RogerBW's Blog

I’ve been doing the Weekly Challenges. The latest involved list comparisons and rearangements. (Note that this ends today.)

I'm bringing a new language on board… well, sort of. Crystal is a compiled language that is heavily inspired by, and broadly compatible with, Ruby. The aim is to offer compiled-language speed but the more forgiving syntax of Ruby compared with something like Rust or C. Let's see what happens.

Task 1: Magic Number

You are given two arrays of integers of same size, @x and @y.

Write a script to find the magic number that when added to each elements of one of the array gives the second array. Elements order is not important.

My thinking in this started with "oh, right, O(n²) comparison". But with the constraint that this is transformation is a classical function with domain and codomain, I realised that I only actually need to look at one value in each array, because e.g. m +(min(x)) is necessarily min(y).

So it turned into a one-liner. Python:

def magicnumber(a, b):
  return min(b) - min(a)

Even in PostScript (though listmin is a convenient shorthand method for a call to my reduce library function, since the built-in min only takes two values).

/magicnumber {
    listmin exch listmin sub
} bind def

Task 2: Number Game

You are given an array of integers, @ints, with even number of elements.

Write a script to create a new array made up of elements of the given array. Pick the two smallest integers and add it to new array in decreasing order i.e. high to low. Keep doing until the given array is empty.

Again most of the solving is in the mental transformation, not in the coding. Step one, sort the list. Step two, step forward and backwards, picking elements 1, 0, 3, 2, 5, 4, etc.

Pulling two items off a list is something various languages approach in different ways. Raku just lets you say "two things please", which is easiest.

sub numbergame(@a0) {
    my @out;
    my @a = @a0.sort({$^a <=> $^b});
    for @a -> $i, $j {
        @out.push($j);
        @out.push($i);
    }
    return @out;
}

Ruby has a function for it:

def numbergame(a0)
  out = []
  a = a0.sort
  a.each_slice(2) do |s|
    out.push(s[1])
    out.push(s[0])
  end
  return out
end

and the only difference for Crystal is declaring out = Array(Int32).new . Scala, Kotlin and Perl (with List::Utils) also take this approach, as does Python in the latest bleeding-edge version (but not in Debian/stable's 3.11).

In Rust I get to play with iterators. (I think one could do something with splitting and chaining but I didn't get it working.)

fn numbergame(a0: Vec<i32>) -> Vec<i32> {
    let mut out = Vec::new();
    let mut a = a0;
    a.sort();
    let mut ai = a.iter();
    while let Some(i) = ai.next() {
        out.push(*ai.next().unwrap());
        out.push(*i);
    }
    out
}

My fallback is just to step through the list two indices at a time and pull out values individually. In Lua here, but also JavaScript and Python.

function numbergame(a0)
   local out = {}
   local a = a0
   table.sort(a)
   for i = 1, #a, 2 do
      table.insert(out, a[i + 1])
      table.insert(out, a[i])
   end
   return out
end

And finally in PostScript, where I already have a perfectly good stack, so I just twiddle each chunk then rotate it to the bottom.

/numbergame {
    0 dict begin
    quicksort
    [ exch
      aload length /l exch def
      l 2 idiv {
          exch
          l 2 roll
      } repeat
    ]
    end
} bind def

Full code on github.