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Ranges Part 1 - Basics

Posted: 2013-06-16 - Link

Iteration is one of the most common activities in programming. Few programming tasks can be accomplished without looping or recursing over some set of values, whether it be a stream of bytes from a file, elements of an array, rows in a database, or nodes in an implicitly generated graph. Programs need to iterate, and programming languages need to provide idioms for iteration.

In D, iteration is achieved through the use of ranges. Broadly speaking, a range defines a sequence of values. There are many styles of ranges. For example, some ranges lazily generate their values instead of iterating over data; some ranges are infinite; some ranges can be iterated from both ends; and some ranges allow you to jump around to any index (like arrays). For the most part, the details of how a particular range operates is up to the range implementor.

So how do you implement a range? At the most basic level, a range is just a type with three member functions: front, empty, and popFront.

As a simple example, here is a range that counts from 0 to ‘n’.

struct Iota
{
    private int m_current = 0;
    private int m_target;

    this(int n) { m_target = n; }
    @property int front() { return m_current; }
    @property bool empty() { return m_current == m_target; }
    void popFront() { m_current++; }
}

(The name Iota is a Greek letter. In a tradition started by the programming language APL, it is used to represent consecutive integers in several languages)

Notice that there is no need to inherit from any sort of IRange interface. There’s nothing magic about ranges, they are just simple types with those functions defined.

To iterate over a range, you can manually call those functions using a for loop, or use the built-in foreach loop in D, which knows about ranges.

void main()
{
    import std.stdio;
    foreach (x; Iota(10))
        writeln(x);
}

As expected, this will print out the numbers 0 through 9, one per line. Of course, writeln knows about ranges too, so writeln(Iota(10)) works as well, and will print out [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], formatted like an array.

Writing functions that work with ranges is not difficult. Suppose we want to write a function to count the number of elements in a range. This can be achieved by accepting the range as a template parameter and iterating the range using popFront() explicitly, like so:

size_t count(Range)(Range r)
{
    size_t n = 0;
    while (!r.empty)
    {
        ++n;
        r.popFront();
    }
    return n;
}

unittest
{
    assert(count(Iota(10)) == 10);
}

By using templates, we can create range types that are templated on other range types. For example, consider a skip range that skips every second element in another range. You could define it like this:

struct Skip(Range)
{
    private Range m_range;

    this(Range r) { m_range = r; }

    // front and empty just forward to the sub-range.
    @property auto ref front() { return m_range.front; }
    @property bool empty() { return m_range.empty; }

    // popFront also forwards to the sub-range, but pops off two
    // elements at a time, instead of one.
    void popFront()
    {
        m_range.popFront();
        if (!m_range.empty)
            m_range.popFront();
    }
}

We can then use Skip in conjunction with Iota to skip through consecutive integers.

void main()
{
    import std.stdio;
    writeln(Skip!Iota(Iota(10))); // [0, 2, 4, 6, 8]
}

Notice that we have to redundantly specify Iota as a template parameter to Skip. This is because D doesn’t currently support type inference for template type constructors. A common workaround is to create a module-level function that wraps the constructor:

auto skip(Range)(Range r) { return Skip!Range(r); }
auto iota(int n) { return Iota(n); }

We can then use these like so:

writeln(iota(10).skip());

The above code also makes use of another D feature: uniform function call syntax, which basically means f(x) can be written as x.f() as if f were a member function of x. If you come from the C# world, you can kind of think of it as every free function being an Extension Method. We could have also written 10.iota().skip() or just plain skip(iota(10)). There’s no difference, so choose whatever you think is most readable.

The ability to combine arbitrary ranges is perhaps their most powerful feature. There’s no reason we need to stop at combining just two ranges:

writeln(iota(10)); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
writeln(iota(10).skip()); // [0, 2, 4, 6, 8];
writeln(iota(10).skip().skip()); // [0, 4, 8];
writeln(iota(10).skip().skip().skip()); // [0, 8];

It shouldn’t be hard to imagine the kind of flexibility and expressiveness that can be achieved once you have a large vocabulary of ranges at your disposal.

In Part 2 we’ll look at some of the different categories of ranges.

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