Related
We define the sigmoidal function
σ(t) = 1 / (1+e−t)
It has the derivative σ′(t) = σ(t)(1 − σ(t)). The module gauss_newton contains a function generate_data(gamma=0) which generates a data set (ti , αi ) where ti ∈ R and αi ∈ R with
αi = σ(6ti + 1) + εiγ.
for i = 1, . . . , 10. The values εi ∼ N (0, 1) are independently normally distributed and the real value γ ∈ R controls the influence of εi.
(i) Solve the problem min (1/2(∥F(x)∥^2),
with Fi(x) = σ(x1ti + x2) − αi for i = 1,...,10 and γ = 0 using the Gauss Newton algorithm . Iterate until the size of the search direction is sufficiently small, i.e. until ∥∆xk ∥ < δ for some tolerance δ > 0.
I am trying to calculate integral of function f(x) = log(2 + sin(e^x)) in interval of between 3 and 10. I have to use the Monte Carlo method which approximates the result by multiplying function (f(x) mean by the interval of 7 (or 10 - 3 in this case).
using scipy.integrate.quad the answer comes to
4.375668121469594
My code:
import scipy.stats as sps
x = sps.uniform.rvs(size=1000)
f = np.log(2 + np.sin(np.exp(x)))
def Monte_Carlo (f, a, b):
h = (b-a)
result = h * np.mean(f)
return result
I guess what needs to happen is for f to loop through all instances of x and store results in a list
I'm trying to get x and y coordinates for points along a line (segment) at even intervals. In my test case, it's every 16 pixels, but the idea is to do it programmatically in ActionScript-3.
I know how to get slope between two points, the y intercept of a line, and a2 + b2 = c2, I just can't recall / figure out how to use slope or angle to get a and b (x and y) given c.
Does anyone know a mathematical formula to figure out a and b given c, y-intercept and slope (or angle)? (AS3 is also fine.)
You have a triangle:
|\ a^2 + b^2 = c^2 = 16^2 = 256
| \
| \ c a = sqrt(256 - b^2)
a | \ b = sqrt(256 - a^2)
| \
|__________\
b
You also know (m is slope):
a/b = m
a = m*b
From your original triangle:
m*b = a = sqrt(256 - b^2)
m^2 * b^2 = 256 - b^2
Also, since m = c, you can say:
m^2 * b^2 = m^2 - b^2
(m^2 + 1) * b^2 = m^2
Therefore:
b = m / sqrt(m^2 + 1)
I'm lazy so you can find a yourself: a = sqrt(m^2 - b^2)
Let s be the slop.
we have: 1) s^2 = a^2/b^2 ==> a^2 = s^2 * b^2
and: 2) a^2 + b^2 = c^2 = 16*16
substitute a^2 in 2) with 1):
b = 16/sqrt(s^2+1)
and
a = sqrt((s^2 * 256)/(s^2 + 1)) = 16*abs(s)/sqrt(s^2+1)
In above, I assume you want to get the length of a and b. In reality, your s is a signed value, so a could be negative. Therefore, the incremental value of a will really be:
a = 16s/sqrt(s^2+1)
The Point class built in to Flash has a wonderful set of methods for doing exactly what you want. Define the line using two points and you can use the "interpolate" method to get points further down the line automatically, without any of the trigonometry.
http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/geom/Point.html#interpolate()
The Slope is dy/dx. Or in your terms A/B.
Therefore you can step along the line by adding A to the Y coordinate, and B to the X coordinate. You can Scale A and B to make the steps bigger or smaller.
To Calculate the slope and get A and B.
Take two points on the line (X1,Y1) , (X2,Y2)
A= (Y2-Y1)
B= (X2-X1)
If you calculate this with the two points you want to iterate between simply divide A and B by the number of steps you want to take
STEPS=10
yStep= A/STEPS
xStep= B/STEPS
for (i=0;i<STEPS;i++)
{
xCur=x1+xStep*i;
yCur=y1+yStep*i;
}
Given the equation for a line as y=slope*x+intercept, you can simply plug in the x-values and read back the y's.
Your problem is computing the step-size along the x-axis (how big a change in x results from a 16-pixel move along the line, which is b in your included plot). Given that you know a^2 + b^2 = 16 (by definition) and slope = a/b, you can compute this:
slope = a/b => a = b * slope [multiply both sides by b]
a^2 + b^2 = 16 => (b * slope)^2 + b^2 = 16 [by substitution from the previous step]
I'll leave it to you to solve for b. After you have b you can compute (x,y) values by:
for x = 0; x += b
y = slope * x + intercept
echo (x,y)
loop
Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
The Challenge
Calculate the Date of the Greek Orthodox Easter (http://www.timeanddate.com/holidays/us/orthodox-easter-day) Sunday in a given Year (1900-2100) using the least amount of characters.
