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NIL is a constant (think of it as the zero of list structures).
NULL is a Boolean function or predicate that test to see if it's argument is NIL.
However you will not need to write any for CSci620.
If you are really curious see
[ scheme.lsp ]
and notice the weird way the ",(.....)" is an inverse
You don't have to figure it all out!
Does the Quadratic Equation functions in the handout work?
I thought they did. This is copied and pasted from the copy
on my laptop hard disk
[ quad.lsp ]
that I downloaded and tested.
Explain the definition of NTH in the handout
Hmmmm there may be an error! It looks like I fixed this in my
laptop but did not get the latest version duplicated. The
NTH is a function that picks out an item in a list as if it was an array:
Page 170 why is (4, 5, 6) in the list and nonatomic.
The list is
But (4,5,6) starts with a "(" and can not be an atomic
value. Atomic values are numbers and words.
Does the blip "'" do anything?
It is a special abbreviation:
At the bottom of page 171, is there a blip missing?
No. You can leave off a blip on a single number because the
value of a number is itself:
Functions like cadar are also OK and in this case extracts the first item of the tail of the first item of a list.
It is worth drawing the tree structures and tracing the a/d paths. I will be giving you a function that takes it's data apart like this:
> (show '( ( a b ) 2 ( x y )))
cr = ((A B) 2 (X Y))
car = (A B)
caar = A
cdar = (B)
cadar = B
cddar = NIL
cdr = (2 (X Y))
cadr = 2
cddr = ((X Y))
caddr = (X Y)
caaddr = X
cdaddr = (Y)
cadaddr = Y
cddaddr = NIL
cdddr = NIL
Why is (car(cdr(car '((1 2 3 4 ) 5 6 7))) equal to 2?
You can try to write a translator from infix or postfix to prefix if you wish. We will see an interpretter for postfix expressions in the near future.
Why do we use T in place of ELSE
First: LISP was invented in the 1950's and the idea or "ELSE"
was invented in the 1960's (Algol 60). Second, people had got into
the habit of using T. Third, T is used in too much code for the
language to lose it. Fourth, Scheme uses ELSE! Finally, you save
3 characters by using T in place of ELSE.
If you want to fix it try
So for example
> (cons 1 2)
(1 . 2)
> (apply 'cons '(1 2))
(1 . 2)
> (funcall 'cons 1 2)
(1 . 2)
> (funcall cons 1 2)
error: unbound variable - CONS
We use one or the other when we have an expression that returns a function. For example: (lambda(x) (* x x)).
[ fun.lsp ]
(stupid funcalls "make it so"),
[ apply.lsp ]
(experiments with apply),
[ sortfun.lsp ]
(using apply for a version of quicksort).
Are there any object oriented aspects to LISP
Scheme provides a neat way to create an object that is
surrounded by functions that manipulated. There may be more to
it than I've heard.
Common Lisp had objects added to it in the 1980's creating
CLOS is a very dynamic object oriented language: full inheritance and
polymorphism, PLUS the ability for classes to gain methods as
a program runs, and for objects to change classes(I think). I have
an examples in
[ Objects%20in%20XLISP in index ]
What was the machine with a CAR and CDR?
I think it was an early IBM Mainframe: IBM 701 or 704. This was
when machines (in the states) didn't have names but numbers.
When we end a COND with T is this like a default in a switch?
What is the difference between Common LISP and Scheme?
Very different philosophies. Common LISP has everything that was in
any popular version of LISP. Scheme has the absolute minimum.
Scheme also introduce static scoping.
Finally, Scheme changed some function names:
|And so on|
Adding variables and assignments and we move away from functional programming.
Why is functional programming good?
See Strachey's classic paper in the 1966 Scientific American.
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History of Functional Programming
Strachey's work had lead to a very powerful language called CPL.
There are very few interpreters or compilers for it.
However, it inspired Basic BCPL and Functional Decomposition methods for data processing systems!
In turn BCPL was used (with Algol 68), at the AT&T Bell Labs (now Lucent) as a basis for the language B, which was superceded by C as the UNIX system programming language. You will notice that C makes a lot of use of functions, even tho' it also includes imperative features like assignments.
Exercises based on expressions and function in reading:
[ lab14.html ]
Study Chapter 7 pages 178-185 (more LISP)
[ 15.html ]
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