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Held: Monday, 28 January 2008
Summary:
Today we begin your exploration of the Scheme programming language and
the environment in which you will be devleoping algorithms in Scheme.
Related Pages:
Notes:
- Sorry, my weekend got eaten by statistics. Expect responses to assignment 1 later this week.
- Are there questions on assignment 2? If you work in a group, you should just turn in one assignment for the group, with everyone's name at the top.
Overview:
- Why use programming languages?
- Scheme basics.
- Scheme history.
- Computer scientists often express their algorithms in programming
languages, languages designed by other computer scientists to
express algorithms.
- Most programming languages can also be
understood
by computers.
- In this class, you will express your algorithms in the programming
language Scheme.
- Some people wonder why we need artificial languages like Scheme, Pascal,
C, Java, and the ilk, given that we should be able to write algorithms
in English.
- In part, it's probably because the computer elite want to maintain
their sense of superiority over the masses.
- In greater part, it's because English and other
natural
languages
can be ambiguous. At the very least, they have many similar structures
that are interpreted very differently. Consider the classic pair of
- Time flies like an arrow.
- Fruit flies like a banana.
- Remember: Computers are sentient and malicious.
- From the programmer's perspective, it often seems that they'll do
their best to misinterpret whatever it is you write.
- Scheme is generally used in an interactive environment:
You type syntactically valid expressions and Scheme returns
their values.
- Later this semester, we'll see how to use Scheme in different
environments.
- Scheme's syntax is fairly simple: Since almost everything in
Scheme involves the application of a function (a.k.a. operation, a.k.a.
procedure) to some arguments (a.k.a. parameters),
you write:
- an open parenthesis;
- the name of the function;
- a space;
- the arguments to the function (separated by spaces); and
- a close parenthesis.
- For example,
(+ 2 3 4)
(sqrt 4)
(expt 2 3)
- If you remember this basic syntactic structure, you'll rarely get
syntactic errors from Scheme.
- And if you believe that claim, I have a bridge to sell you.
-
+: Sum its parameters.
-
*: Compute the product of its parameters.
-
-: Subtract the second parameter from the first.
-
(sqrt val): Compute the square root of val.
-
(expt v p): Compute vp.
-
(abs v): Compute the absolute value of v.
-
(max v1 v2 ... vn): Determine the largest of the given values.
-
(min v1 v2 ... vn): Determine the smallest of the given values.
- Scheme is a variant of Lisp (the List processing language).
- Lisp is one of the oldest high-level programming languages in
active use
- It's from the late 1950's.
- That is, it's from about the time of the invention of Cobol and only a few
years after the invention of Fortran.
- John McCarthy (of MIT and then Stanford) designed Lisp to provide a
language for programming in artificial intelligence.
- At the time, many people believed that intelligence was grounded
in symbolic processing.
- Lisp added many things to languages of the day:
- Symbolic values
- Dynamic lists as built-in data structures
- Automatic memory management for those built-in data structures
- Functions as values (we'll return to this in a few weeks)
- In later reflection, McCarthy indicated that some of these things
were just luck. For example, the
lambda that you'll
learn about soon was just a hmmm ... that sounds interesting, I'll
put it in
fluke.
- Scheme was designed in the 1970's as
a variant of Lisp more appropriate for teaching computer science
(and doing many other things; it's more elegant).
- Scheme has a slightly clearer semantics than Lisp (and a formal
semantics, which is important).
- Scheme adds some nifty features for advanced programmers.
- But, at least for 151, you're doing Lisp-like programming.
- A variant of Scheme called Script-Fu was added to the Gimp in its
early development, so it is tightly integrated with the Gimp.
- Now that you have learned DrScheme and some background, it's time for the
lab.
- I hope to spend the first few minutes of the next class giving you a
chance to reflect on your first real experiences with Scheme.
;
