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Part 2 Summer 2014 Web.pdf
Part_2_Summer_2014_Web.pdf
Showing 21-26 out of 53
Part 2 Summer 2014 Web.pdf-Part II: Functional Pro...
Part_2_Summer_2014_Web.pdf-Part II: Functional Programming with LISP
Part 2 Summer 2014 Web.pdf-Part II:...
Part_2_Summer_2014_Web.pdf-Part II: Functional Programming with LISP
Page 21
21
41
Counted Loop
(
defun
abc (x y)
(
let
((result 1))
(
dotimes
(count y result)
(
setf
result (* x result))
)
)
)
(abc 2 3) ;Output: 8
abc is calculating x^y.
The value of dotimes expression is result.
42
Iterate over List
•
Iterating over a list with
dolist
:
(
dolist
(
<var> <list-to-iterate-over>
<optional-return-value>
)
<expression>
)
Page 22
22
43
Iterate over List
•
dolist
evaluates the list-to-iterate-over, then one by one sets
var
to each element in the list, and evaluates the expression.
•
dolist
then returns the optional return value, else nil if none is
provided
.
(
defun
abc (lst)
(
let
(new-list)
;initially nil:
(
let
(
(
new-list ‘()
)
)
(
dolist
(x lst)
(
setf
new-list (
cons
x new-list))
)
new-list
)
)
•
(abc '(a b c d e f g))
;Output: g f e d c b a
abc reverses the list it has as argument.
44
Functions as Objects
•
We can pass functions as arguments.
•
One function that takes a function as an argument is
apply
. It takes a function and a list of arguments for it
and returns the result of applying the function to the
arguments
:
(
apply function
+ ‘(1 2 3)) => 6
or
(
apply
#’+
‘(1 2 3)) => 6 ;shorthand notation
(
sort
(list 8 4 0 2 9 7)
#’<
) => (0 2 4 7 8 9)
(
mapcar
#’*
‘(2 3 7) ‘(5 6)) => (10 18)
Page 23
23
Lists
46
Some Operations
•
We have already seen
cons
,
car
and
cdr
.
•
What
cons
really does is combine two objects into a
two-part object.
•
Conceptually a
cons
is a pair of pointers; the first one is
the head of a list (accessible through
car
) and the
second one is the tail of the list (accessible through
cdr
).
e.g. (
setf
x (
cons
‘a
nil
))
(
car
x) => a
(
cdr
x) => nil
nil
a
a
or
Page 24
24
47
Example
(
setf
y (list ‘a ‘b ‘c))
(
cdr
y) => (b c)
a
b
c
48
a
d
b
c
Example
(
setf
z
(
list
‘a
(
list
‘b ‘c
)
‘d
)
)
;; Output: (a (b c) d)
•
This list has three elements. It just happens that the
second element of
z
is also a list.
•
z
is an example of a
nested
list (as opposed to a
flat
list)
(
car
(
cdr
z)) => (b c)
Page 25
25
49
Example
•
How to construct the list (a (b c) d e)
using cons?
•
(cons ‘a (cons (cons ‘b (con ‘c ‘()))
(cons ‘d (cons ‘e ‘()))))
50
Equality
•
Each time you call
cons
, LISP allocates a new piece of
memory with room for two pointers.
•
If we call
cons
twice with the same arguments, we get
back two values that look the same but are in fact
distinct objects.
(
eql
(
cons
‘a
nil
) (
cons
‘a
nil
))
nil
(
setf
x (
cons
‘a
nil
))
(
eql
x x) => T
(
equal
x (
cons
‘a
nil
)) => T
Page 26
26
51
Equality
•
Every value is conceptually a pointer.
(
setf
x '(a b c))
(
setf
y
x)
(
eql
x y) => T
a
c
b
x
y
52
Building Lists
•
The function
copy-list
takes a list and returns a copy
of it. The new list will have the same elements but
contained in a new address:
(setf x ‘(a b c)
y (copy-list x))
•
(defun our-copy-list (lst)
(if (atom lst) ;or (null lst)
lst ; or ‘(), but not (list ‘())
(cons (car lst) (our-copy-list (cdr lst)))))
a
c
b
x
a
c
b
y
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