Lecture 17 - March 20^th 2007

Reminder: if you haven't seen Professor Harris to apply to be a major, you should do it right away.

Talked about the life and death of John Backus, leader of the team that developed FORTRAN... See articles at

URLs below:

http://www...nytimes...com/2007/03/20/business/20backus...html?_r=1&ref=technology&oref=slogin

http://www.msnbc.msn.com/id/17704662/

Went back to Lecture 16 notes and covered the following

Here’s another: The set of all (finite-length ) strings of symbols over a finite alphabet A is denoted by A*... The recursive definition of A* is

1. The empty string l (string with no symbols) belongs to A*

2. Any single member of A belongs to A*

3. If x and y are strings in A*, so is xy, the concatenation of strings x and y...

Here’s another In a given programming language, identifiers can be alphanumeric strings of arbitrary length but must begin with a letter... A recursive definition for the set of such strings is

1. A single letter is an identifier...

2. If A is an identifier, so is the concatenation of A and any letter or digit...

A more symbolic notation for describing sets of strings that are recursively defined is called Backus-Naur form or BNF, which was originally developed by John Backus and Peter Naur to define the programming language ALGOL...

General Characteristics of Recursive methods

· Recursive methods get their solutions by calling themselves...

· They have what is known as a stopping case which keeps them from calling themselves forever...

· They call themselves with a smaller problem each time...

Most common problem:

· Infinite recursion

· Caused by

o Omitting stopping case

o Not having successive cases getting smaller

You should review Dr... Bernstein's notes and the Big Java notes on recursion found in Lecture16...

...
...	...
...	...	...
...	...	...	...
...	...	...	...	...
...	...	...	...	...	...
...	...	...	...	...	...	...

We then discussed three different ways of computing the area of the shaded boxes.

1. recursively - discussion and code here

2. iteratively - using a FOR loop

3. using the formula n(n+1)

2

The rest of the class was devoted to the following 3 group activities.

1. determining the output of the substring example

TestSubstring.doc

2. determining the output of the call to mystery ("DELIVER")

public static String mystery.doc

3. determining the output of the call to goFigure (60)

public static int goFigure.doc

4. determining the output of the call to scramble("BARBARA")

public static String scramble (String str)

{

if (str.length() >= 2)

{

int n = str.length() / 2;

str = scramble(str.substring(n)) + str.substring (0,n);

}

return str;

} // end scramble

You need to study and understand all of this code in order to do tomorrow's lab.