The school tournament

Four school teams play each other once. Six results become a league table in twenty lines, with X, Z, [+], » and a junction.

Four teams from a school play a small football tournament: every team plays every other team once. A win gives three points, a draw one. The program takes the six results and prints the table.

Before you run it

Two teams finish with the same number of points. In which order does the program print them? And how many goals were scored in the whole tournament?

The text

my @teams = <Eagles Herons Owls Swans>;
my @games = @teams.combinations(2);    # every pair plays once
my @goals = (2, 1), (0, 0), (3, 1), (1, 2), (2, 2), (0, 1);

my %points = @teams X=> 0;

for @games Z @goals -> (($home, $away), ($home-goals, $away-goals)) {
    given $home-goals <=> $away-goals {
        when More { %points{$home} += 3 }
        when Less { %points{$away} += 3 }
        default   { %points{$home} += 1; %points{$away} += 1 }
    }
}

for %points.sort({ -.value, .key }) {
    say .fmt('%-8s %d');
}

say "Goals: {[+] @goals».sum}";
say 'Every team has a point.' if all(%points.values) > 0;

Eagles   7
Owls     4
Swans    4
Herons   1
Goals: 15
Every team has a point.

Words

• .combinations — all the ways to choose elements from a list. .combinations(2) gives every pair, each pair once.
• X=> — the cross operator X combined with =>: it makes a pair of every element on the left with every element on the right.
• Z — the zip operator: takes the first element of each list, then the second of each, and so on.
• <=> — compares two numbers and gives Less, Same or More.
• More — one of the three values of the type Order, together with Less and Same.
• .fmt — formats a value with a pattern, like sprintf. On a pair, the pattern gets the key and the value.
• [+] — the reduction operator: puts + between all elements of a list.
• » — the hyper operator: calls the method that follows on every element of the list.
• all — makes a junction: a value that stands for all the elements at once. A comparison with it is true only if it is true for each of them.

Reading notes

my @teams = <Eagles Herons Owls Swans>;
my @games = @teams.combinations(2);    # every pair plays once
my @goals = (2, 1), (0, 0), (3, 1), (1, 2), (2, 2), (0, 1);

@games gets six pairs, in this order: Eagles–Herons, Eagles–Owls, Eagles–Swans, Herons–Owls, Herons–Swans, Owls–Swans. @goals holds the results in the same order: the Eagles beat the Herons 2:1, and so on. The first team of each pair is the home team.

my %points = @teams X=> 0;

X=> pairs every team with every element on the right. There is only one element there, 0, so you get Eagles => 0, Herons => 0, Owls => 0, Swans => 0, and every team starts in the table with zero points, even if it never wins.

for @games Z @goals -> (($home, $away), ($home-goals, $away-goals)) {

Z walks along both lists together. Each step gives a list of two elements, a game and its result: ((Eagles Herons) (2 1)) the first time. The signature of the block takes that list apart in one go. The outer parentheses take the two elements, and the inner ones take each element apart again, into four variables.

    given $home-goals <=> $away-goals {
        when More { %points{$home} += 3 }
        when Less { %points{$away} += 3 }
        default   { %points{$home} += 1; %points{$away} += 1 }
    }

<=> turns the score into one of three answers. More means the home team scored more and wins three points; Less means the away team wins. The only other answer is Same, a draw, and default gives both teams a point.

for %points.sort({ -.value, .key }) {
    say .fmt('%-8s %d');
}

The table is sorted by two things. The block returns a list for each team: minus the points, so that more points come first, and then the name. When the points are equal, as for the Owls and the Swans, the names decide. %-8s writes the name in eight characters, so the points line up.

say "Goals: {[+] @goals».sum}";
say 'Every team has a point.' if all(%points.values) > 0;

@goals».sum adds up each result on its own and gives (3 0 4 3 4 1). [+] adds those six numbers. The last line asks whether all the points are greater than zero. They are, because even the Herons drew once.

Questions

1. How many points do the Swans get, and from which games?
2. What would say @teams.combinations(2).elems; print for six teams?
3. What would the table look like if the last result were (1, 0) instead of (0, 1)?

Try this

Give three points for a win and nothing for a draw. Which line of the output disappears then?