Input is just a year in the form '2010'. It's not relevant where you get it (Input, CommandLineArgs etc.) but it must be dynamic!
Output should be in the form day-month-year (say dd/mm/yyyy or d/m/yyyy)
Restrictions No standard functions, such as Mathematica's EasterSundayGreekOrthodox or PHP's easter_date(), which return the (not applicable gregorian) date automatic must be used!
Examples
2005 returns 1/5/2005
2006 returns 23/4/2006
2007 returns 8/4/2007
2008 returns 27/4/2008
2009 returns 19/4/2009
2010 returns 4/4/2010
2011 returns 24/4/2011
2012 returns 15/4/2012
2013 returns 5/5/2013
2014 returns 20/4/2014
2015 returns 12/4/2015
Code count includes input/output (i.e full program).
Edit:
I mean the Eastern Easter Date.
Reference: http://en.wikipedia.org/wiki/Computus
Python (101 140 132 115 chars)
y=input()
d=(y%19*19+15)%30
e=(y%4*2+y%7*4-d+34)%7+d+127
m=e/31
a=e%31+1+(m>4)
if a>30:a,m=1,5
print a,'/',m,'/',y
This one uses the Meeus Julian algorithm but since this one only works between 1900 and 2099, an implementation using Anonymous Gregorian algorithm is coming right up.
Edit: Now 2005 is properly handled. Thanks to Mark for pointing it out.
Edit 2: Better handling of some years, thanks for all the input!
Edit 3: Should work for all years in range. (Sorry for hijacking it Juan.)
PHP CLI, no easter_date(), 125 characters
Valid for dates from 13 March 1900 to 13 March 2100, now works for Easters that fall in May
Code:
<?=date("d/m/Y",mktime(0,0,0,floor(($b=($a=(19*(($y=$argv[1])%19)+15)%30)+(2*($y%4)+4*$y%7-$a+34)%7+114)/31),($b%31)+14,$y));
Invocation:
$ php codegolf.php 2010
$ php codegolf.php 2005
Output:
04/04/2010
01/05/2005
With whitespace:
<?=date("d/m/Y", mktime(0, 0, 0, floor(($b = ($a = (19 * (($y = $argv[1]) % 19) + 15) % 30) + (2 * ($y % 4) + 4 * $y % 7 - $a + 34) % 7 + 114) / 31), ($b % 31) + 14, $y));
This iteration is no longer readable thanks to PHP's handling of assignments. It's almost a functional language!
For completeness, here's the previous, 127 character solution that does not rely on short tags:
Code:
echo date("d/m/Y",mktime(0,0,0,floor(($b=($a=(19*(($y=$argv[1])%19)+15)%30)+(2*($y%4)+4*$y%7-$a+34)%7+114)/31),($b%31)+14,$y));
Invocation:
$ php -r 'echo date("d/m/Y",mktime(0,0,0,floor(($b=($a=(19*(($y=$argv[1])%19)+15)%30)+(2*($y%4)+4*$y%7-$a+34)%7+114)/31),($b%31)+14,$y));' 2010
$ php -r 'echo date("d/m/Y",mktime(0,0,0,floor(($b=($a=(19*(($y=$argv[1])%19)+15)%30)+(2*($y%4)+4*$y%7-$a+34)%7+114)/31),($b%31)+14,$y));' 2005
C#, 155 157 182 209 212 characters
class P{static void Main(string[]i){int y=int.Parse(i[0]),c=(y%19*19+15)%30,d=c+(y%4*2+y%7*4-c+34)%7+128;System.Console.Write(d%31+d/155+"/"+d/31+"/"+y);}}
Python 2.3, 97 characters
y=int(input())
c=(y%19*19+15)%30
d=c+(y%4*2+y%7*4-c+34)%7+128
print"%d/%d/%d"%(d%31+d/155,d/31,y)
This also uses the Meeus Julian algorithm (and should work for dates in May).
removed no longer necessary check for modern years and zero-padding in output
don't expect Easters in March anymore because there are none between 1800-2100
included Python 2.3 version (shortest so far)
Mathematica
<<Calendar`;a=Print[#3,"/",#2,"/",#]&##EasterSundayGreekOrthodox##&
Invoke with
a[2010]
Output
4/4/2010
Me too: I don't see the point in not using built-in functions.
