RogerBW's Blog

I’ve been doing the Weekly Challenges. The latest involved array slicing and point filtering. (Note that this ends today.)

Task 1: Range Sum

You are given a list o integers and pair of indices..

Write a script to return the sum of integers between the given indices (inclusive).

Every language I'm using except for Lua has some kind of array slice function (sometimes start and length, sometimes start and end), and most of them have a sum function. Which together make this a one-liner. Crystal:

def rangesum(a, s, e)
  a[s .. e].sum
end

Scala:

def rangesum(a: List[Int], s: Int, e: Int): Int = {
  a.slice(s, e + 1).sum
}

etc.

Task 2: Nearest Valid Point

You are given current location as two integers: x and y. You are also given a list of points on the grid.

A point is considered valid if it shares either the same x-coordinate or the same y-coordinate as the current location.

Write a script to return the index of the valid point that has the smallest Manhattan distance to the current location. If multiple valid points are tied for the smallest distance, return the one with the lowest index. If no valid points exist, return -1.

The Manhattan distance between two points (x1, y1) and (x2, y2) is calculated as: |x1 - x2| + |y1 - y2|.

This could probably be done with a series of maps and filters operating on the list of points, but I found it easier to do in a more traditionally imperative style.

Raku:

sub nearestvalidpoint($x, $y, @points) {

Set default return index value, and an invalid distance.

    my $ix = -1;
    my $minmhd = -1;

Iterate over points.

    for @points.kv -> $i, @p {

Check that either X or Y coordinate matches (validity test).

        if (@p[0] == $x || @p[1] == $y) {

Calculate the Manhattan distance.

            my $mhd = abs(@p[0] - $x) + abs(@p[1] - $y);

If it's the first valid point, or it's closer than any previous point, store its index and the minimum distance found so far.

            if ($minmhd == -1 || $mhd < $minmhd) {
                $minmhd = $mhd;
                $ix = $i;
            }
        }
    }

Return the result.

    $ix;
}

And in all the other languages it looks essentially the same.

Full code on github.