Weather in four cities

Today’s temperature in Riga, Amsterdam, Turin and Belgrade, in Celsius and Fahrenheit, with a word for how it feels.

A weather report for four European cities. The temperatures are in Celsius, and the program also gives them in Fahrenheit, for a friend who lives in the United States.

Before you run it

In which order do the cities come out? And how does 59 look in the output: 59, 59.0 or 59.00?

The text

sub fahrenheit($celsius) {
    return $celsius * 9 / 5 + 32;
}

sub feels($celsius) {
    if $celsius < 5 {
        return 'cold';
    }
    if $celsius < 12 {
        return 'cool';
    }
    return 'mild';
}

my @cities = 'Riga', 'Amsterdam', 'Turin', 'Belgrade';
my %today = Riga => 4, Amsterdam => 11, Turin => 15, Belgrade => 9;

for @cities -> $city {
    my $c = %today{$city};
    say "$city: $c °C, {fahrenheit($c)} °F, {feels($c)}";
}

Riga: 4 °C, 39.2 °F, cold
Amsterdam: 11 °C, 51.8 °F, cool
Turin: 15 °C, 59 °F, mild
Belgrade: 9 °C, 48.2 °F, cool

Words

• sub — defines a function: a piece of code with a name that you can call again and again.
• $celsius — a parameter. When you call fahrenheit(4), the value 4 arrives in the sub as $celsius.
• return — ends the sub and gives a value back to the code that called it.
• @cities — an array: a list of values in a variable. Array names start with @.
• -> $city — gives a name to the current element of the loop.
• {fahrenheit($c)} — inside double quotes, { } runs the code, here a call of the sub, and puts the result into the string.

Reading notes

sub fahrenheit($celsius) {
    return $celsius * 9 / 5 + 32;
}

The first sub converts a temperature: multiply by 9, divide by 5, add 32. Defining a sub does not run it. It waits until something calls it.

sub feels($celsius) {
    if $celsius < 5 {
        return 'cold';
    }
    if $celsius < 12 {
        return 'cool';
    }
    return 'mild';
}

The second sub gives a word for the temperature. As soon as one return runs, the sub is over, so the checks below it are skipped. This is why the second if does not need to check that the temperature is 5 or more: if it were below 5, the sub would have returned already.

Both subs call their parameter $celsius. These are two different variables: each sub has its own.

my @cities = 'Riga', 'Amsterdam', 'Turin', 'Belgrade';
my %today = Riga => 4, Amsterdam => 11, Turin => 15, Belgrade => 9;

The cities are in an array, and today’s temperatures are in a hash, with the city as the key.

for @cities -> $city {
    my $c = %today{$city};
    say "$city: $c °C, {fahrenheit($c)} °F, {feels($c)}";
}

The loop goes over the array, so the cities come out in the order of the array, not in alphabetical order. For each city, it looks up the temperature and prints one line with the results of both subs.

4 * 9 / 5 + 32 is 39.2. 15 * 9 / 5 + 32 is exactly 59, and Raku prints it as 59, without a decimal point.

Questions

1. Which sub is called first when the loop prints the line for Riga?
2. What would feels(12) return?
3. If the second if in feels checked $celsius < 10, which city’s line would change?

Try this

Add Tallinn with 2 °C. You need to change two lines. Then add a third word to feels: freezing for temperatures below 0.