--- layout: language-war date: 2018-10-11 title: "Hackerrank solutions: Python 3 and Perl 6 (part 2)" tags: - Hackerrank - Perl6 - Python - Python3 - Programming - Raku --- {% markdown %} # Hackerrank solutions: Python 3 and Perl 6 (part 2) {% endmarkdown %} {% markdown %} As a continuation of the [previous part](/post/2018/09/13/hackerrank-solutions-python3-and-perl6-part-1/) of this series, I will be continuing to work through some Hackerrank challenges for Python 3, and compare the solutions to how I would solve them in a language I'm more proficient in, Perl 6. In this post, I will work through some of the Python 3 string challenges from Hackerrank. Raiph [posted a comment on Reddit](https://www.reddit.com/r/perl6/comments/9ffc2p/hackerrank_solutions_python_3_and_perl_6_part_1/e5xml3m) suggesting a slightly different layout, which I will be using for this post. Additional comments are always welcome as I try to improve the format. {% endmarkdown %} {% admonition_md Disclaimer %} Once again I'd like to make clear I'm trying to stick to the original Hackerrank challenges by not using any imports not specifically used in the original challenge. If you have suggestions for Python 3 or Perl 6 modules to make a given task easier, I still appreciate them, but I won't update my solutions to use a module. {% endadmonition_md %} {% markdown %} ## Challenges {% endmarkdown %}
{% markdown %} ### String Split and Join This challenge involves a string containing spaces, where the spaces are to be replaced with dashes (`-`) instead. {% endmarkdown %}
{% highlight python3 tio=https://tio.run/##K6gsycjPM/7/PyU1TaG4ICezJD4xLyU@Kz8zTyMnMy9V04pLAQiKUktKi/IUlHSV9OBSemDlGkoKSpqa/wuKMvNKNNAMyMwrKC3R0ARKp@XnKyQlFgFxFQA %} def split_and_join(line): return "-".join(line.split(" ")) {% endhighlight %}
{% markdown %} I personally am not too fond that `join` takes a list of words to join together, whereas `split` takes a word to split with. It feels a little inconsistent. It also doesn't allow me to read the code logically from left to right. {% endmarkdown %}
{% highlight perl6 tio=https://tio.run/##K0gtyjH7/7@4NEmhuCAns0Q3MS9FNys/M09BQyUnMy9VU6GaSwEIwBw9sBINJQUlTT2QGg0lXSVNrtr/KlqefnrFOaVFBXpqqKboFSdW/k/Lz1dISiwC4ioA %} sub split-and-join ($line) { $line.split(" ").join("-") } {% endhighlight %}
{% markdown %} The Perl 6 solution to the challenge does the same as the Python variant. Even the function names are the same! The biggest difference is that I can chain the functions from left to right, leading to clearer code. {% endmarkdown %}
{% markdown %} ### What's Your Name? The next challenge is a simply string formatting task. You get two inputs, a first name and a last name, and have to put it in a string which will be printed to `STDOUT`. {% endmarkdown %}
{% highlight python3 tio=https://tio.run/##XYzBCsIwEETv/YqxUEigePHmD@jZm6cSyRYj22ww2UK/PobqyWEOAzPz0laeEk@1epqR3iGWaVbmKbqFjBvxsOcOTXtl@isxC4bcfMBdFC/NBZ54JY@2EKQdeOwx4Pu39R8bYtJi7IhfsPWiwUu3uoib5KzLBw %} def print_full_name(a, b): print("Hello %s %s! You just delved into python." % (a, b)) {% endhighlight %}
{% markdown %} Before you begin, I know this can be done using `f""` strings, and that was my first attempt to use as well. However, Hackerrank did not accept this, complaining about invalid syntax, so I assume they're running an older Python 3 than I do. That said, this is a simple `printf` formatted string, which then accepts a tuple of arguments to put into the string. `printf` formatted string are very powerful in their possibilities, and it's clear to read. {% endmarkdown %}
