes input from command line. Change to take input from prompt. Make a function to increment an integer Make a function to add two integers

Procedures (methods) import java.util.Scanner; public class MeanApp { public static int numSamples; // Number of samples to be computed. public static float total=0.0f, // Total of all sample values. mean =0.0f; // Mean of the sample values. // Asks user for number of samples and gathers input from the keyboard. public static void gatherInput() { // Create a new KeyboardInput object for input. Scanner keyIn = new Scanner (System.in); int count; // Counts through samples. System.out.print("How many samples? :"); numSamples = keyIn.nextInt(); for (count=1; count <= numSamples; count++) {System.out.print("Sample number " + count + " :"); total += keyIn.nextFloat(); } // Do the mean calculation. mean = total / numSamples;} // Displays the results of sample mean calculations. public static void displayResults() { System.out.println("The mean of the " + numSamples + " samples is " + mean); } public static void main(String[] args){ gatherInput(); displayResults();}}

2D arrays

CSV2DArrayApp This program reads a comma separated file into a two dimensional array. Copy the program and data files, called male.csv and female.csv to your folder. The dimensions of the array are int [][] numbers = new int[13][3]; Change these to [20][20] and then to [12][3], and then to [13][2]

Sorting public static void Sort() { int list[] = {10,7,19,5,16}; sortList(list); }

Sorting do { displayList(list); listChanged = false; for (int i=0; i<list.length-1; i++) { if (list[i] > list[i+1]) {temp = list[i]; list[i] = list[i+1]; list[i+1] = temp; listChanged = true;}} } while (listChanged == true);

Sorting swapping elements temp = list[i]; list[i] = list[i+1]; list[i+1] = temp;

Sorting do { displayList(list); listChanged = false; for (int i=0; i<list.length-1; i++) { if (list[i] > list[i+1]) {temp = list[i]; list[i] = list[i+1]; list[i+1] = temp; listChanged = true;}} } while (listChanged == true);

Bubble Sort Algorithm Elements of array list during the first iteration Elements of array list during the second iteration

Bubble Sort Algorithm Elements of array list during the third iteration Elements of array list during the fourth iteration

Location Quotients To load CSV files see program CSVLocQuoApp.java Location quotients compare a local area's business composition to that of a larger area. In this case the two CSV files (male.csv and female.csv) hold the number of people employed in various industries in the Dublin area. See notes section below.