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IFS Common Lisp implementation #722

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Merged
merged 6 commits into from
Sep 15, 2021
Merged

IFS Common Lisp implementation #722

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7151153
first working version
Trashtalk217 Jul 3, 2020
f233beb
cleaned up the chaos-game function and added some helpfull comments
Trashtalk217 Jul 4, 2020
1cdbf03
changed the .md file and upped iterations
Trashtalk217 Jul 4, 2020
adc2294
Skeleton of the final solution
Trashtalk217 Dec 27, 2020
68cf274
Update contents/IFS/IFS.md
leios Sep 15, 2021
78045ee
Merge branch 'master' into ifs-lisp
leios Sep 15, 2021
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4 changes: 4 additions & 0 deletions contents/IFS/IFS.md
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Expand Up @@ -134,6 +134,8 @@ Here, instead of tracking children of children, we track a single individual tha
[import:5-12, lang:"python"](code/python/IFS.py)
{% sample lang="c" %}
[import:18-29, lang:"c"](code/c/IFS.c)
{% sample lang="lisp" %}
[import:5-13, lang:"lisp"](code/clisp/ifs.lisp)
{% endmethod %}

If we set the initial points to the on the equilateral triangle we saw before, we can see the Sierpinski triangle again after a few thousand iterations, as shown below:
Expand Down Expand Up @@ -203,6 +205,8 @@ In addition, we have written the chaos game code to take in a set of points so t
[import, lang:"python"](code/python/IFS.py)
{% sample lang="c" %}
[import, lang:"c"](code/c/IFS.c)
{% sample lang="lisp" %}
[import, lang:"lisp"](code/clisp/ifs.lisp)
{% endmethod %}

### Bibliography
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28 changes: 28 additions & 0 deletions contents/IFS/code/clisp/ifs.lisp
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@@ -0,0 +1,28 @@
;;;; Iterated Function System implementation

(defstruct (point (:constructor make-point (x y))) x y)

(defun chaos-game (iterations shape-points)
"Plays a chaos game with a certain shape for a determined amount of iterations"
(loop
repeat iterations
for rand-point = (svref shape-points (random (length shape-points)))
for point = (make-point (random 1.0) (random 1.0)) ; starting point
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I think this creates a new point on every iteration of the loop, which none of the other implementations do. Can you change it?

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Do you mean the (make-point (random 1.0) (random 1.0)), because that's only run once. I don't exactly understand what you mean.

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Well, there must be something here that I don't understand, because the code clearly works. I'm fine with it then.

then (make-point
(* 0.5 (+ (point-x rand-point) (point-x point)))
(* 0.5 (+ (point-y rand-point) (point-y point)))) ; every subsequent point
collect point))

(defparameter *shape-points*
(map
'vector
(lambda (e) (apply #'make-point e))
;; the backquote allows us to selectively evaluate (sqrt 0.75) with the comma
`((0 0) (0.5 ,(sqrt 0.75)) (1 0))))

;; output the data to the "out.dat" file
(with-open-file (out "out.dat" :direction :output :if-exists :supersede)
(flet ((format-point (p)
;; this is not very clean, but it's the simplest way to insert a tab into a string.
(format nil "~f~c~f" (point-x p) #\tab (point-y p))))
(format out "~{~a~%~}" (map 'list #'format-point (chaos-game 10000 *shape-points*)))))