{% highlight perl6 tio=https://tio.run/##K0gtyjH7/7@4NEmhoCgzr0Q3rTQnRzcvMTdVQyVRR0ElSVOhmksBCIoTKxWUPFJzcvIVVBKB4ooKkfmlClmlxSUKKak5ZakpCkDd@QoBQPMUzPSUuGr/Y5in5emnl55aAjQVytL875NYVFTJFZ6YkwMA %} sub print-full-name($a, $b) { say "Hello $a $b! You just delved into Perl 6." } {% endhighlight %}
{% markdown %} Perl 6 has double-quote semantics that many people may be familiar with from other languages. When you insert a variable in a double-quoted string, it's `.Str` value will be used. That is to say, the value will be converted to a `Str` if required, and then put into the string. If you need it or want it for clarity, you can also use `"Hello {$a}"` in Perl 6, allowing you to use it similarly to Python 3's `f""` strings. {% endmarkdown %}
{% markdown %} ### Mutations You are given a string _string_, an integer _position_ and a character _character_. In _string_, replace the character at _position_ to the given _character_. The position is counted from starting point 0, so I don't have to think about differences between what a human or computer considers to be position 1 in a string. {% endmarkdown %}
{% highlight python3 tio=https://tio.run/##VY5BC8IwDIXv/RVhpxaKIN4Ef4mIjNm5iCYlTQ/79bXrpmIuyeN975E468R0KOUWRnhl7TVckwrS3a7LQ@SEikwehqmXftAg7migzqITnOCJSTfc/YzzJ3ipyDdqGiBBsxB03e7BSLbxrsTaoPb/C6SY1ToPi7WJptbLlZHZ7I28AQ %} def mutate_string(string, position, character): chars = list(string) chars[position] = character return "".join(chars) {% endhighlight %}
{% markdown %} This is basically what the example showed as well that came with the challenge, so wasn't too hard to solve. My only complaint was that I couldn't call my list "list", because that's a reserved keyword. {% endmarkdown %}
{% highlight perl6 tio=https://tio.run/##K0gtyjH7/7@4NEkht7QksSRVt7ikKDMvXUFDBcLQUVApyC/OLMnMzwMykzMSixKTS1KLNBWquRSAILdSwSEns7hEwVYBqkEvOT83yRosCZaJhuuPBSmCm2DNhVCjl5WfmWfNVfsfxQ0aKlqefnrpqSVAizFYmnrFiZX/0/LzuQy5igA %} sub mutate-string ($string, $position, $character) { my @list = $string.comb; @list[$position] = $character; @list.join; } {% endhighlight %}
{% markdown %} The Perl 6 variant does the same things as the Python variant. `comb` without arguments converts a string to a list of characters, and `join` without arguments joins a list together to a string. {% endmarkdown %}
{% markdown %} ### Find a String In the following challenge you are given a string _string_, and a substring _sub\_string_. The challenge is to find how often a substring occurs in the _string_. The substrings may overlap one another, so the string `"ABCDCDC"` contains the substring `"CDC"` twice. {% endmarkdown %}
{% highlight python3 tio=https://tio.run/##XU67CsMwDNz9FTfaxENKt4CHPv4ilNCHkwqKbBx76Ne7bhxIqYSQdNzp5N/x6Xif88OOuLvEcZjTbY6BeJK1aRRkqLPqBEosRBi0YllHF0AgRrjyZGWr8bK8qtUq@QaNqGBPHaGprO32Bcb8eG26zbIx2FXPYGMKXOHsCz/K//eJfYpSaayDUvlwPJ1LilIf %} def count_substring(string, sub_string): count = 0 for i in range(0, len(string)): if string[i:i + len(sub_string)] == sub_string: count += 1 return count {% endhighlight %}
{% markdown %} As solution to this challenge I loop through the entire _string_, and check whether it contains the _sub\_string_ at that point. If it does, I increment _count_ by 1. Now, I learned that Python also has the inline `if`, just like Perl 6 does, however, it also *needs* an `else` block. That put me off from using it in this situation. I think it puts me off from using it in most situations, actually. With an `else` coming after it, it just becomes messy to read, in my opinion. {% endmarkdown %}
