Running Python functions in CLISP

Question

Are there any CLISP sources available? I could only find references to Stack.

I'm trying to run a python definition in CLISP - any suggestions on how I go about doing this? I'm fairly familiar in writing definitions in CLISP, but not in python... for starters I'm looking to rewrite this from python:

def f(x):
if x <= 1/2:
    return 2 * x
if x > 1/2:
    return 2*x - 1

Just to see how CLISP deals with computing fractions as a comparison I was going to run this, which I took from an online computational algebra course:

x = 1/10
for i in range(80):
    print(x)
    x = f(x)

Grateful for any suggestions...

Cheers.

please include any links you found so we can make heads or tails of your question. what is Stack that you refer to? I assume you mean this CLISP? (as is the tag, so, probably) By sources, do you mean its source code? if so, why should you want it? or do you mean language's specs, like this? or do you mean any common lisp source code snippets to learn from? is the specific question here how to re-write your Python code as Common Lisp code? — Will Ness
– Will Ness, Commented Apr 18, 2020 at 12:02
How would you write the python code in CLISP format? I've tried but keep getting errors. Cheers. — johnny_trfc
– johnny_trfc, Commented Apr 18, 2020 at 14:54
seeing your attempt would definitely help answering. please include it in your post. — Will Ness
– Will Ness, Commented Apr 18, 2020 at 15:03

Will Ness · Accepted Answer · 2020-04-19 10:34:05Z

3

So I don't really understand what you are trying to achieve, but let's try to run the Python example first. It is badly formatted currently (maybe there was a copy-paste problem), so here it is with proper indentation:

def f(x):
    if (x <= 1/2):
        return 2 * x
    else:
        return (2 * x) - 1

x = 1/10
for i in range(80):
    print(x)
    x = f(x)

In Python 2 this prints only zeroes, because you are not working with rational numbers, but just doing an integer division that truncates to zero (followed by multiplications by 2). In Python 3 the result would be a series of floats which converge towards 1.0, because floats are approximate (thanks @ex nihilo).

CLISP is one specific implementation of the Common Lisp standard, among other ones (in no particular order Lispworks, ECL, SBCL, Allegro CL, Clozure Common Lisp, ABCL, CLASP, ...). Typically Common Lisp is abbreviated CL, or sometimes just Lisp (this is a somewhat controversial opinion), for the reason that other branches of the family decided to follow quite different philosphies and were named differently (Scheme, Clojure, or even Julia). They all belong however to the "Lisp family" of languages.

If by CLISP you mean Common Lisp then an idiomatic way to code this would be:

(defun f (x)
  (if (<= x 1/2)
      (* 2 x)
      (1- (* 2 x))))

(loop
   for i below 80
   for x = 1/10 then (f x)
   collect x)

This evaluates to⁽*⁾:

(1/10 1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 
      1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 
      1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5
      1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5
      1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 1/5 2/5 4/5 3/5 1/5 2/5 4/5)

So if you want something equivalent in Python, you need to use fractions:

from fractions import Fraction

def f(x):
    if (x <= Fraction(1, 2)):
        return 2 * x
    else:
        return (2 * x) - 1

x = Fraction(1,10)
r = list()
for i in range(80):
    r.append(x)
    x = f(x)

The resulting list is:

[Fraction(1, 10), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5), Fraction(3, 5), Fraction(1, 5), Fraction(2, 5), Fraction(4, 5)]

⁽*⁾ but

(loop
   for i below 80
   for x = 0.1 then (f x)
   collect x)

evaluates to

(0.1 0.2 0.4 0.8 0.6 0.20000005 0.4000001 0.8000002 0.6000004 0.20000076
 0.40000153 0.80000305 0.6000061 0.2000122 0.4000244 0.8000488 0.60009766
 0.20019531 0.40039063 0.80078125 0.6015625 0.203125 0.40625 0.8125 0.625 0.25
 0.5 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0)

and converting this back to rationals reveals it to be

(13421773/134217728 13421773/67108864 13421773/33554432 13421773/16777216
 5033165/8388608 838861/4194304 838861/2097152 838861/1048576 314573/524288
 52429/262144 52429/131072 52429/65536 19661/32768 3277/16384 3277/8192 
 3277/4096 1229/2048 205/1024 205/512 205/256 77/128 13/64 13/32 13/16 5/8 
 1/4 1/2 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 
 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 
 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1)

Indeed if we use 13421773/134217728 as the starting point in the loop, the resulting sequence is exactly the same.

edited Apr 19, 2020 at 10:34

Will Ness

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answered Apr 18, 2020 at 19:48

coredump

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5 Comments

johnny_trfc Over a year ago

Amazing! Thanks :). How did you develop your CLISP skills? I currently have Peter Seibel's book 'Practical Common Lisp'. Do you have any recommendations, please? Looks like my effort at writing the function was close but I had the (<= x 1/2) section the wrong way around. Much appreciated!

coredump Over a year ago

Thanks, there are a lot of books, tutorial, I think the cliki has good resources for beginners: cliki.net/Lisp%20books I mostly read a lot and practiced writing small to medium programs. CL was not my first language to learn, "your mileage may vary".

ad absurdum Over a year ago

"but just doing an integer division that truncates to zero" -- in Python 2 this would be integer division, but in Python 3 / does floating point division and yields a float when its operands are integers; OP code will run under either.

coredump Over a year ago

@exnihilo Thanks, I forgot the behaviour was different in both versions, I edited the answer.

Will Ness Over a year ago

this turned out to be a nice little thingy, huh. thanks for answering.

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