Java - 252 196 190 chars
Update 1: The first algo was for Western Gregorian Easter. Fixed to Eastern Julian Easter now. Saved 56 chars :)
Update 2: Zero padding seem to not be required. Saved 4 chars.
class E{public static void main(String[]a){long y=new Long(a[0]),b=(y%19*19+15)%30,c=b+(y%4*2+y%7*4-b+34)%7+(y>1899&y<2100?128:115),m=c/31;System.out.printf("%d/%d/%d",c%31+(m<5?0:1),m,y);}}
With newlines
class E{
public static void main(String[]a){
long y=new Long(a[0]),
b=(y%19*19+15)%30,
c=b+(y%4*2+y%7*4-b+34)%7+(y>1899&y<2100?128:115),
m=c/31;
System.out.printf("%d/%d/%d",c%31+(m<5?0:1),m,y);
}
}
JavaScript (196 characters)
Using the Meeus Julian algorithm. This implementation assumes that a valid four-digit year was given.
y=~~prompt();d=(19*(y%19)+15)%30;x=d+(2*(y%4)+4*(y%7)-d+34)%7+114;m=~~(x/31);d=x%31+1;if(y>1899&&y<2100){d+=13;if(m==3&&d>31){d-=31;m++}if(m==4&&d>30){d-=30;m++}}alert((d<10?"0"+d:d)+"/0"+m+"/"+y)
Delphi 377 335 317 characters
Single line:
var y,c,n,i,j,m:integer;begin Val(ParamStr(1),y,n);c:=y div 100;n:=y-19*(y div 19);i:=c-c div 4-(c-((c-17)div 25))div 3+19*n+15;i:=i-30*(i div 30);i:=i-(i div 28 )*(1-(i div 28)*(29 div(i+1))*((21 -n)div 11));j:=y+y div 4 +i+2-c+c div 4;j:=j-7*(j div 7);m:=3+(i-j+40 )div 44;Write(i-j+28-31*(m div 4),'/',m,'/',y)end.
Formatted:
var
y,c,n,i,j,m:integer;
begin
Val(ParamStr(1),y,n);
c:=y div 100;
n:=y-19*(y div 19);
i:=c-c div 4-(c-((c-17)div 25))div 3+19*n+15;
i:=i-30*(i div 30);
i:=i-(i div 28 )*(1-(i div 28)*(29 div(i+1))*((21 -n)div 11));
j:=y+y div 4 +i+2-c+c div 4;j:=j-7*(j div 7);
m:=3+(i-j+40 )div 44;
Write(i-j+28-31*(m div 4),'/',m,'/',y)
end.
Tcl
Eastern Easter
(116 chars)
puts [expr 1+[incr d [expr ([set y $argv]%4*2+$y%7*4-[
set d [expr ($y%19*19+15)%30]]+34)%7+123]]%30]/[expr $d/30]/$y
Uses the Meeus algorithm. Takes the year as a command line argument, produces Eastern easter. Could be a one-liner, but it's slightly more readable when split...
Western Easter
(220 chars before splitting over lines)
interp alias {} tcl::mathfunc::s {} set;puts [expr [incr 3 [expr {
s(2,(s(4,$argv)%100/4*2-s(3,(19*s(0,$4%19)+s(1,$4/100)-$1/4-($1-($1+8)/25+46)
/3)%30)+$1%4*2-$4%4+4)%7)-($0+11*$3+22*$2)/451*7+114}]]%31+1]/[expr $3/31]/$4
Uses the Anonymous algorithm.
COBOL, 1262 chars
WORKING-STORAGE SECTION.
01 V-YEAR PIC S9(04) VALUE 2010.
01 V-DAY PIC S9(02) VALUE ZERO.
01 V-EASTERDAY PIC S9(04) VALUE ZERO.
01 V-CENTURY PIC S9(02) VALUE ZERO.
01 V-GOLDEN PIC S9(04) VALUE ZERO.
01 V-GREGORIAN PIC S9(04) VALUE ZERO.
01 V-CLAVIAN PIC S9(04) VALUE ZERO.
01 V-FACTOR PIC S9(06) VALUE ZERO.
01 V-EPACT PIC S9(06) VALUE ZERO.
PROCEDURE DIVISION
XX-CALCULATE EASTERDAY.
COMPUTE V-CENTURY = (V-YEAR / 100) + 1
COMPUTE V-GOLDEN= FUNCTION MOD(V-YEAR, 19) + 1
COMPUTE V-GREGORIAN = (V-CENTURY * 3) / 4 - 12
COMPUTE V-CLAVIAN
= (V-CENTURY * 8 + 5) / 25 - 5 - V-GREGORIAN
COMPUTE V-FACTOR
= (V-YEAR * 5) / 4 - V-GREGORIAN - 10
COMPUTE V-EPACT
= FUNCTION MOD((V-GOLDEN * 11 + 20 + V-CLAVIAN), 30)
IF V-EPACT = 24
ADD 1 TO V-EPACT
ELSE
IF V-EPACT = 25
IF V-GOLDEN > 11
ADD 1 TO V-EPACT
END-IF
END-IF
END-IF
COMPUTE V-DAY = 44 - V-EPACT
IF V-DAY < 21
ADD 30 TO V-DAY
END-IF
COMPUTE V-DAY
= V-DAY + 7 - (FUNCTION MOD((V-DAY + V-FACTOR), 7))
IF V-DAY <= 31
ADD 300 TO V-DAY GIVING V-EASTERDAY
ELSE
SUBTRACT 31 FROM V-DAY
ADD 400 TO V-DAY GIVING V-EASTERDAY
END-IF
.