{% highlight perl6 tio=https://tio.run/##K0gtyjH7/7@4NEkhOb80r0QXyCouKcrMS1fQUIEwdBRUgIK6EI6mQjUXZ2pOam6xAlRaoa5OIdcqvwxoUGKBPpJSfa7a/2hmaqhoefrppaeWAM2EsjT1ihMr/zs6ObsAIRcQAwA %} sub count-substring ($string, $sub-string) { elems $string ~~ m:overlap/$sub-string/ } {% endhighlight %}
{% markdown %} The Perl 6 version makes use of some regex magic, and the `elems` subroutine. `elems` returns the number of elements in a list, which in this case would be the number of matches found by the regex. The `m:overlap//` makes a regex to *m*atch, with *overlap*ping strings. {% endmarkdown %}
{% markdown %} ### String Validators In the following challenge, the program is given a string _s_, and have to validate a number of properties on this string. These are, in order, whether they contain - alphanumeric characters (a-z, A-Z or 0-9), - alphabetic characters (a-z or A-Z), - digits (0-9), - lowercase characters (a-z), - uppercase characters (A-Z). If any character in the given string passes a validation, it must print `"True"`, otherwise it must print `"False"`. {% endmarkdown %}
{% highlight python3 tio=https://tio.run/##hZLNioMwEIDvPsXQPdTAIrS9CR72sk@wt1JCVtMaqklIIqWUPrubn6p1MZib38x8zmQi76YW/ND37AwYc9JSjKEoYItxSxjHeJsnYI@GAhiXnUlR4kFZ0/Lq6MN/urMhDe/aTQ7fpNH0853Lmizwil2YWeCNuFG1wDspJ@7xMzRzFso2RJRtERqmTapRPpbZybgwr4aPryZPQHjlazKmPUrfSqYBp/wCflRHk7jWzfhPa1FcG/JXtOGKZlqPYtohf0Ubbnim9SimHfJXtGFBM61HMe2QH7QQvFd6d@/K7zEkZg6lCI3hTAs1vkS3fAvd7l1s@pNUjA@Kow2dUN9//ZbVbn/4@AM %} if __name__ == '__main__': s = input() checks = { "alnum": False, "alpha": False, "digit": False, "lower": False, "upper": False } for char in list(s): if not checks["alnum"] and char.isalnum(): checks["alnum"] = True if not checks["alpha"] and char.isalpha(): checks["alpha"] = True if not checks["digit"] and char.isdigit(): checks["digit"] = True if not checks["lower"] and char.islower(): checks["lower"] = True if not checks["upper"] and char.isupper(): checks["upper"] = True keys = list(checks.keys()) keys.sort() for key in keys: print(checks[key]) {% endhighlight %}
{% markdown %} As stated in the disclaimer, I don't want to make use of any `import` statements unless these are explicitly given in the original challenges. This means I can't use regexes, as these are stuffed away in the `re` packages in Python. Luckily, Python has the correct check available as a method on the string object, so I can still check them in a single line. I first tried to call the methods on _s_ directly, but this seemed to require the entire string to match the check, instead of just any character in the string. So I had to loop through the string by character, which I did. If any character is found to validate, the appropriate key in the _checks_ dict will be set to `True`. Once I've walked through the entire string, I sort the _keys_ from _checks_ so I can be sure they're printed in the right order. {% endmarkdown %}
{% highlight perl6 tio=https://tio.run/##K0gtyjH7/7@4NEnB19HTT6GaSwEIcisVVIoVbBVUtDz99NJTS6y5wMLFiZUKxfkgqbo6BX2bxJy80lw7fWvscgUZiTjkUjLTM0twyOXkl6cW4ZArLSiAyNX@/@@YlJxiaGSsDAA %} sub MAIN { my $s = $*IN.get; say so $s ~~ //; say so $s ~~ //; say so $s ~~ //; say so $s ~~ //; say so $s ~~ //; } {% endhighlight %}
