1 Repetition structures [cont’d] Overview l Nested Loops l Sentinel-Controlled loop l Avoiding Number Format exception

2 Nested Loops l One of the statements inside a loop (for, while or do-while) could be another while loop statement – such situation is called Nested Loops. l The following example prints the indices of a 3x3 matrix: public class NestedWhileLoop { public static void main(String[] args) { int i=1,j; while(i<=3) { j=1; while(j<=3) { System.out.print(i+","+j+"\t"); j++; } System.out.println(); i++; }

3 Nested Loops (cont’d) l The following example is a for-loop version of the previous example: public class NestedForLoop { public static void main(String[] args) { int i,j; for (i=1; i<=3; i++) { for (j=1; j<=3; j++) System.out.print(i+","+j+"\t"); System.out.println(); }

4 Nested Loops (cont’d) l The following example prints a table of x y for x from 1 to 10 and y from 1 to 5: public class Table { public static void main(String[] args) { final int COLUMN_WIDTH = 7; for (int x = 1; x <= 10; x++) { for (int y = 1; y <= 5; y++) { int p = (int)Math.pow(x, y); // convert value to string String pstr = "" + p; // pad with spaces while (pstr.length() < COLUMN_WIDTH) pstr = " " + pstr; System.out.print(pstr); } System.out.println(); }

5 Nested Loops (cont’d)

6 Sentinel-Controlled Loops l It is a common practice to write loops that repeat until a particular data value is read. Such data value is called Sentinel and the loop is called sentinel-controlled loop. l A problem usually encountered in writing such loop is how to choose the sentinel – What if the sentinel is part of the data? l The following example shows how this may be solved – by choosing the sentinel to be of type character. import java.io.BufferedReader; import java.io.InputStreamReader; public class SentinelLoop { public static void main(String[] args) throws java.io.IOException { BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in)); System.out.println("Enter data (Q to finish):");

7 Sentinel-Controlled Loops (cont’d) double sum = 0; int count = 0; boolean done = false; while (!done) { String inputLine = stdin.readLine(); if (inputLine.equalsIgnoreCase("Q")) done = true; else { double x = Double.parseDouble(inputLine); sum = sum + x; count++; } if (count == 0) System.out.println("No data"); else System.out.println("Average = " + sum/count); }

8 Avoiding Number Format Exception l If a string that contains non-numeric characters is passed to the methods Integer.parseInt, Double.ParseDouble, etc, they throw NumberFormatException. l This can be solved by enclosing a try-and-catch statement inside a loop. import java.io.BufferedReader; import java.io.InputStreamReader; public class NumberFormat { public static void main(String[] args) throws java.io.IOException { BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in)); boolean inputOk = false; do { try { String inputLine = stdin.readLine(); int n = Integer.parseInt(inputLine.trim()); inputOk = true; } catch(NumberFormatException e) { System.out.println("Input Error. Try again."); } } while (!inputOk); }