XX-EXIT.
EXIT.
Note: Not mine, but I like it
EDIT: I added a char count with spaces but I don't know how spacing works in COBOL so I didn't change anything from original. ~vlad003
UPDATE: I've found where the OP got this code: http://www.tek-tips.com/viewthread.cfm?qid=31746&page=112. I'm just putting this here because the author deserves it. ~vlad003
C, 128 121 98 characters
Back to Meeus' algorithm. Computing the day in Julian, but adjusting for Gregorian (this still seems naive to me, but I cannot find a shorter alternative).
main(y,v){int d=(y%19*19+15)%30;d+=(y%4*2+y%7*4-d+34)%7+128;printf("%d/%d/%d",d%31+d/155,d/31,y);}
I have not found a case where floor(d/31) would actually be needed. Also, to account for dates in May, the m in Meeus' algorithm must be at least 5, therefore the DoM is greater than 154, hence the division.
The year is supplied as the number of program invocation arguments plus one, ie. for 1996 you must provide 1995 arguments. The range of ARG_MAX on modern systems is more than enough for this.
PS. I see Gabe has come to the same implementation in Python 2.3, surpassing me by one character. Aw. :(
PPS. Anybody looking at a tabular method for 1800-2099?
Edit - Shortened Gabe's answer to 88 characters:
y=input()
d=(y%19*19+15)%30
d+=(y%4*2+y%7*4-d+34)%7+128
print"%d/%d/%d"%(d%31+d/155,d/31,y)
BASIC, 973 chars
Sub EasterDate (d, m, y)
Dim FirstDig, Remain19, temp 'intermediate results
Dim tA, tB, tC, tD, tE 'table A to E results
FirstDig = y \ 100 'first 2 digits of year
Remain19 = y Mod 19 'remainder of year / 19
' calculate PFM date
temp = (FirstDig - 15) \ 2 + 202 - 11 * Remain19
Select Case FirstDig
Case 21, 24, 25, 27 To 32, 34, 35, 38
temp = temp - 1
Case 33, 36, 37, 39, 40
temp = temp - 2
End Select
temp = temp Mod 30
tA = temp + 21
If temp = 29 Then tA = tA - 1
If (temp = 28 And Remain19 > 10) Then tA = tA - 1
'find the next Sunday
tB = (tA - 19) Mod 7
tC = (40 - FirstDig) Mod 4
If tC = 3 Then tC = tC + 1
If tC > 1 Then tC = tC + 1
temp = y Mod 100
tD = (temp + temp \ 4) Mod 7
tE = ((20 - tB - tC - tD) Mod 7) + 1
d = tA + tE
'return the date
If d > 31 Then
d = d - 31
m = 4
Else
m = 3
End If
End Sub
Credit: Astronomical Society of South Australia
EDIT: I added a char count but I think many spaces could be removed; I don't know BASIC so I didn't make any changes to the code. ~vlad003
I'm not going to implement it, but I'd like to see one where the code e-mails the Pope, scans any answer that comes back for a date, and returns that.
Admittedly, the calling process may be blocked for a while.
Javascript 125 characters
This will handle years 1900 - 2199. Some of the other implementations cannot handle the year 2100 correctly.
y=prompt();k=(y%19*19+15)%30;e=(y%4*2+y%7*4-k+34)%7+k+127;m=~~(e/31);d=e%31+m-4+(y>2099);alert((d+=d<30||++m-34)+"/"+m+"/"+y)
Ungolfed..ish
// get the year to check.
y=prompt();
// do something crazy.
k=(y%19*19+15)%30;
// do some more crazy...
e=(y%4*2+y%7*4-k+34)%7+k+127;
// estimate the month. p.s. The "~~" is like Math.floor
m=~~(e/31);
// e % 31 => get the day
d=e%31;
if(m>4){
d += 1;
}
if(y > 2099){
d += 1;
}
// if d is less than 30 days add 1
if(d<30){
d += 1;
}
// otherwise, change month to May
// and adjusts the days to match up with May.
// e.g., 32nd of April is 2nd of May
else{
m += 1;
d = m - 34 + d;
}
// alert the result!
alert(d + "/" + m + "/" + y);
A fix for dates up to 2399.
