At the market

Five things from the market and what they cost. In what order does the program list them?

We came back from the market with five things. The program lists what we bought, adds up the prices, and checks the budget.

Before you run it

The program lists the five items with their prices. In what order? Is it the order in which they are written in the program?

The text

# What we bought at the market, with prices in euro.
my %prices =
    bread  => 2.15,
    cheese => 4.75,
    apples => 3.25,
    milk   => 1.15,
    coffee => 6.45;

my $total = 0;
for %prices.keys.sort -> $item {
    say "$item: %prices{$item}";
    $total = $total + %prices{$item};
}

say "Items: {%prices.elems}";
say "Total: $total";
say "Average: {$total / %prices.elems}";

unless $total > 20 {
    say 'We stayed within our budget of 20 euro.';
}

apples: 3.25
bread: 2.15
cheese: 4.75
coffee: 6.45
milk: 1.15
Items: 5
Total: 17.75
Average: 3.55
We stayed within our budget of 20 euro.

Words

• %prices — a hash: a table that connects keys to values. Hash names start with %.
• => — makes a pair of a key and a value: bread => 2.15. The key on the left does not need quotes.
• .keys — gives the keys of a hash: here, the names of the items.
• .sort — gives the elements of a list in order: numbers from small to large, strings in alphabetical order.
• -> $item — gives a name to the current element of a for loop, instead of $_.
• %prices{$item} — the value for the key $item. Inside double quotes, Raku replaces it with the value.
• {%prices.elems} — inside double quotes, the code in { } runs and its result goes into the string. .elems counts the elements.

Reading notes

my %prices =
    bread  => 2.15,
    cheese => 4.75,
    apples => 3.25,
    milk   => 1.15,
    coffee => 6.45;

The hash is filled with five pairs. The statement takes six lines, but it is one statement: it ends with the semicolon after 6.45. The spaces that line up the arrows are only there for the eye.

my $total = 0;
for %prices.keys.sort -> $item {
    say "$item: %prices{$item}";
    $total = $total + %prices{$item};
}

A hash does not remember the order in which you put things in. If you want an order, you ask for one: %prices.keys gives the five names, and .sort puts them in alphabetical order. That is why apples comes first, although it is third in the program.

For each name, the loop prints the name and the price and adds the price to $total.

say "Items: {%prices.elems}";
say "Total: $total";
say "Average: {$total / %prices.elems}";

$total can go straight into the string. %prices.elems cannot: inside double quotes, a hash name is only replaced when it is followed by { } with a key. So the program puts the code in { }, and Raku runs it. The same goes for the division on the last line.

The average is exactly 3.55. Raku keeps numbers like 2.15 exact, as fractions, so sums and averages of prices have no rounding errors in them.

unless $total > 20 {
    say 'We stayed within our budget of 20 euro.';
}

unless is the opposite of if: the block runs when the condition is false. The total is 17.75, which is not more than 20, so the line is printed.

Questions

1. Which item is printed last, and why?
2. What would say "Items: %prices.elems"; print, without the { }?
3. What would change in the output if cheese cost 7.25?

Try this

Print the items from the most expensive to the cheapest. Hint: .sort can take a block that says what to sort by, for example .sort({ %prices{$_} }), and .reverse turns a list around.