Frank Mitchell

Computer Science 310 notes, part 2

More notes from Bruce Bolden’s Computer Science 310 course, “Programming Languages”. Part 1 can be found someplace else.

True crimes of lexical analysis

Lexical analysis in FORTRAN

FORTRAN variable names:

Typo:

Scanner generators

Extended Backus-Naur Form (EBNF)

Adds some conventinet symbols:

Regular expression (RE or RegEx) program

Java language specification

http://java.sun.com/reference/docs/

25 February 2005

[Discussion on reusing written material, and the waiting game that’s played in an engineering environment. Next assignment is a summary of of a language that’s interesting to you, something we won’t study in class.]

Book recommendation: Who Moved My Cheese?

[More discussion on the assignment: formatting, language choice, etc.]

“Given enough time, you can do anything with string and Tinker Toys.”

Bruce Bolden

One of our next topics is lex. Read the paper about it (on the website) sometime in the near future.

28 February 2005

Check your numbers, and check the numbers they give you too.

“Some people, when confronted with a problem, think ‘I know, I’ll use regular expressions.’ Now they have two problems.”

Jamie Zawinski, in comp.lang.emacs

Regular expressions

A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions, by using various operators to combine smaller expressions.

Widely used in many programing languages and editors:

grep - The Unix tool, grep, uses regular expressions extensively to search file(s) for the specified expressions.

Usage:

grep <regex> file(s)
egrep  grep -E extended
fgrep  grep -F fixed strings (aka fast grep)

Details:

man (grep|egrep|fgrep)

Metacharacters

The caret ^ and dollar sign $ are metacharacters that respectivly match the empty string at the begining and end of a line.

A bracket expression is a list of characters enclosed by [ and ]. It matches any single character in that list; if the first character of the list is the caret ^, then it matches any character not in the list.

Operators

Regular expressions make use of three operators: concatenation, alternation, and closure.

Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole subexpression may be enclosed in parenthesis to override these precedence rules.

Repetition operators

A regular expression may be followed by one or serveral repetition operators:

Using metacharacters

In basic regular expressions, the metacharacters lose their special meaning: instead use the backslashed (escaped) versions \?, \+, \{, \}, \|, \(, \), \^, \$, \*, \., \[, \], \\ etc.

Examples

File (test.txt):

Sylvester is a cat.
Fido is a dog.
Dexter is a boy.
I like dogs, but am not particularly fond of cats.
Cats are fun to tease.

% grep cat test.txt
Sylvester is a cat.
I like dogs, but am not particularly fond of cats.

Single quotes prevent evaluation.

% grep cat.*dog test.txt
grep: No match.

% grep 'cat.*dog' text.txt
nothing

% grep dog.*cat test.txt
grep: No match.

% grep 'dog.*cat' test.txt
I like dogs, but am not particularly fond of cats.

grep [cC] [Notes incomplete here. Bruce moved on to the next slide before I could get it all down.]

Searching through your mailbox

May have to use grep instead of egrep here.

egrep '^Subject:.*CS.*310' mbox

What does egrap '^Subject:.*CS.*310' mbox | wc -l do?

Searching for variable names

How long can a variable name be in C? How long can a variable name be in Java?

Searching for money

Dollar amount with optional cents:

\$[0-9]+(\.[0-9][0-9])?

Won’t match $.49. Why?

What time is it?

Matches time of day:

[0-9]?[0-9]:[0-9][0-9] (am|pm)

Works for both 9:17 am and 12:30 pm, but also allows 99:99 pm. First digit must be a 1.

/1?[0-9]/ allows an hour of 19 and an hour of 0. So, break the hour part into two possibilities:

(1[012]|[1-9]):[0-5][0-9] (am|pm)

AM or PM? Military time?

Strings with double quotes

"[^"]*" the quotes at either end are to match the open and close quotes of the string.

Check out the O'Reilly book on regular expressions. Also see awk, Perl, Tcl, and Ruby.

2 March 2005

Discussion on Area 51, whether it does or doesn’t exist, the mutated animals that might or might not live there. All this was started by a background check done on some students.

Short little quiz on Friday. Probably a test next week.

“How to get bad student ratings. Give a test before spring break.”

Bruce Bolden

Lexical analysis

What is lexical analysis?

Process of breaking input into tokens. Sometimes called “scanning” or “tokenizing”. The lexical analyzer scans the input stream and converts sequences of characters into tokens. Identifies tokens in input string (stream).

lex

lex file format

%% is used to separate the sections.

definitions

%%

rules (regular expressions and associated actions)

%%

user routines (C functions)

Definitions section

You can put C code in your lex files.

%{
C code
%}

%definition(s)

Notes for this lecture are all online. You can grab the PDF file and take a look for yourself. Any annotations here will reference that file.

The lex library is named l, so you link it in with -ll. Kind of like the math library being named m and linking in with -lm.

Special lex variables

Once upon a time with the early C compilers, the max variable length name was eight characters.

Example 2: Name matching

“We could use Shakespeare, but most of you probably wouldn’t recognize the characters….How much Tom Clancy have you read?…Spongebob made the cut back before he was evil….South Park, now there we have truly evil.”

Bruce Bolden

Example 3: Word count (wc)

Notice the comments in the source file. You need to have comments in yours as well.

In this example, period (.) functions as a catch all.

“There’s not a lot of glory in text processing. It’s a lot cooler if you have a GUI and graphics so people go ‘Oooh’ and ‘Aaah’.”