I'm sure there is a way to algorithmically calculate dates beyond this but I don't want to figure it out.
y=prompt();k=(y%19*19+15)%30;e=(y%4*2+y%7*4-k+34)%7+k+127;m=~~(e/31);d=e%31+m-4+(y<2200?0:~~((y-2000)/100));alert((d+=d<30||++m-34)+"/"+m+"/"+y)
'VB .Net implementation of:
'http://aa.usno.navy.mil/faq/docs/easter.php
Dim y As Integer = 2010
Dim c, d, i, j, k, l, m, n As Integer
c = y \ 100
n = y - 19 * (y \ 19)
k = (c - 17) \ 25
i = c - c \ 4 - (c - k) \ 3 + 19 * n + 15
i = i - 30 * (i \ 30)
i = i - (i \ 28) * (1 - (i \ 28) * (29 \ (i + 1)) * ((21 - n) \ 11))
j = y + y \ 4 + i + 2 - c + c \ 4
j = j - 7 * (j \ 7)
l = i - j
m = 3 + (l + 40) \ 44
d = l + 28 - 31 * (m \ 4)
Easter = DateSerial(y, m, d)
Is there an algorithm that can calculate the digits of a repeating-decimal ratio without starting at the beginning?
I'm looking for a solution that doesn't use arbitrarily sized integers, since this should work for cases where the decimal expansion may be arbitrarily long.
For example, 33/59 expands to a repeating decimal with 58 digits. If I wanted to verify that, how could I calculate the digits starting at the 58th place?
Edited - with the ratio 2124679 / 2147483647, how to get the hundred digits in the 2147484600th through 2147484700th places.
OK, 3rd try's a charm :)
I can't believe I forgot about modular exponentiation.
So to steal/summarize from my 2nd answer, the nth digit of x/y is the 1st digit of (10n-1x mod y)/y = floor(10 * (10n-1x mod y) / y) mod 10.
The part that takes all the time is the 10n-1 mod y, but we can do that with fast (O(log n)) modular exponentiation. With this in place, it's not worth trying to do the cycle-finding algorithm.
However, you do need the ability to do (a * b mod y) where a and b are numbers that may be as large as y. (if y requires 32 bits, then you need to do 32x32 multiply and then 64-bit % 32-bit modulus, or you need an algorithm that circumvents this limitation. See my listing that follows, since I ran into this limitation with Javascript.)
So here's a new version.
function abmody(a,b,y)
{
var x = 0;
// binary fun here
while (a > 0)
{
if (a & 1)
x = (x + b) % y;
b = (2 * b) % y;
a >>>= 1;
}
return x;
}
function digits2(x,y,n1,n2)
{
// the nth digit of x/y = floor(10 * (10^(n-1)*x mod y) / y) mod 10.
var m = n1-1;
var A = 1, B = 10;
while (m > 0)
{
// loop invariant: 10^(n1-1) = A*(B^m) mod y
if (m & 1)
{
// A = (A * B) % y but javascript doesn't have enough sig. digits
A = abmody(A,B,y);
}
// B = (B * B) % y but javascript doesn't have enough sig. digits
B = abmody(B,B,y);
m >>>= 1;
}
x = x % y;
// A = (A * x) % y;
A = abmody(A,x,y);
var answer = "";
for (var i = n1; i <= n2; ++i)
{
var digit = Math.floor(10*A/y)%10;
answer += digit;
A = (A * 10) % y;
}
return answer;
}
(You'll note that the structures of abmody() and the modular exponentiation are the same; both are based on Russian peasant multiplication.)
And results:
js>digits2(2124679,214748367,214748300,214748400)
20513882650385881630475914166090026658968726872786883636698387559799232373208220950057329190307649696
js>digits2(122222,990000,100,110)
65656565656
js>digits2(1,7,1,7)
1428571
js>digits2(1,7,601,607)
1428571
js>digits2(2124679,2147483647,2147484600,2147484700)
04837181235122113132440537741612893408915444001981729642479554583541841517920532039329657349423345806
edit: (I'm leaving post here for posterity. But please don't upvote it anymore: it may be theoretically useful but it's not really practical. I have posted another answer which is much more useful from a practical point of view, doesn't require any factoring, and doesn't require the use of bignums.)
#Daniel Bruckner has the right approach, I think. (with a few additional twists required)
Maybe there's a simpler method, but the following will always work:
Let's use the examples q = x/y = 33/57820 and 44/65 in addition to 33/59, for reasons that may become clear shortly.
Step 1: Factor the denominator (specifically factor out 2's and 5's)
Write q = x/y = x/(2a25a5z). Factors of 2 and 5 in the denominator do not cause repeated decimals. So the remaining factor z is coprime to 10. In fact, the next step requires factoring z, so you might as well factor the whole thing.
