The departure board

Five trains, one small class, and three questions to the timetable — all trains in order, the trains to Rotterdam, and the first one after nine.

A station has five trains this morning. The program describes a train as a class, puts five trains in a list, and asks that list three questions.

Before you run it

The trains are written in the program in no particular order. Which train does the last line name? Read the condition on that line carefully.

The text

class Train {
    has $.number;
    has $.to;
    has $.departs;
    has $.platform;

    method board {
        "$.departs  $.to (train $.number), platform $.platform";
    }
}

my @trains =
    Train.new(number => 3014, to => 'Utrecht',   departs => '09:12', platform => 5),
    Train.new(number => 1745, to => 'Rotterdam', departs => '08:47', platform => 2),
    Train.new(number => 3020, to => 'Utrecht',   departs => '09:42', platform => 5),
    Train.new(number => 2241, to => 'Haarlem',   departs => '09:05', platform => 8),
    Train.new(number => 1749, to => 'Rotterdam', departs => '09:17', platform => 2);

say 'All departures:';
for @trains.sort(*.departs) -> $train {
    say $train.board;
}

say '';
say 'To Rotterdam:';
for @trains.grep(*.to eq 'Rotterdam') -> $train {
    say $train.board;
}

say '';
my $next = @trains.sort(*.departs).first(*.departs gt '09:00');
say "First train after nine: {$next.number} to {$next.to}";

All departures:
08:47  Rotterdam (train 1745), platform 2
09:05  Haarlem (train 2241), platform 8
09:12  Utrecht (train 3014), platform 5
09:17  Rotterdam (train 1749), platform 2
09:42  Utrecht (train 3020), platform 5

To Rotterdam:
08:47  Rotterdam (train 1745), platform 2
09:17  Rotterdam (train 1749), platform 2

First train after nine: 2241 to Haarlem

Words

• class — describes a kind of object: what it knows and what it can do.
• has — declares an attribute, a piece of data that every object of the class carries.
• $.number — an attribute with a dot: Raku also makes a method with the same name, so other code can ask a train for its number.
• method — a sub that belongs to a class and works on one object.
• Train.new — makes a new object. You give the attributes as named arguments: number => 3014.
• *.departs — the whatever star. *.departs is a short way to write a small piece of code: “take a thing and give me its .departs“.
• .grep — keeps the elements of a list for which a condition is true.
• .first — gives the first element for which a condition is true.
• gt — “greater than” for strings: compares them in alphabetical order.

Reading notes

class Train {
    has $.number;
    has $.to;
    has $.departs;
    has $.platform;

Every train knows four things: its number, where it goes, when it leaves, and from which platform. The times are strings, like '09:12'.

    method board {
        "$.departs  $.to (train $.number), platform $.platform";
    }
}

The method makes the line that the departure board shows. Inside double quotes, $.departs and the other attributes are replaced by their values. There is no return: a method gives back the value of its last statement.

my @trains =
    Train.new(number => 3014, to => 'Utrecht',   departs => '09:12', platform => 5),
    ...

Five calls of Train.new, separated by commas, make a list of five trains.

for @trains.sort(*.departs) -> $train {
    say $train.board;
}

.sort(*.departs) sorts the trains by their departure time. The array itself does not change: .sort gives you a new, sorted list. Times written as '08:47' and '09:12' sort correctly as strings, because they always have two digits for the hour and two for the minutes.

for @trains.grep(*.to eq 'Rotterdam') -> $train {
    say $train.board;
}

.grep keeps the trains whose destination is Rotterdam. It does not sort: the two trains come out in the order of the array, which happens to be the order of their times.

my $next = @trains.sort(*.departs).first(*.departs gt '09:00');
say "First train after nine: {$next.number} to {$next.to}";

The last question chains two methods: sort the trains by time, then take the first one that leaves after '09:00'. That is train 2241 to Haarlem, at 09:05. Without the .sort, .first would find train 3014, because it is first in the array and leaves at 09:12.

Questions

1. What does $next.platform give?
2. What would the program print under “To Rotterdam:” if the condition were *.platform == 5?
3. Why does the method board not need return?

Try this

Ask the timetable a fourth question: print the numbers of all trains that leave from platform 2, sorted by time. One line of code is enough, for example with .grep, .sort and .map(*.number).