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Held: Friday, January 26, 2007
Summary:
Today we begin your exploration of the Scheme programming language (and
of algorithm development) in earnest.
Related Pages:
Due
Notes:
- A few of you have asked about extra credit. I will distribute a checklist at the end of the semester, and you can check what you've done. (How's that for an honor system?)
- Last-minute recommendations ate my Thursday. Expect responses to HW1 and HW2 over the weekend.
- There are two readings for Monday.
- HW3 is now ready.
Overview:
- Why use programming languages?
- Scheme basics.
- Scheme history.
- Lab.
- In this class, you will express your algrorithms in Scheme.
- Some people wonder why we need computer languages like Scheme, Pascal,
C, Java, and the ilk.
- 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.
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 to some arguments,
you write:
- an open parenthesis;
- the name of the procedure;
- a space;
- the parameters to the procedure (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.
-
+: 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 about 1960.
- 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 managment 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.
- Now that you have learned DrScheme and some background, it's time for the
Beginning Scheme lab.
- I hope to spend the last few minutes of class giving you a chance to reflect
on your first real experiences with Scheme.
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