Calculate a10 = max(a2, a5) which is the smallest exponent of 10 that is a multiple of the factors of 2 and 5 in y.
In our example 57820 = 2 * 2 * 5 * 7 * 7 * 59, so a2 = 2, a5 = 1, a10 = 2, z = 7 * 7 * 59 = 2891.
In our example 33/59, 59 is a prime and contains no factors of 2 or 5, so a2 = a5 = a10 = 0.
In our example 44/65, 65 = 5*13, and a2 = 0, a5 = a10 = 1.
Just for reference I found a good online factoring calculator here. (even does totients which is important for the next step)
Step 2: Use Euler's Theorem or Carmichael's Theorem.
What we want is a number n such that 10n - 1 is divisible by z, or in other words, 10n ≡ 1 mod z. Euler's function φ(z) and Carmichael's function λ(z) will both give you valid values for n, with λ(z) giving you the smaller number and φ(z) being perhaps a little easier to calculate. This isn't too hard, it just means factoring z and doing a little math.
φ(2891) = 7 * 6 * 58 = 2436
λ(2891) = lcm(7*6, 58) = 1218
This means that 102436 ≡ 101218 ≡ 1 (mod 2891).
For the simpler fraction 33/59, φ(59) = λ(59) = 58, so 1058 ≡ 1 (mod 59).
For 44/65 = 44/(5*13), φ(13) = λ(13) = 12.
So what? Well, the period of the repeating decimal must divide both φ(z) and λ(z), so they effectively give you upper bounds on the period of the repeating decimal.
Step 3: More number crunching
Let's use n = λ(z). If we subtract Q' = 10a10x/y from Q'' = 10(a10 + n)x/y, we get:
m = 10a10(10n - 1)x/y
which is an integer because 10a10 is a multiple of the factors of 2 and 5 of y, and 10n-1 is a multiple of the remaining factors of y.
What we've done here is to shift left the original number q by a10 places to get Q', and shift left q by a10 + n places to get Q'', which are repeating decimals, but the difference between them is an integer we can calculate.
Then we can rewrite x/y as m / 10a10 / (10n - 1).
Consider the example q = 44/65 = 44/(5*13)
a10 = 1, and λ(13) = 12, so Q' = 101q and Q'' = 1012+1q.
m = Q'' - Q' = (1012 - 1) * 101 * (44/65) = 153846153846*44 = 6769230769224
so q = 6769230769224 / 10 / (1012 - 1).
The other fractions 33/57820 and 33/59 lead to larger fractions.
Step 4: Find the nonrepeating and repeating decimal parts.
Notice that for k between 1 and 9, k/9 = 0.kkkkkkkkkkkkk...
Similarly note that a 2-digit number kl between 1 and 99, k/99 = 0.klklklklklkl...
This generalizes: for k-digit patterns abc...ij, this number abc...ij/(10k-1) = 0.abc...ijabc...ijabc...ij...
If you follow the pattern, you'll see that what we have to do is to take this (potentially) huge integer m we got in the previous step, and write it as m = s*(10n-1) + r, where 1 ≤ r < 10n-1.
This leads to the final answer:
s is the non-repeating part
r is the repeating part (zero-padded on the left if necessary to ensure that it is n digits)
with a10 =
0, the decimal point is between the
nonrepeating and repeating part; if
a10 > 0 then it is located
a10 places to the left of
the junction between s and r.
For 44/65, we get 6769230769224 = 6 * (1012-1) + 769230769230
s = 6, r = 769230769230, and 44/65 = 0.6769230769230 where the underline here designates the repeated part.
You can make the numbers smaller by finding the smallest value of n in step 2, by starting with the Carmichael function λ(z) and seeing if any of its factors lead to values of n such that 10n ≡ 1 (mod z).