{% markdown %} Perl 6 does have regexes available in the main namespace by default, so that made this challenge a lot easier to work with. `$*IN` in a special variable that refers to `STDIN`, and the `.slurp` method reads all remaining data from the buffer. The next 5 lines all do a `say`, which acts like `print` in Python 3. The `so` function coerces a value to become a `Bool`. When a `Bool` is given to `say`, it will be coerced to a string representation again, and become either `"True"` or `"False"`. The smartmatch operator `~~` has already been covered in the previous post, so I recommend you read that as well if you haven't yet. In Perl 6, regexes are (usually) delimited by the `/` character. The ``, `` etcetera parts are [predefined character classes][classes] in Perl 6 regexes. These check for exactly what we need in the challenges, so were a good pick to solve them. [classes]: https://docs.perl6.org/language/regexes.html#Predefined_character_classes {% endmarkdown %}
{% markdown %} ### Text Wrap You are given a string _s_ and a width _w_. The string should be split over multiple lines so it is never more wide than _w_. {% endmarkdown %}
{% highlight python3 tio=https://tio.run/##K6gsycjPM/7/PzO3IL@oRKEktaKkvCixgIsrJTVNAcTSKC4pysxL11HITayIL89MKcnQtOJSAIKi1JLSojwFpZg8Jb2s/Mw8DZhePRzaNP8XAIVKNMDSmXkFpSUamjoKIBEoBwj@Ozo5u7i6uXt4enn7@Pr5BwQGBYeEhoVHREZxmQIA %} import textwrap def wrap(string, max_width): return "\n".join(textwrap.wrap(string, max_width)) {% endhighlight %}
{% markdown %} This challenge introduces the first Python module: `textwrap`. This makes the challenge very easy to solve as well, using the `wrap` function exposed by the module. This function makes a list of strings, each no longer than the given width. I then join these together with newlines to get the desired output. {% endmarkdown %}
{% highlight perl6 tio=https://tio.run/##K0gtyjH7/7@4NEmhvCixQEFDpbikKDMvXUdBpTwzpSRDU6GaSwEIoMJ6yfm5SRpQKb2s/Mw8DaWYPCVNrtr/IO0aKlqefnrpqSVA7VCWnmdeiaZecWLlf0cnZxdXN3cPTy9vH18//4DAoOCQ0LDwiMgoLlMA %} sub wrap ($string, $width) { $string.comb($width).join("\n") } {% endhighlight %}
{% markdown %} For the Perl 6 solution, I have not used an additional module, as all the functionality are in the core namespace. I actually made a module in Perl 6 for a less primitive wrapping functionality, called [`String::Fold`][string::fold]. In this solution, I use `comb` with the `$width` argument. This returns a list of strings, each no longer than the given width, just like Python's `textwrap.wrap`. I can then join these together with newlines as well to get the same result. [string::fold]: https://modules.perl6.org/dist/String::Fold:cpan:TYIL {% endmarkdown %}
{% markdown %} ### Designer Door Mat This challenge is more complex than previous challenges. The task at hand is to "draw" a certain "design" as the output. For the input, you are given both a height _y_ and a width _x_, however _x_ must always be _y_ × 3, so you can ignore the second argument. This one is much simpler to explain using two examples. The first example is the output if the input were `7 21`. {% endmarkdown %} {% highlight text %} ---------.|.--------- ------.|..|..|.------ ---.|..|..|..|..|.--- -------WELCOME------- ---.|..|..|..|..|.--- ------.|..|..|.------ ---------.|.--------- {% endhighlight %} {% markdown %} In the second example, the input is `11 33`. {% endmarkdown %} {% highlight text %} ---------------.|.--------------- ------------.|..|..|.------------ ---------.|..|..|..|..|.--------- ------.|..|..|..|..|..|..|.------ ---.|..|..|..|..|..|..|..|..|.--- -------------WELCOME------------- ---.|..|..|..|..|..|..|..|..|.--- ------.|..|..|..|..|..|..|.------ ---------.|..|..|..|..|.--------- ------------.|..|..|.------------ ---------------.|.--------------- {% endhighlight %}
