Andrew Shitov Weekly Review: Challenge - 263

Monday, Apr 8, 2024| Tags: Raku

Raku Solutions Weekly Review

Task 1

In the first task, you have an array of integers @ints and a target number $k. You need to print the indices of those elements, which are equal to the target, but you need to search in the sorted source data.


What does Raku offer that can be used in this task? First, the sort routine, which you can use as a method or as a stand-alone function:

my @ints = 1, 5, 3, 2, 4, 2;

say @ints.sort;  # (1 2 2 3 4 5)
say sort(@ints); # (1 2 2 3 4 5)

By the way, the sort routine in Raku is smart enough to take care of correclty sorting both numbers and strings. Compare the following two sortings:

my @ints = 1, 2, 10, 20;
say @ints.sort;

my @strs = '1', '2', '10', '20';
say @strs.sort;

The result is expectedly correct:

(1 2 10 20)
(1 10 2 20)

Back to the task, and the second mean in Raku to be used here is the grep routine that finds the elements with the given condition. Our condition is the equality with the target value $k, so in terms of Raku you can choose one of the following approaches:

my @data = @ints.sort;
my $k = 2;
say @data.grep: * == $k;    # (2 2)
say @data.grep({$_ == $k}); # (2 2)

The tricky point here is to get the indices of the elements that match the condition. You cannot do it directly after you have already filtered the data. Instead, you may think of making a loop over the values and gathering the indices:

say gather {
    for ^@data -> $index {
        take $index if @data[$index] == $k;
} # (1 2)

Raku, nevertheless, has a better solution. Just use the :k flag of grep. In this case, instead of returing the values, grep returns the indices of the matched elements, which is exactly what is needed.

say @data.grep: * == $k, :k; # (1 2)


All of the above steps can be chained to get a construction like this, as you can see in the solutions propopsed by a number of authors. Notice that you even don’t have to explicitly use comparison and thus the condition can be shortened to a bare minumum: $k.

@ints.sort.grep: $k, :k

Jan Krňávek does effectively the same but he prefers to use andthen instead of dots:

sub target-index (+ints,:$k) {
    andthen .sort
    andthen .grep: { $_ == $k },:k

Luca Ferrari uses the smartmatch ~~ oprartor for comparison. In our case, it compares numbers, so it is equivalent to ==.

@nums.sort.grep( * ~~ $k, :k ).say;

A slightly different algorithm is used by some authors. Instead of grepping and looking for an index, they use a tricky condition to grep indices directly:

my @sorted = @in.sort;
my @out = grep {@sorted[$_] == $target}, 0..@sorted.end;

You can code it differently, for example:

sub targetindex(@a0, $k) {
    my @a = @a0.sort({$^a <=> $^b});
    return [(0 .. @a.end).grep({@a[$_] == $k})];

Further, let’s look at the solutions with a loop, for example:

for @sorted.kv -> $i, $v {
    next unless $v == $k;

Athanasius uses a loop too, but it is always interesting to see that Raku allows completely different style of programming with more control over the types of the variables, function arguments and return values.

sub find-target-indices( List:D[Int:D] $ints, Int:D $k --> List:D[UInt:D] )
    my UInt @indices;
    my Int  @sorted = $ints.sort;

    for 0 .. @sorted.end -> UInt $i
        @indices.push: $i if @sorted[ $i ] == $k;

    return @indices;

BarrOff demonstrates another appoach. The program uses the indices function. It works on strings and returns the indices of the matching characters, so you need to convert the array of integers to a string first. The method works with the test samples, where all the numbers are below 10, so they can be spellt with a single character.

sub target-index(Int:D $k, @ints --> Positional) {
    indices(@ints.sort.join, $k)


As a bonus, examine the following alternative code submitted by Bruce Gray in Raku:

sub task1_single_pass_without_sorting ( UInt $k, @ns ) {
    return flat .{Less} + ^.{Same} given ( @ns »<=>» $k ).Bag;

Open the source code to find additional information about this algorithm and the link to the solution by Peter Campbell Smith in Perl .


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