Decimals to fractions, and some other stuff i was playing with and forgot about.

dec2frac.py

# https://www.quora.com/How-do-you-determine-sum-limits_-n-1-infty-frac-n+1-4-n-by-hand/answer/Job-Bouwman

from __future__ import division, print_function  # Fix Python 2.

def dec2frac(f):
	"From scratch, 7dec15. Handles negative input since 9apr17."
	of = f
	f = abs(f)
	n = 1
	d = 2
	while 1:
		diff = n/d - f
		#print(n, d, diff, f)
		if abs(diff) < 0.00001: break
		if diff > 0:
			n -= 1
			d += 1
		else: n += 1
	return (-n if of < 0 else n, d)

print(dec2frac(0.235))  # => 47/200
print(dec2frac(0.3333333333333))  # => 1/3
print(dec2frac(0.35))  # => 7/20
print(dec2frac(0.13))  # => 13/100
print(dec2frac(0.37))  # => 37/100
print(dec2frac(7/8))  # => 7/8

1/0  # lazy exit

def gcd(a, b):
	"https://en.wikipedia.org/wiki/Greatest_common_divisor"
	gcd = int(a)
	while gcd > 1:
		if a%gcd == 0 and b%gcd == 0: break
		gcd -= 1
	return gcd

def dec_to_frac(f):
	"http://www.purplemath.com/modules/percents2.htm"
	if f == 0: return 0
	sf = str(f%1)[2:]
	lf = len(sf)
	repeating = False
	for i, c in enumerate(sf):
		if sf.find(c) < i:
			lf = i
			repeating = True
			break
	x = 10**(lf)
	print("f:%s len:%s x:%s"%(str(f), lf, x))
	a = int(x*f)#-f)
	if repeating:
		a = int(x*f-f)
		b = x -1
	else:
		a = int(x*f)
		b = x
	print("gcd(x*f, x)", x*f)
	d = gcd(a, x)
	#print(a, b, d)
	return (a//d, b//d)
	
	
sum = 0
for i in range(5):
	n = i+1
	sum += (n+1)/(4**n)
	print(i, sum)

print(7/9)
for i in range(10):
	print("{}/{}={}, {}".format(i, i+1, i/(i+1), dec2frac(i/(i+1))))

Python can do this since 4 months ago:

>>> Decimal('-3.14').as_integer_ratio()
(-157, 50)