Calculating large fractions in Python?

Question

I am trying to calculate fractions in Python 2.7. The limit_denominator method works great for the first 15 iterations of this code. However, then the code gets stuck in a loop, outputting denominators less than 1,000,000

Fraction = 1217471/860882

When I don't use limit_denominator, I get repeat outputs like this:

Fraction = 141421356237/100000000000

Eventually I want to iterate i to 1000, so my fractions will be very large. Any help?

from fractions import *
i = 0
x = 1/2.0
x1 = 0
count = 0
while i < 20:
    (y) = (1.0 + (x))
    (x) = (1 / (2.0 + (x)))
    y1 = Fraction(str(y)).limit_denominator()
    print("
Fraction = " + str(y1))
    i += 1

Answer 1

The values converge to sqrt(2.0), which gives you a narrow range of fractions that will accurately represent the 64-bit float value. Your rational fraction cannot be more accurate than the float you give it.

If you want larger denominators, then you have to specify a larger denominator limit. You're still limited by the float accuracy: once you converge within the accuracy of your computational type (likely float64), you will not get more accuracy in your rational representation thereof. If you want greater accuracy, convert the entirety to fraction computations:

from fractions import *

x = Fraction(1,2)

for i in range(40):
    y = Fraction(1) + x
    x = Fraction(1) / (Fraction(2) + x)
    print("Fraction = " + str(y))

Output:

Fraction = 3/2
Fraction = 7/5
Fraction = 17/12
Fraction = 41/29
Fraction = 99/70
Fraction = 239/169
Fraction = 577/408
Fraction = 1393/985
Fraction = 3363/2378
Fraction = 8119/5741
Fraction = 19601/13860
Fraction = 47321/33461
Fraction = 114243/80782
Fraction = 275807/195025
Fraction = 665857/470832
Fraction = 1607521/1136689
Fraction = 3880899/2744210
Fraction = 9369319/6625109
Fraction = 22619537/15994428
Fraction = 54608393/38613965
Fraction = 131836323/93222358
Fraction = 318281039/225058681
Fraction = 768398401/543339720
Fraction = 1855077841/1311738121
Fraction = 4478554083/3166815962
Fraction = 10812186007/7645370045
Fraction = 26102926097/18457556052
Fraction = 63018038201/44560482149
Fraction = 152139002499/107578520350
Fraction = 367296043199/259717522849
Fraction = 886731088897/627013566048
Fraction = 2140758220993/1513744654945
Fraction = 5168247530883/3654502875938
Fraction = 12477253282759/8822750406821
Fraction = 30122754096401/21300003689580
Fraction = 72722761475561/51422757785981
Fraction = 175568277047523/124145519261542
Fraction = 423859315570607/299713796309065
Fraction = 1023286908188737/723573111879672
Fraction = 2470433131948081/1746860020068409