Lecture 9 - Example class

Question 1

Write a recursive algorithm which takes an int n and prints a row of asterisks of length n.

Answer 1

Here we can return the String or just print either works.

1. Stopping criteria is n equals 0 print nothing and return else
2. The recursive method, a method calling itself repeatedly.

Question 2

Write a recursive algorithm that takes a string s and writes the characters vertically

Answer 2

Recursion must have 1., a stopping case and 2., a recursive method where the method is calling itself.

In this case, we can return the String or print it. Both work.

1. Stopping criteria is when there is if the length of the String =s. (Note: we can’t say look for the last point in the string because each time the method is called it doesn’t know that there was a char already printed out and that it is a substring)
2. The recursive method is to print the first character and call the vertical substring.

Question 3

When might you get a java.lang.StackOverflowError

Answer 3

When the method has been called so many times that it has ran out of space in memory.

This would happen when there is no stopping case.

Question 4

How do you write lines from a file in reverse order?

Answer 4

public class ReverseFile extends ConsoleProgram {
 public void run() {
 println("This program reverses the lines in a file.");
 BufferedReader rd = openFileReader("Enter input file: ");
 String[] lines = readLineArray(rd);
 for (int i = lines.length - 1; i >= 0; i--) {
 println(lines[i]);
 }