Bruce Bolden

Example 4: Yet another world count

If you’re going to do nothing, put a comment in there.

Test next Wednesday! Maybe.

4 March 2005

For the assignment, tabs become four spaces, and a single space becomes &nbsp;.

Side note: Star Wars: Episode III comes out on May 19th.

“It’s not necessary that you always know (except maybe on a test), but that you know where to find it, and how to find it fast.”

Bruce Bolden [not an exact quote, but you get the idea]

More thoughts on the assignment: How do you want to store your keywords? Hash table, alphabetized binary trees, there are lots of options. Pick a solution that’s fast and extendable. Let keywords be keywords too. You don’t want if to be bolded in the middle of a comment.

HTML keywords: http://w3schools.com/html/html_reference.asp

C keywords: http://www.phim.unibe.ch/comp_doc/c_manual/C/SYNTAX/keywords.html

You’re encouraged to abstract away the header and footer. All the code you submit should be in a single lex file.

7 March 2005

  1. No class on Friday!
  2. lex program still due Friday.
  3. Remember written / oral reports.
  4. Test Friday after spring break.

Yacc

More with the online note goodness. Go download the Yacc PDF and follow along.

Example: A -> Bc is written in yacc as A: B 'c';

Rules

Code snippet update:

 rule1: action1
 rule2: action2
 ...
 ruleN: actionN

Right recursive vs. left recursive rules:

A :: A B // Left recursive
A :: B A // Right recursive

Side note: If you take Dr. Heckendorm’s compilers class, start your assignemnts early and send him email if you’ve got questions.

Simple calculator

The yywrap function is invoked when the program wraps up. You can put memory cleanup stuff in here. Try including it if you can’t get your program to compile.

int yywrap( void )
{
    return 1;
}

main() invokes yyparse() that’s the function built up by our yacc code.

http://dinosaur.compilertools.net/

9 March 2005

Side notes: The next generation XBox will use three 3 Ghz PowerPC chips. This means we might see a 3 GHz Mac out in a little bit. Of course the real question is can I install OS X on it?

http://wired.com/news/games/0,2101,61065,00.html?tw=wn_tophead_1

According to said Wired article, Microsoft’s going to use Virtual PC technology to provide backwards compatibility with old XBox games. No wonder they’re going to need three processors. Virtual PC runs like molasses on anything less.

The ten worst engineering pitfalls

  1. The solution is more problematic than the problem it was created to solve.
  2. Basing the design on your own motives rather than on users' needs.
  3. Neglecting to handle all possible failure cases gracefully.
  4. Failing to protect users' privacy.
  5. Expecting that users will (or should have to) read anything.
  6. Expecting that users will (or should have to) possess technical knowledge or jargon.
  7. Expecting that users will (or should have to) configure something before using it.
  8. Challenging or attempting to guess the user’s intent.
  9. Not knowing when to re-architect (either doing it pointlessly, or avoiding it when needed).
  10. Failing to make the implementation as maintainable and understandable as possible.

http://osnews.com/story.php?news_id=9915

21 March 2005

http://www.fincher.org/Misc/Pennies/

Got to go find some pennies now. Lots of pennies.

We got our assignments back. Yay! I got 23 out of 25 on the first assignment and 19 out of 20 on the second. Points docked for not handling the edge case of no solution on the maze solver and lines that were too long.

We’ve got a test this Friday. We’ll talk more about that on Wednesday. Make sure you know how to do linked lists and trees in C. Four programing related questions. Something on a short lex program; overall, no gory details. Language concepts will be back with a vengeance.

http://slashdot.org/article.pl?sid=03/03/07/173242

http://perso.wanadoo.fr/pascal.brisset/kernel3d/kernel3d.html

ML (Objective) generates the graphics.

Scheme

Lisp is the language of choice in the computational linguistics area.

Side note about life in the real world: lots of Visual Basic out there. Learn it, even if you don’t love it.

http://drscheme.org/

You want version 2.09 or whatever the current version is. This is the program that Bolden will use to test, so it’s probably what you should be writing in. It’s an integrated environment with a very nice help system. They’ve been using this for teaching at RICE University for a number of years.

http://drpython.sourceforge.net/

“No one respects size anymore. 100 megs? Oh, I’ve got time to go get a drink.”

Bruce Bolden

Examples of scheme

Statements have the form:

(function arg1 arg2...argN)

> (+ 2 3)
5

Define a symbol or function:

> (define PI 3.14159)
PI
> PI
3.14159
> (PI)
procedure application: expected procedure, given: 3.14159 (no arguments)
> pi
#i3.141592653589793 ;The #i tells you the type; in this case indeterminate.

Interesting aside:

> (+ 2 1 3)
6

Symbols don’t have a value unless they’re assigned. Names are global!

More errors and oddities.

> E
reference to undefined identifier: E

> (/ 33 99)
;Returns 0.3 with a bar over the 3.

23 March 2005

What’s on the test

Focus on terminology and concepts.

Test format:

Basic concepts:            20
    True / False
    Fill in blank

Concepts:                  20
    Short answer

C coding (4 functions):    20
    Arrays
    Strings
    Lists, Stacks, Queues
    Trees
    Files (guaranteed)

Grammar derivations:       15

Regular expressions:       10
    3 or 4 fairly simple
    No specific regex

lex file problem:          15

The test is going to be long, and it’s going to test your knowledge. Comment the code you write.

Onwards to part 3