# As many digits as you like

I was playing with arbitrary precision rational numbers in Perl 6, irrationally perhaps by playing with the natural base, e, also known as Euler’s Number. That’s a builtin value in Perl 6 but with limited precision (that’s probably good enough for most people):

$perl6 > e 2.71828182845905  I want more digits so I calculate them myself. One approximation of e is an infinite series: $$e = \sum\limits_{n=0}^{\infty}\frac{1}{n!}$$ I can turn that into Perl 6 as a program that specifies the number of digits past the decimal place I’d like to calculate. In this code, you don’t see a factorial explicitly. There’s no need for a function (recursive or otherwise) because I can keep the result of the factorial in one iteration for the next: sub MAIN ( Int$precision = 30 ) {
loop {
state $sum = FatRat.new( 0, 1 ); # the term in the sequence state$i         = 0;

# The factorial, remembered for the next iteration
state $last = 1;$sum += FatRat.new( 1, $last );$last *= ++$i; put "$i: ({$sum.nude})\n\t " ~$sum;
last if $sum.Str.chars >$precision + 2; # +2 for the 2 and the .
}
}


I use the .nude method to get the numerator and denominator of the fraction that Perl 6 tracks. I output that with the iteration number. Under that, I output part of the stringified version of the FatRat:

$perl6 euler.p6 20 1: (1 1) 1 2: (2 1) 2 3: (5 2) 2.5 4: (8 3) 2.666667 5: (65 24) 2.708333 6: (163 60) 2.716667 7: (1957 720) 2.718056 8: (685 252) 2.718254 9: (109601 40320) 2.718279 10: (98641 36288) 2.718282 11: (9864101 3628800) 2.71828180 12: (13563139 4989600) 2.71828183 13: (260412269 95800320) 2.718281828 14: (8463398743 3113510400) 2.71828182845 15: (47395032961 17435658240) 2.718281828458 16: (888656868019 326918592000) 2.7182818284590 17: (56874039553217 20922789888000) 2.718281828459042 18: (7437374403113 2736057139200) 2.71828182845905 19: (17403456103284421 6402373705728000) 2.71828182845904523 20: (82666416490601 30411275102208) 2.718281828459045 21: (6613313319248080001 2432902008176640000) 2.71828182845904523534 22: (69439789852104840011 25545471085854720000) 2.718281828459045235359  Look at iterations 19 and 20. Each iteration should make the approximation larger because it only adds a positive value to the previous value. But, the value goes from 2.71828182845904523 to the slightly less 2.718281828459045. This isn’t a rounding error because one is two digits shorter: 19: (17403456103284421 6402373705728000) 2.71828182845904523 20: (82666416490601 30411275102208) 2.718281828459045  Notice what happened to the fraction in iteration 20. There are fewer digits in the denominator for that iteration. Perl 6 uses the number of digits in the denominator to decide how many digits to put into the stringification. You can see this in the Rational.Str method in Rakudo. It’s a minimum of 6 digits, but otherwise one more than the number of denominator digits. But, the fraction can change. Perl 6 normalizes that numerator and denominator to be co-prime. They share no common factors. This normalization is still the same number. If I make a Rat with 2 and 4, Perl 6 reduces that to 1 and 2: $ perl6
To exit type 'exit' or '^D'
> Rat.new( 2, 4 )
0.5
> Rat.new( 2, 4 ).nude
(1 2)


When this happens in my Euler example, some iterations seem to backtrack in digits because of the digit length of the normalized fraction. No big whoop.

There’s another way to write this program, though. Often we do these sorts of things because we pretend that we don’t already know the answers. I could simply put the answer in the program and output the parts of it that I want. Maybe I don’t need 10,000 digits but they are there if I want them:

#!/Applications/Rakudo/bin/perl6

# https://www.math.utah.edu/~pa/math/e.html
my $digits = q/ 71828 18284 59045 23536 02874 71352 66249 77572 47093 69995 95749 66967 62772 40766 30353 54759 45713 82178 52516 64274 27466 39193 20030 59921 81741 35966 29043 57290 03342 95260 59563 07381 32328 62794 34907 63233 82988 07531 95251 01901 15738 34187 93070 21540 89149 93488 41675 09244 76146 06680 82264 80016 84774 11853 74234 54424 37107 53907 77449 92069 55170 27618 38606 26133 13845 83000 75204 49338 26560 29760 67371 13200 70932 87091 27443 74704 72306 96977 20931 01416 92836 81902 55151 08657 46377 21112 52389 78442 50569 53696 77078 54499 69967 94686 44549 05987 93163 68892 30098 79312 77361 78215 42499 92295 76351 48220 82698 95193 66803 31825 28869 39849 64651 05820 93923 98294 88793 32036 25094 43117 30123 81970 68416 14039 70198 37679 32068 32823 76464 80429 53118 02328 78250 98194 55815 30175 67173 61332 06981 12509 96181 88159 30416 90351 59888 85193 45807 27386 67385 89422 87922 84998 92086 80582 57492 79610 48419 84443 63463 24496 84875 60233 62482 70419 78623 20900 21609 90235 30436 99418 49146 31409 34317 38143 64054 62531 52096 18369 08887 07016 76839 64243 78140 59271 45635 49061 30310 72085 10383 75051 01157 47704 17189 86106 87396 96552 12671 54688 95703 50354 02123 40784 98193 34321 06817 01210 05627 88023 51930 33224 74501 58539 04730 41995 77770 93503 66041 69973 29725 08868 76966 40355 57071 62268 44716 25607 98826 51787 13419 51246 65201 03059 21236 67719 43252 78675 39855 89448 96970 96409 75459 18569 56380 23637 01621 12047 74272 28364 89613 42251 64450 78182 44235 29486 36372 14174 02388 93441 24796 35743 70263 75529 44483 37998 01612 54922 78509 25778 25620 92622 64832 62779 33386 56648 16277 25164 01910 59004 91644 99828 93150 56604 72580 27786 31864 15519 56532 44258 69829 46959 30801 91529 87211 72556 34754 63964 47910 14590 40905 86298 49679 12874 06870 50489 58586 71747 98546 67757 57320 56812 88459 20541 33405 39220 00113 78630 09455 60688 16674 00169 84205 58040 33637 95376 45203 04024 32256 61352 78369 51177 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71039 76521 46960 27662 58359 90519 87042 30017 94655 3679 /;$digits.subst-mutate( /\s+/, '', :g );

sub MAIN ( $places = 15 ) { # could constrain further my$fractional = substr $digits, 0,$places;
put 2 + FatRat.new( +$fractional, 10**($fractional.chars) );
}