Chapter 6: Repetition

Continued

2

Validity ChecksWhat’s weak about the following code?

do

{

s1 = JOptionPane.showInputDialog (“Enter a number: ”);

idNum = Integer.parseInt(s1);

}

While (idNum < 100 || idNum > 1999);

3

Alternative Codedo

{

s1 = JOptionPane.showInputDialog (“Enter a number: ”);

idNum = Integer.parseInt(s1);

if (idNum < 100 || idNum > 1999)

JOptionPane.showMessageDialog(null, “Error bla bla bla”, JOptionPane.ERROR_MESSAGE);

else

break; // a valid id num was entered

} while (true); // expression is always true

4

Recursion

• It is possible for a method to call itself• Methods that call themselves are referred to as:

– Self-referential methods

– Recursive methods

• Direct recursion– A method invokes itself

• Indirect or mutual recursion– A method can invoke a second method, which in turn

invokes the first method

5

Mathematical Recursion

• The recursive concept is that the solution to a problem can be stated in terms of “simple” versions of itself

• Factorial of number n:

– Denoted as n!, where n is a positive integer

1! = 1

n! = n * (n * 1)! for n > 1

6

Mathematical Recursion (continued)

• General considerations that must be specified include:

– What is the first case?

– How is the nth case related to the (n - 1) case?

7

Pseudocode for Method factorial

If n = 1

factorial = n

Else

factorial = n * factorial(n - 1)

8

Recursive Example

public class Recursive{ public static void main(String[] ags) { int n = 3; long result;

result = factorial(n); System.out.println("The factorial of " + n + " is " + result); }

public static long factorial(int n) { if (n == 1) return (n); else return (n * factorial(n-1)); }}

9

How the Computation Is Performed

• A Java method can call itself due to:

– Java’s allocation of new memory locations for all method arguments and local variables as each method is called

• The allocation is made dynamically in a memory area referred to as the stack

• The stack is memory used for rapidly storing and retrieving data

10

Recursion Versus Iteration

• The recursive method can be applied to any problem in which the solution is represented in terms of solutions to simpler versions of the same problem

• Recursive methods can always be written in a non-recursive manner using an iterative solution

11

Recursion Versus Iteration (continued)

• If a problem solution can be expressed iteratively or recursively with equal ease, the iterative solution is preferable because it:– Executes faster– Uses less memory

• There are times when recursive solutions are preferable– Some problems are easier to visualize using a recursive

algorithm– Sometimes a recursive solution provides a much simpler

solution

12

Applications: Random Numbers and Simulations

• Random numbers– Series of numbers whose order cannot be predicted

– Hard to find in practice

• Pseudorandom numbers– Sufficiently random for task at hand

• Java compilers provide general-purpose method for creating random numbers – Defined in the Math class

– Named random()

13

public class RandomNumbers{ public static void main(String[] args) { double randValue; int i;

for (i = 1; i <= 10; i++) { randValue = Math.random(); System.out.println(randValue); } } }

14

Scaling

• A method for adjusting random numbers produced by a random number generator to reside within ranges, such as 1 to 100

– Accomplished using the expression:

(int) (Math.random() * N)

15

Simulations

• Common use of random numbers:

– Simulate events rather than going through time and expense of constructing a real-life experiment

• Coin Toss Simulation

– Use random number generator to simulate coin tosses

• Elevator Simulation

– Simulate the operation of an elevator

16

Common Programming Errors

• Creating a loop that is “off by one”• Repetition statements should not test for equality

when testing floating-point (real-values) operands• Placing a semicolon at the end of either a while or

for loop’s parentheses• Using commas to separate items in a for statement

instead of the required semicolons• Omitting a final semicolon from a do-while

statement

17

Summary

• A section of repeating code is referred to as a loop

– Types of loops:

• while

• for

• do-while

• A while statement checks a condition before any other statement in a loop

18

Summary (continued)

• A for statement is extremely useful in creating loops that must be executed a fixed number of times

• A do-while statement checks its expression at the end of the loop