{% highlight python3 tio=https://tio.run/##zZDNS8MwGMbv@SseMwbJlrbrehCEnWS3DS@CBxGpNFsCJQ1p5xD832s@Nh2I7GpyCHmf3/u8H/ZjUJ2pxnFyg@LQu@JNm0Kad9gkEKKk3qsBK2gzMKaNPQyM571ttX/58@KFk6NuBuWJEzpDRVTd7s45p3CGkqPAkhMywWNnERiy6xxabaRH4Wqzl6wUUcHc83cE/pjOvDZ1r2TvLRmL@MwbRctIfKuxYOonA7tI9E3xc/mQYV1A6bQPl2IKRjPqqYQL0PwzD/8fCx@7IHgcY6ubppVxAPLbMRZKSSxoqa8idV5xkfSn9eb@Ybum4gpOUslNd5Tur92FsMBCIPvfuxvHWyzLLw %} #! /usr/bin/env python3 height = int((input().split())[0]) width = height * 3 half = int((height - 1) / 2) # Top half for line in range(1, half + 1): non_dashes = ((line * 2) - 1) dashes = int((width - (non_dashes * 3)) / 2) print("%s%s%s" % ("-" * dashes, ".|." * non_dashes, "-" * dashes)) # Middle line print("%s%s%s" % ( "-" * (int(width / 2) - 3), "WELCOME", "-" * (int(width / 2) - 3) )) # Lower half for line in range(half, 0, -1): non_dashes = ((line * 2) - 1) dashes = int((width - (non_dashes * 3)) / 2) print("%s%s%s" % ("-" * dashes, ".|." * non_dashes, "-" * dashes)) {% endhighlight %}
{% markdown %} I split the code up in a top half, middle line and lower half, to make it easier to reason about. The `for` loops contain some logic to get the right output on every line. I found out that `range` supports a third argument, allowing me to count down with it as well, which was perfect for this situation. {% endmarkdown %}
{% highlight perl6 tio=https://tio.run/##zZBBTsMwEEX3PsXgdpEgxVFSqSwiVqiLSi1skFiiIE@xpeBEdtpQhZyDo3CAHix0UpKGSuzxypp5/8/XL9Bm87adXEG4dTZ80SZEs4OCxoy97WGqUL@qEm5her28Fy7b2kJUuZVOKEylWJoy6bhKy1IR9iM4fMLstFFptgkGG68HAoh8OHxBnDA2gce8AALZJrcQCfFLVTM4PvIyuQlk6hS6zuqZzsQ@eSUDM9qfQgXH30hIyfzhNKlcugde84DDey9vai4@RDc4S5sLhiesofBrLWWGkGmDbOzl9QnoFqWc@c3TYnX3sF7U8DfCu0ZWeYX23Il3UYovLO7QOvzX7bTtDcTRNw %} #! /usr/bin/env perl6 my $height = $*IN.slurp.words.head.Int; my $width = $height × 3; my $half-height = ($height - 1) ÷ 2; # Top half for 1..$half-height { my $non-dashes = ($_ × 2) - 1; my $dashes = ($width - ($non-dashes × 3)) ÷ 2; say "{"-" x $dashes}{".|." x $non-dashes}{"-" x $dashes}"; } # Middle line say "{"-" x (($width ÷ 2) - 3)}WELCOME{ "-" x (($width ÷ 2) - 3)}"; # Lower half for (1..$half-height).reverse { my $non-dashes = ($_ × 2) - 1; my $dashes = ($width - ($non-dashes × 3)) ÷ 2; say "{"-" x $dashes}{".|." x $non-dashes}{"-" x $dashes}"; } {% endhighlight %}
{% markdown %} As usual, the code is functionally the same. I must admit I like the functional style to get an `Int` from the first argument much more than the way I do it in Python, though. A thing I learned is that the `..` operator that generates a sequence does not have a way to make a sequence that counts down, so I had to use `.reverse` on a sequence that counts up. I had expected this to Just Work as I expected and count down if the left hand side would be larger than the right hand side. You may notice some fancy Unicode characters in the source, namely `×` for multiplication, and ÷ for division. Perl 6 allows Unicode characters in the source files, which can oftentimes lead to prettier code. In this particular instance, there's no big difference in code readability, though. And for those who don't yet have a modern editor that can make Unicode characters, do not worry, as the ASCII equivalents (`*` and `/` respectively) still work as well. {% endmarkdown %}
