RogerBW's Blog

Perl Weekly Challenge 122: Basketball Stream 22 July 2021

I’ve been doing the Weekly Challenges. The latest involved a progressive average and additive compositions. (Note that this is open until 25 July 2021.)

TASK #1 › Average of Stream

You are given a stream of numbers, @N.

Write a script to print the average of the stream at every point.

Bearing in mind that I'm answering these as functions, rather than use a stream, I take an array and return another array with the sets of averages (arithmetic means, to be specific). Fairly straightforward:

sub aos(@m) {
  my $n=0;
  my $t=0;
  my @o;
  for @m -> $i {
    $t+=$i;
    $n++;
    push @o,floor($t/$n);
  }
  return @o;
}

PostScript, using a bunch of variables rather than constant stack furkling. (I haven't used arrays in PostScript before. How quaint, having to declare a maximum capacity.)

/aos {
    /n 0 def
    /t 0 def
    dup length array /o exch def
    {
        n add /n exch def
        o t
        /t t 1 add def
        n t div cvi put
    } forall
    o
} def

Extending my very basic test harness to compare the arrays was harder work…

TASK #2 › Basketball Points

You are given a score $S.

You can win basketball points e.g. 1 point, 2 points and 3 points.

Write a script to find out the different ways you can score $S.

Seems like a call for my standard loop-search pattern. Which meant I couldn't be bothered to do it in Raku. Here's the Rust.

fn bp (n: u32) -> Vec<Vec<u32>> {
    let mut o=vec![];

Some of the other languages make it unreasonably difficult to declare a list with an empty list as its only member. Not Rust.

    let mut p=vec![vec![]];
    while p.len() > 0 {
        let s=p.pop().unwrap();

Also its sum() defaults to 0. (Yeah, I could store the sum along with the list of values that make that sum and it would run a bit faster, but that would be more work.)

        let t: u32=s.iter().sum();
        if t==n {
            o.push(s);
        } else {
            let mut mx=n-t;
            if mx > 3 {
                mx=3;
            }
            for i in 1..=mx {
                let mut q=s.clone();
                q.push(i);
                p.push(q);
            }
        }
    }

This produces a set of outputs sorted by highest first number, then highest second number, etc.; so we reverse it to match the input example.

    o.reverse();
    return o;
}

The number of entries for increasing $S matches the Tribonacci numbers, where each number is the sum of the three previous values. This might suggest alternative ways of finding the results; this problem reminded me somewhat of the "Find Possible Paths" of challenge #117.

Full code on github.

See also:
Perl Weekly Challenge 117: Missing Possibilities


  1. Posted by RogerBW at 06:44pm on 26 July 2021

    There aren't many different ways to do part 1, though if one operates on a stream from STDIN some languages provide a line counter. Some bloggers didn't bother to avoid repeated summing. Raku of course can potentially generate an infinite stream with supply

    Most people tackled part 2 recursively. I suppose I really ought to use recursion again some time; my experiences with it have mostly been that it doesn't gain you much if any performance (all that stack winding/unwinding of the whole calling context rather than just one variable) and it's a pain to debug. Still, most of that experience was with different languages…

    Another approach was to build cross-products of (1..3) at all lengths up to $S, then see which ones produced the right score.

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