CS151.01 2009F Functional Problem Solving
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[CSC151 2008S (Rebelsky)]
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Back to Preconditions, Revisited. On to Numeric Recursion.
This outline is also available in PDF.
Held: Tuesday, 27 October 2009
Summary: We explore why and how one writes local recursive procedures.
Related Pages:
Notes:
Overview:
letrec
.reverse
.Iowa's Great Contribution to Programming: The Husk-and-Kernel approach
let
and let*
.
let
nor let*
works for recursive procedures.
letrec
.
let
called named let.
letrec
letrec
expression has the format
(letrec ((name1 exp1) (name2 exp2) ... (namen expn)) body)
letrec
is evaluated using the following series
of steps.
name1
through
namen
into the binding table.
(Note that no corresponding values are entered.)
exp1
through
expn
, giving you results
result1
through
resultn
.
namei
and
resulti
for each
reasonable i.
let
, except
that the order of entry into the binding table is changed.
let
let
is somewhat stranger, but is handy for
some problems.
let
has the format
(let name ((param1 exp1) (param2 exp2) ... (paramn expn)) body)
param1
...
paramn
and body body
.
name
.
exp1
through
expn
.
(letrec ((name (lambda (param1 ... paramn ) body))) (name val1 ... valn))
reverse
.
(define reverse (lambda (lst) (reverse-kernel lst null))) (define reverse-kernel (lambda (remaining so-far) (if (null? remaining) so-far (reverse-kernel (cdr remaining) (cons (car remaining) so-far)))))
reverse-kernel
a local procedure.
(define reverse (letrec ((kernel (lambda (remaining so-far) (if (null? remaining) so-far (kernel (cdr remaining) (cons (car remaining) so-far)))))) (lambda (lst) (kernel lst null))))
create a kernel and call itis so common that the named let exists simply as a way to write that more concisely.
(define reverse (lambda (lst) (let kernel ((remaining lst) (so-far null)) (if (null? remaining) so-far (kernel (cdr remaining) (cons (car remaining) so-far))))))
Back to Preconditions, Revisited. On to Numeric Recursion.
[Skip to Body]
Primary:
[Front Door]
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[Academic Honesty]
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Current:
[Outline]
[EBoard]
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Groupings:
[Assignments]
[EBoards]
[Examples]
[Exams]
[Handouts]
[Labs]
[Outlines]
[Projects]
[Readings]
References:
[A-Z]
[By Topic]
-
[Scheme Report (R5RS)]
[R6RS]
[TSPL4]
Related Courses:
[CSC151.02 2009S (Weinman)]
[CSC151.02 2009S (Davis)]
[CSC151 2008S (Rebelsky)]
Misc:
[SamR]
[MediaScript]
[GIMP]
Disclaimer:
I usually create these pages on the fly
, which means that I rarely
proofread them and they may contain bad grammar and incorrect details.
It also means that I tend to update them regularly (see the history for
more details). Feel free to contact me with any suggestions for changes.
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