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December 12, 2013 22:28
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This is just a draft of sympy/ntheory/egyptian_fractions.py . Please tell me how should I arrange the docstrings.
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from __future__ import print_function, division | |
from sympy import Rational | |
from fractions import gcd | |
from random import randint | |
def egypt(rat,choice): | |
def egypt_greedy(): | |
""" | |
Greedy algorithm for Egyptian fraction expansion | |
Also called the Fibonacci-Sylvester algorithm | |
At each step, extract the largest unit fraction less | |
than the target and replace the target with the remainder | |
Inputs: x/y is the target fraction, not necessarily in lowest terms | |
y is any natural number | |
x is any integer strictly between 0 and y | |
References | |
========= | |
- https://en.wikipedia.org/wiki/Greedy_algorithm_for_Egyptian_fractions | |
Examples | |
======== | |
>>> from sympy import Rational | |
>>> from sympy.ntheory.egyptian_fraction import egypt_greedy | |
>>> egypt_greedy(Rational(7,12)) | |
[2, 12] | |
>>> egypt_greedy(Rational(2,3)) | |
[2, 6] | |
See Also | |
======= | |
egypt_graham_jewett(Rational) : Uses Graham and Jewett Algorithm | |
egypt_takenouchi(Rational) : Uses Takenouchi Algorithm | |
""" | |
x, y = rat.as_numer_denom() | |
if (x == 1): | |
return [y] | |
else: | |
a = (-y) % (x) | |
b = y * (int(y/x) + 1) | |
c = gcd(a, b) | |
if c > 1: | |
num, denom = int(a/c), int(b/c) | |
else: | |
num, denom = a, b | |
return [int(y/x) + 1] + egypt(Rational(num,denom),"Greedy") | |
def egypt_graham_jewett(): | |
""" | |
The algorithm suggested by the result of Graham and Jewett. | |
Note that this has a tendency to blow up: the length of the resulting expansion | |
is always 2**(x/gcd(x,y)) - 1, | |
Same arguments as egypt_greedy(Rational). | |
References | |
========= | |
- http://en.wikipedia.org/wiki/Egyptian_fraction#Modern_number_theory | |
Examples | |
======== | |
>>> from sympy import Rational | |
>>> from sympy.ntheory.egyptian_fraction import egypt_graham_jewett | |
>>> egypt_graham_jewett(Rational(2,3)) | |
[3, 4, 12] | |
>>> egypt_graham_jewett(Rational(3,7)) | |
[7, 8, 9, 56, 57, 72, 3192] | |
See Also | |
======== | |
egypt_takenouchi(Rational) : Uses Takenouchi Algorithm | |
egypt_greedy(Rational) : Uses Fibonacci-Sylvester Algorithm | |
""" | |
x, y = rat.as_numer_denom() | |
l = [y] * x | |
#l is now a list of integers whose reciprocals sum to x/y. | |
#We shall now proceed to manipulate the elements of l without | |
#changing the reciprocated sum until all elements are unique. | |
while len(l) != len(set(l)): | |
l.sort() #So the list has duplicates. Find a smallest pair | |
for i in range(len(l) - 1): | |
if l[i] == l[i + 1]: | |
break | |
#We have now identified a pair of identical elements: l[i] and l[i+1]. | |
#Now comes the application of the result of Graham and Jewett: | |
l[i + 1] = l[i] + 1 | |
l.append(l[i]*(l[i] + 1)) #And we just iterate that until the list has no duplicates. Ta da! | |
return l | |
def egypt_takenouchi(): | |
""" | |
The algorithm suggested by Takenouchi (1921). | |
Differs from the Graham-Jewett algorithm only in the handling of duplicates. | |
References | |
========== | |
- http://www.ics.uci.edu/~eppstein/numth/egypt/conflict.html | |
Examples | |
======== | |
>>> from sympy import Rational | |
>>> from sympy.ntheory.egyptian_fraction import egypt_takenouchi | |
>>> egypt_takenouchi(Rational(3,7)) | |
[4, 28, 7] | |
>>> egypt_takenouchi(Rational(7,23)) | |
[6, 12, 23, 138, 276] | |
See Also | |
======== | |
egypt_greedy(Rational) : Uses Fibonacci-Sylvester Algorithm | |
egypt_graham_jewett(Rational) : Uses Graham and Jewett Algorithm | |
""" | |
x, y = rat.as_numer_denom() | |
l = [y] * x | |
while len(l) != len(set(l)): | |
l.sort() | |
for i in range(len(l) - 1): | |
if l[i] == l[i + 1]: | |
break | |
k = l[i] | |
if k % 2 == 0: | |
l[i] = (l[i] // 2) | |
del l[i + 1] | |
else: | |
l[i],l[i + 1] = ((k + 1)//2), (k*(k + 1)//2) | |
return l | |
if choice == "Greedy": | |
return egypt_greedy() | |
elif choice == "Graham Jewett": | |
return egypt_graham_jewett() | |
elif choice == "Takenouchi": | |
return egypt_takenouchi() |
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