This article has been moved from «perl6.eu» and updated to reflect the language rename in 2019.

This is my response to the Perl Weekly Challenge #2.

Write a script or one-liner to remove leading zeros from positive numbers. |

Leading zeros means that the value
is a string. All we have to do, is coerce it to a number. The shortest way
to do that is using the `+`

(Numeric) prefix.

As a one liner (with single or double quotes):

$ raku -e 'say +@*ARGS[0]' 0000012
12
$ raku -e "say +@*ARGS[0]" 0000000009
9

And as a script:

File: sans-zerosay +@*ARGS[0];

This will choke on non-numeric input, and removes leading zeroes from all numeric values.

We can write it as a more traditional looking progam with a `MAIN`

procedure,
and add the `Numeric`

type constraint on the input value. The type constraint
doesn't exclude leading zeroes, as the input from the command line is of the
`IntStr`

type (if it looks like an integer) or the `RatStr`

type
(if it looks like a rational number).

This will allow any numeric value, and removes leading zeroes:

File: sans-zero2sub MAIN (Numeric $number)
{
# say $number.WHAT;
say +$number;
}

See
docs.raku.org/language/create-cli#index-entry-MAIN
for more information about the special `MAIN`

procedure.

Uncomment the first `say`

line, and run the program to see the types:

$ raku sans-zero2 12
(IntStr)
12
$ raku sans-zero2 12.1
(RatStr)
12.1

The reason that `IntStr`

and some others exist is that Raku
gets the values from the command line without any quotes, as the shell removes them. In
Raku `"12"`

is a string, whereas `12`

is an integer. When we get
the value from the command line (without the quotes), we must be able to treat it as both
- without an error. Hence the special type.

The task was to remove the leading zeroes from *positive* numbers only, so
we'll fix that:

multi sub MAIN (Numeric $number where $number >= 0)
{
say +$number;
}
multi sub MAIN ($value)
{
say $value;
}

Positive numbers with leading zeroes, are stripped of leading zeroes. The challenge didn't say what to do with other values, so in this version they are printed unchanged.

Note that `multi sub`

gives us
multiple dispatch, where we set up procedures with the same name, but with different
signatures. The version to use is decided when called, depending on the arguments.

See
docs.raku.org/syntax/multi for
more information about `multi sub`

.

Note the `where`

clause on the type. It excludes
negative values. (I have chosen to consider zero as a positive value, so that
any leading zeros are removed from it as well.)

See
docs.raku.org/type/Signature
for more information about `where`

clauses.

Write a script that can convert integers to and from a base35 representation, using the characters 0-9 and A-Y. |

Raku has built in support for this, so this is easy(ish):

`:<value>`

syntax:
Try it out in REPL:

$ raku
> :2<1111>
15
> :16<FF>
255

As a program:

File: base35to10sub MAIN (Str $number)
{
say :35($number);
}

Running it:

$ base35to10 12X
1328
$ ./base35to10 99x
11373

See the top of
docs.raku.org/type/Int for
more information about `:<`

.*value*>

We can also use the `parse-base`

method:

sub MAIN (Str $number)
{
say $number.parse-base: 35;
}

See
docs.raku.org/routine/parse-base for
more information about `parse-base`

.

The built in support recognizes both upper- and lower case letters. The challenge stated upper case letters only, but I'll disregard that for now.

`base`

method:
File: base10to35
sub MAIN (Str $number)
{
say $number.base: 35;
}

Running it:

$ base10to35 11373
99X

The value round trips. `base`

uses upper case letters, so we are good.

See
docs.raku.org/routine/base for
more information about `base`

.

This program assumes base 10 if there are digits only in the given value, and base 35 otherwise:

File: base1035multi sub MAIN (Int $number where $number !~~ /\D/)
{
ay $number.base: 35;
}
multi sub MAIN (Str $number)
{
say $number.parse-base: 35;
}

The problem is that a sequence of digits can be a valid base 35 value. The next version fixes that, by allowing the use of the «--base=35» flag to force the usage of base 35:

File: base1035-2multi sub MAIN (Int $number where $number !~~ /\D/, :$base is copy = 10)
{
$base == 35
?? say $number.parse-base: $base
!! say $number.base: $base;
}
multi sub MAIN (Str $number)
{
say $number.parse-base: 35;
}

It is unneccessary to have both «multi»s handle base 35. We can rewrite it, and fix the problem of allowing other values than 10 and 35 as base at the same time:

File: base1035-3subset Base10or35 of Int where { $_ == 10 || $_ == 35 };
sub MAIN (Str $value, Base10or35 :$base is copy = 10)
{
$base = 35 if $value ~~ /\D/;
$base == 35
?? say $value.parse-base: $base # base 35 -> 10
!! say $value.base: $base # base 10 -> 35
}

See
docs.raku.org/language/typesystem#subset
for more information about `subset`

.

I should address the problem that we allow *lower case letters* in
the base 35 values. This version fixes that:

subset BaseValue of Str where { /^<[ 0 .. 9, A .. Y ]>+$/ }
subset Base10or35 of Int where { $_ == 10 || $_ == 35 }
sub MAIN (BaseValue $value, Base10or35 :$base is copy = 10)
{
$base = 35 if $value ~~ /\D/;
$base == 35
?? say $value.parse-base: $base # base 35 -> 10
!! say $value.base: $base # base 10 -> 35
}

The built in support in Raku stops at base 36, as we only have ten digits and 26 letters to choose from. But what if we want to go higher? Let's say to base 42, also known as Duoquadragesimal.