update: For the curious, the Python interpeter seems to be the easiest way to calculate with bignums. (pow(x,y) calculates xy, and // and % are integer division and remainder, respectively.) Here's an example:
>>> N = pow(10,12)-1
>>> m = N*pow(10,1)*44//65
>>> m
6769230769224
>>> r=m%N
>>> r
769230769230
>>> s=m//N
>>> s
6
>>> 44/65
0.67692307692307696
>>> N = pow(10,58)-1
>>> m=N*33//59
>>> m
5593220338983050847457627118644067796610169491525423728813
>>> r=m%N
>>> r
5593220338983050847457627118644067796610169491525423728813
>>> s=m//N
>>> s
0
>>> 33/59
0.55932203389830504
>>> N = pow(10,1218)-1
>>> m = N*pow(10,2)*33//57820
>>> m
57073676928398478035281909373919059149083362158422691110342442061570390868211691
45624351435489450017295053614666205465236942234520927014873746108612936700103770
32168799723279142165340712556208924247665167762020062262193012798339674852992044
27533725354548599100657212037357315807679003804911795226565202352127291594603943
27222414389484607402282947077135939121411276374956762365963334486336907644413697
68246281563472846765824974057419578000691802144586648218609477689380837080594949
84434451746800415081286751988931165686613628502248356969906606710480802490487720
51193358699411968177101349014181943964026288481494292632307160152196471809062608
09408509166378415773088896575579384296091317883085437564856451054998270494638533
37945347630577654790729851262538913870632998962296783120027672085783465928744379
10757523348322379799377378069872016603251470079557246627464545140089934278796264
26841923209961950882047734347976478727084053960567277758561051539259771705292286
40608785887236250432376340366655136630923555863023175371843652715323417502594258
04219993081978554133517813905223106191629194050501556554825319958491871324801106
88343133863714977516430300933932895191975095122794880664130058803182289865098581
80560359737115185
>>> r=m%N
>>> r
57073676928398478035281909373919059149083362158422691110342442061570390868211691
45624351435489450017295053614666205465236942234520927014873746108612936700103770
32168799723279142165340712556208924247665167762020062262193012798339674852992044
27533725354548599100657212037357315807679003804911795226565202352127291594603943
27222414389484607402282947077135939121411276374956762365963334486336907644413697
68246281563472846765824974057419578000691802144586648218609477689380837080594949
84434451746800415081286751988931165686613628502248356969906606710480802490487720
51193358699411968177101349014181943964026288481494292632307160152196471809062608
09408509166378415773088896575579384296091317883085437564856451054998270494638533
37945347630577654790729851262538913870632998962296783120027672085783465928744379
10757523348322379799377378069872016603251470079557246627464545140089934278796264
26841923209961950882047734347976478727084053960567277758561051539259771705292286
40608785887236250432376340366655136630923555863023175371843652715323417502594258
04219993081978554133517813905223106191629194050501556554825319958491871324801106
88343133863714977516430300933932895191975095122794880664130058803182289865098581
80560359737115185
>>> s=m//N
>>> s
0
>>> 33/57820
0.00057073676928398479
with the overloaded Python % string operator usable for zero-padding, to see the full set of repeated digits:
>>> "%01218d" % r
'0570736769283984780352819093739190591490833621584226911103424420615703908682116
91456243514354894500172950536146662054652369422345209270148737461086129367001037
70321687997232791421653407125562089242476651677620200622621930127983396748529920
44275337253545485991006572120373573158076790038049117952265652023521272915946039
43272224143894846074022829470771359391214112763749567623659633344863369076444136
97682462815634728467658249740574195780006918021445866482186094776893808370805949
49844344517468004150812867519889311656866136285022483569699066067104808024904877
20511933586994119681771013490141819439640262884814942926323071601521964718090626
08094085091663784157730888965755793842960913178830854375648564510549982704946385
33379453476305776547907298512625389138706329989622967831200276720857834659287443
79107575233483223797993773780698720166032514700795572466274645451400899342787962
64268419232099619508820477343479764787270840539605672777585610515392597717052922
86406087858872362504323763403666551366309235558630231753718436527153234175025942
58042199930819785541335178139052231061916291940505015565548253199584918713248011
06883431338637149775164303009339328951919750951227948806641300588031822898650985
8180560359737115185'
As a general technique, rational fractions have a non-repeating part followed by a repeating part, like this:
nnn.xxxxxxxxrrrrrr
xxxxxxxx is the nonrepeating part and rrrrrr is the repeating part.
Determine the length of the nonrepeating part.
If the digit in question is in the nonrepeating part, then calculate it directly using division.
If the digit in question is in the repeating part, calculate its position within the repeating sequence (you now know the lengths of everything), and pick out the correct digit.
The above is a rough outline and would need more precision to implement in an actual algorithm, but it should get you started.
AHA! caffiend: your comment to my other (longer) answer (specifically "duplicate remainders") leads me to a very simple solution that is O(n) where n = the sum of the lengths of the nonrepeating + repeating parts, and requires only integer math with numbers between 0 and 10*y where y is the denominator.
Here's a Javascript function to get the nth digit to the right of the decimal point for the rational number x/y:
function digit(x,y,n)
{
if (n == 0)
return Math.floor(x/y)%10;
return digit(10*(x%y),y,n-1);
}
It's recursive rather than iterative, and is not smart enough to detect cycles (the 10000th digit of 1/3 is obviously 3, but this keeps on going until it reaches the 10000th iteration), but it works at least until the stack runs out of memory.
