PWC 175

PWC 175

Challenge 1 -- Last Sunday

Challenge 1 asks us to find the last Sunday of every month in a particular year, say 2022.

This is easy to do in Perl thanks to the Date::Manip suite of modules, which bristles with options, including a straightforward way to do this. The specific module useful here is Date::Manip::Recur

With this module it's as easy as saying:

$date_recur_object -> parse("last Sunday of every month in 2022");

which is exactly what I do.

In Raku, it's a bit more difficult, but still easy via Raku's excellent and well-documented built-in Date objects. Pretty much a matter of looping through the last 10 days of each month, picking up the Sundays in that stretch, and then keeping the last Sunday.

Julia is similar to Raku in its Dates handling, except that the Dates are not built-in, but a short library call away. I could have translated my Raku script line-for-line to Julia, but I have the minor difference that I used multiple dispatch in Raku and a single function in Julia.

Here is my Perl 5 script.

Here is my Raku script.

Here is my Julia script. 


Challenge 2

Challenge 2 is again a number-theory exercise. The task is to find the first 20 perfect totient numbers. I had never heard of totients before this exercise, but apparently they are quite a useful concept, used in RSA cryptography for example. The totient function of a positive integer n counts the number of positive integers less than n that are relatively prime to n, i.e., do not have a common factor with n except for 1. To get the perfect totient numbers, we iteratively compute the totients for each candidate n (i.e., totient(n), then totient(totient(n)), ...) and at each stage we add the answer to the sum of the results from previous iterations, stopping when the iteratively-computed totient is 1. A number is a perfect totient number  when the result of this calculation equals the number itself.

The related OEIS page helpfully gives a Perl script to compute the perfect totient numbers. This in turn points to the extensive and useful ntheory library aka Math::Prime::Util and its "euler_phi" function (another name for the totient function).

This is the approach I followed. I used it in Raku too using Inline::Perl 5 to call the Perl 5 library. The Primes module in Julia too has a totient function, so it is easy to apply the same logic in Julia.

Here is my Perl 5 solution

Here is my Raku solution

Here is my Julia solution

Comments

Post a Comment

Popular posts from this blog

PWC 258

PWC 258 Last week (#257) I finished challenge 2 and my blog post for challenge 257 late last week, so had only submitted challenge 1. Here is my blog post for PWC 257 , including the full code for my challenge 2 solution. This week, I have done both challenges in perl 4.019 on DOSBOX. Both scripts also work fine on the latest Perl 5.38. I also did the challenges in Raku, where they are one-liners.  Challenge 1 (Count Even Digits Number) We are given a list of integers, and asked to count the number of elements whose base-10 representation has an even number of digits. In Raku , this is a one-liner. Here it is: raku -e 'say +@*ARGS.grep({$_.chars %% 2})' $@ It could be a one-liner in Perl 4 too, but since I run it on DOSBOX, my script would not be a bash shell script. Here is the short subroutine that does the job.  sub count_even_digits {  scalar( grep( ((length($_) % 2) == 0), @_) ); } The full code is here . Challenge 2 (Sum of values) We are given an integer k  and a list in
Read more

PWC 249

PWC 249 Challenge 1 (Equal pairs) We are given a list of integers with an even number of elements. We are asked to check if the list can be completely divided into sub-lists of size 2 with each sub-list containing the same integer repeated, like say (2,2). If so, we are asked to return the list of such sub-lists. This is straightforward in Perl 5. I use modern conveniences this time, though it could be done using only primitive Perl 4 syntax. Here is my subroutine. It returns a list of array references. local *equal_pairs = sub {     #-- return () if input is of odd length     ((scalar(@_) % 2) > 0) && (return ());          #-- count number of occurrences of each element using a hash     local %_ = ();     map {$_{$_}++} @_;          #-- if any element occurs an odd number of times, return ()     (grep {($_{$_} % 2) > 0} keys %_) && (return ());          #-- loop thru %_ keys, returning pairs of such keys     map {         my $k=$_;         my $num_pairs=( ($_{$k}
Read more

PWC 253

PWC 253 I do both challenges this week using old software on DOSBOX: perl 4.019, and for comparison, also python 1.4 beta. My solutions also work on the current versions of Perl 5 (5.38) and Python 2 (2.7.18) respectively. Challenge 1 (Split Strings) Links to code: perl4.019 python1.4b We are given a list of strings, and a single character say sep . We are asked to return the list, with the strings split into component substrings using sep as a separator. We should not include any empty strings in the output. In perl 4, I first stick a backslash in front of the separator character, so that it is escaped if entered in a regex. This is because the perl split function takes a regex as its first argument indicating the separator. If we have a character like '$' or ',', we want to escape it so that it is read literally when parsing the regex. Here is the key subroutine:   sub split_strings {  local ($separator, @words)=@_;  local (@retval)=(); #array to be returned  #  #--
Read more