The first thing we'd have to, is define the characters to use after the initial 36
ones (0 .. 9, A .. Z). (I have chosen to support upper case letters only, so I could
have used the lower case letters as the next characters. But that would be confusing,
right?) I am from Norway, so using the additional Norwegian letters «Æ», «Ø» and «Å»
seemed natural. I needed three more, and chose the Swedish letters «Ä» and «Ö» and
the German «ß» (which I know is a *lower case* letter).

The module uses *base 42* unless told otherwise.

unit module BaseX;
subset LegalBase of Int where { 2 <= $_ <= 42 };
my @digits = (0 .. 9, "A" .. "Z", "Æ", "Ø", "Å", "Ä", "Ö", "ß").flat;
our sub to-baseX (Int $value is copy, LegalBase :$base = 42)
{
my @result;
while ($value)
{
@result.push($value % $base); # The remainder
$value = $value div $base; # Integer division
}
return @result.reverse.map( { @digits[$_] } ).join;
}

File: baseX (partial)
use lib "lib";
use BaseX;
say BaseX::to-baseX(1234567890); # -> ÄSEZF
say BaseX::to-baseX(256, base => 16); # -> 100
say BaseX::to-baseX(255, base => 2); # -> 11111111

A walk-through may help, starting with the value 123456789:

- 123456789 / 42 = 2939447 * 42 +
**15** - 2939447 / 42 = 69986 * 42 +
**35** - 69986 / 42 = 1666 * 42 +
**14** - 1666 / 42 = 39 * 42 +
**28** - 39 / 42 = 0 * 42 +
**39**

`@result`

array with the values in reverse order:
`15,35,14,28,39`

.
We use `map`

to convert from the values (in correct order:
`39,28,14,35,15`

) to the digits `ÄSEZF`

. Try it in REPL,
to make sure:

$ raku
> my @digits = (0 .. 9, "A" .. "Z", "Æ", "Ø", "Å", "Ä", "Ö", "ß").flat;
> say @digits[39,28,14,35,15];
(Ä S E Z F)

Too much code, perhaps? It is possible to do the conversion
much shorter, with `polymod`

:

our sub to-baseX2 (Int $value is copy, LegalBase :$base = 42)
{
return $value.polymod($base xx 50).reverse.map( { @digits[$_] } ).join;
}

See
docs.raku.org/routine/polymod
for more information about `polymod`

.

This gives us a lot of leading zeroes (the total number of "digits" is 50
- a number we get from `xx 50`

):

0000000000000000000000000000000000000000000000ÄSEZF
000000000000000000000000000000000000000000000000100
000000000000000000000000000000000000000000011111111

The leading zeroes must be removed, and we cannot use `+`

as the value isn't numeric. A Regex does the trick:

our sub to-baseX3 (Int $value is copy, LegalBase :$base = 42)
{
$value.polymod($base xx 50).reverse.map( { @digits[$_] } ).join ~~ /^0*(.*)/;
return $0.Str;
}

`$0`

is a *match
object* and not a string, so we have to stringify it manually.

Sadly it isn't a one liner any more...

**Excercise:** What happens if we specify a number that has more than
50 digits after conversion?

The array slice I used to explain the values in REPL can replace the `map`

:

our sub to-baseX4 (Int $value is copy, LegalBase :$base = 42)
{
return (@digits[$value.polymod($base xx 50).reverse].join ~~ /^0*(.*)/)[0].Str;
}

We can use an index on the match object (which is returned by the regex, but usually not used directly), so we ended up with a one liner after all.

my %values = @digits.map( { $_ => $++ } );
our sub from-baseX (Str $value, LegalBase :$base = 42)
{
my $return = 0;
my $exp = 0;
for $value.flip.comb -> $digit
{
$return += %values{$digit} * $base ** $exp++;
}
return $return;
}

`flip`

flips a string (reverses it), and `comb`

splits a string
into a list of characters, one at a time.

use lib "lib";
use BaseX;
say BaseX::from-baseX("ÄSEZF"); # -> 123456789
say BaseX::from-baseX("FFFF", base => 16); # -> 65535
say BaseX::from-baseX("1000", base => 2); # -> 8

A walk-through may help this time as well, starting with the string "ÄSEZF". The values can be computed like this (in REPL):

> my @digits = (0 .. 9, "A" .. "Z", "Æ", "Ø", "Å", "Ä", "Ö", "ß").flat;
> my %values = @digits.map( { $_ => $++ } );
> say %values{"ÄSEZF".comb};
(39 28 14 35 15)

- "F" -> 15 * 42 ** 0 =
**15** - "Z" -> 35 * 42 ** 1 =
**1470** - "E" -> 14 * 42 ** 2 =
**24696** - "S" -> 28 * 42 ** 3 =
**2074464** - "Ä" -> 39 * 42 ** 4 =
**121356144**

> say sum(<15 1470 24696 2074464 121356144>); # -> 123456789

We can shorten this one as well to a one liner with `map`

and
`sum`

, even without a trick like `polymod`

:

our sub from-baseX2 (Str $value, LegalBase :$base = 42)
{
return $value.flip.comb.map( {%values{$_} * $base ** $++ } ).sum;
}

`$`

is an *Anonymous State
Variable*, starting with zero as value the first time it is used. I have used
it so that I don't have to declare an explicit variable, as that would have
messed up the one liner.

See
docs.raku.org/syntax/$ for more
information about `$`

.