Basically this works because of two facts:
the nth digit of x/y is the (n-1)th digit of 10x/y (example: the 6th digit of 1/7 is the 5th digit of 10/7 is the 4th digit of 100/7 etc.)
the nth digit of x/y is the nth digit of (x%y)/y (example: the 5th digit of 10/7 is also the 5th digit of 3/7)
We can tweak this to be an iterative routine and combine it with Floyd's cycle-finding algorithm (which I learned as the "rho" method from a Martin Gardner column) to get something that shortcuts this approach.
Here's a javascript function that computes a solution with this approach:
function digit(x,y,n,returnstruct)
{
function kernel(x,y) { return 10*(x%y); }
var period = 0;
var x1 = x;
var x2 = x;
var i = 0;
while (n > 0)
{
n--;
i++;
x1 = kernel(x1,y); // iterate once
x2 = kernel(x2,y);
x2 = kernel(x2,y); // iterate twice
// have both 1x and 2x iterations reached the same state?
if (x1 == x2)
{
period = i;
n = n % period;
i = 0;
// start again in case the nonrepeating part gave us a
// multiple of the period rather than the period itself
}
}
var answer=Math.floor(x1/y);
if (returnstruct)
return {period: period, digit: answer,
toString: function()
{
return 'period='+this.period+',digit='+this.digit;
}};
else
return answer;
}
And an example of running the nth digit of 1/700:
js>1/700
0.0014285714285714286
js>n=10000000
10000000
js>rs=digit(1,700,n,true)
period=6,digit=4
js>n%6
4
js>rs=digit(1,700,4,true)
period=0,digit=4
Same thing for 33/59:
js>33/59
0.559322033898305
js>rs=digit(33,59,3,true)
period=0,digit=9
js>rs=digit(33,59,61,true)
period=58,digit=9
js>rs=digit(33,59,61+58,true)
period=58,digit=9
And 122222/990000 (long nonrepeating part):
js>122222/990000
0.12345656565656565
js>digit(122222,990000,5,true)
period=0,digit=5
js>digit(122222,990000,7,true)
period=6,digit=5
js>digit(122222,990000,9,true)
period=2,digit=5
js>digit(122222,990000,9999,true)
period=2,digit=5
js>digit(122222,990000,10000,true)
period=2,digit=6
Here's another function that finds a stretch of digits:
// find digits n1 through n2 of x/y
function digits(x,y,n1,n2,returnstruct)
{
function kernel(x,y) { return 10*(x%y); }
var period = 0;
var x1 = x;
var x2 = x;
var i = 0;
var answer='';
while (n2 >= 0)
{
// time to print out digits?
if (n1 <= 0)
answer = answer + Math.floor(x1/y);
n1--,n2--;
i++;
x1 = kernel(x1,y); // iterate once
x2 = kernel(x2,y);
x2 = kernel(x2,y); // iterate twice
// have both 1x and 2x iterations reached the same state?
if (x1 == x2)
{
period = i;
if (n1 > period)
{
var jumpahead = n1 - (n1 % period);
n1 -= jumpahead, n2 -= jumpahead;
}
i = 0;
// start again in case the nonrepeating part gave us a
// multiple of the period rather than the period itself
}
}
if (returnstruct)
return {period: period, digits: answer,
toString: function()
{
return 'period='+this.period+',digits='+this.digits;
}};
else
return answer;
}
I've included the results for your answer (assuming that Javascript #'s didn't overflow):
js>digit(1,7,1,7,true)
period=6,digits=1428571
js>digit(1,7,601,607,true)
period=6,digits=1428571
js>1/7
0.14285714285714285
js>digit(2124679,214748367,214748300,214748400,true)
period=1759780,digits=20513882650385881630475914166090026658968726872786883636698387559799232373208220950057329190307649696
js>digit(122222,990000,100,110,true)
period=2,digits=65656565656
Ad hoc I have no good idea. Maybe continued fractions can help. I am going to think a bit about it ...
UPDATE
From Fermat's little theorem and because 39 is prime the following holds. (= indicates congruence)
10^39 = 10 (39)
Because 10 is coprime to 39.
10^(39 - 1) = 1 (39)
10^38 - 1 = 0 (39)
[to be continued tomorow]
I was to tiered to recognize that 39 is not prime ... ^^ I am going to update and the answer in the next days and present the whole idea. Thanks for noting that 39 is not prime.
The short answer for a/b with a < b and an assumed period length p ...
calculate k = (10^p - 1) / b and verify that it is an integer, else a/b has not a period of p
calculate c = k * a
convert c to its decimal represenation and left pad it with zeros to a total length of p
the i-th digit after the decimal point is the (i mod p)-th digit of the paded decimal representation (i = 0 is the first digit after the decimal point - we are developers)
Example
a = 3
b = 7
p = 6
k = (10^6 - 1) / 7
= 142,857
c = 142,857 * 3
= 428,571
Padding is not required and we conclude.
3 ______
- = 0.428571
7