{% markdown %} ### String Formatting In this challenge, you are to produce a table with four columns. The columns should contain the decimal, octal, hexadecimal and binary values of the row numbers. The function receives an int _number_. The table should contain that many rows, starting with row number 1. {% endmarkdown %}
{% highlight python3 tio=https://tio.run/##jZFNCoMwEIX3nmIQhIQG0XYn9CwSTawpJoYYqVJ6dht/W7uQDlll3vveTKJ7W9bqMgyMF6CNUDYtaiOptZwh1cqMG5x44ErSLn0IZku4QsUV8p9Rkr38cJavWuxNYncJAoQCQ9WNo5jA3IcTxAtvLMZzIWnliI72YQkcmnvbWLRl4s1S53Y1JPV/lpJ3dJeUdDtjqzU36ACQCUVNv3iz49DNND0m8oMG1uNDAGgZhMyLkO/hyBKE8fD7EyNKKN1ahF33HL0B %} def print_formatted(number): max_width = len("{0:b}".format(number)) for i in range(1, number + 1): decimal = "{0}".format(i).rjust(max_width) octal = "{0:o}".format(i).rjust(max_width) hexadecimal = "{0:x}".format(i).upper().rjust(max_width) binary = "{0:b}".format(i).rjust(max_width) print("%s %s %s %s" % (decimal, octal, hexadecimal, binary)) {% endhighlight %}
{% markdown %} In the Python 3 solution I first calculate the max width I need to take into account. Then I loop from 1 until _number_ to get the right amount of rows. Each iteration, I format the number correctly, and then print it out using a printf format string. The hardest part of this challenge was to get formatting right the way Hackerrank wanted it. But I guess that was the entire point of the challenge. {% endmarkdown %}
{% highlight perl6 tio=https://tio.run/##XY9NCsIwEIX3PcUQIiRiB1tEBPEA3bhxK0iq0VZMWpIUK1KvXmNb/HurGWbeN29KaS7ztrVVCqXJtQuPhVHCOXkARnWlUmk43APwUjegStThNT@4DFYwjDEVVrKY48YZ3GfC2OV7vYeF1nn0yVsYGd0/jMYSqGuYcTwXuWYECIcHkK0my6BDeDtEiMOhIcZLv2Dskh8Z3U2A7vo8C/6po/lXE3Pe52uCpv17mdFxssaTdJhox9t4@gQ %} sub print-formatted ($number) { my $max-width = $number.base(2).Str.chars; my $format-string = ("%{$max-width}s" xx 4).join(" ") ~ "\n"; for 1..$number { $format-string.printf($_, $_.base(8), $_.base(16), $_.base(2)); } } {% endhighlight %}
{% markdown %} The Perl 6 solution starts of the same, in that it first calculates the max width I need to take into account. Next, however, I generate the format string using the `$max-width` to make the `printf` subroutine pad it for me. The `xx` operator makes a total of 4 such strings, and puts them into a list, which I can then `join` together with a space character, and add a `\n` at the end of it (the `~` operator is for string concatenation). I'm assuming something similar is possible in Python 3 as well, and I would like to have an example so I can compare it more fairly. In the Perl 6 solution I am also able to make use of the `base` method to convert the numbers into the right base, something I could not find for Python 3. {% endmarkdown %}
{% markdown %} ## Wrap-up This time I did not do all of the challenges, as the post would probably get too long. I still did 8 of them, and might do the rest of the string challenges in a later part anyway. I still find Perl 6 to produce much cleaner code, which is shown best with the first challenge. In Perl 6 (`$line.split(" ").join("-")`), I can read from left to right to see what I'm doing: I have a `$line`, which I split, and then join. In the Python variant (`"-".join(line.split(" "))`), it is much less clear what the actual item I'm working on is, as it's hidden inbetween the `join` and `split` calls. Of course, I'm still not an expert on Python 3 code, so I'm sure that there are many parts that could be written in a cleaner fashion. I'm still open for feedback to improve my Python 3 skills (hence I'm publishing these posts), so please let me know if you know better ways to solve some challenges. {% endmarkdown %}