Design Patterns: Interpreter

The Interpreter Design Pattern was created to solve an issue interpreting data from a context. By having separate classes, one for each type of interpretation, it is possible to have well-defined classes with single purposes.

To do this design our application needs a structure similar to the following:

Context <-- Client --> Expression1
`-> Expression2

The client has a context that is sent to a list of expression interpreters and they set the output to the final result.

For example, we can have expressions to parse decimal numbers into roman numerals.

public class RomanContext
{
    public RomanContext(int input)
    {
        Input = input;
    }

    public int Input { get; set; }

    public string Output { get; set; }
}

public abstract class Expression
{
    public abstract void Interpret(RomanContext value);
}

public class RomanOneExpression : Expression
{
    public override void Interpret(RomanContext value)
    {
        while ((value.Input - 9) >= 0)
        {
            value.Output += "IX";

            value.Input -= 9;
        }

        while ((value.Input - 5) >= 0)
        {
            value.Output += "V";

            value.Input -= 5;
        }

        while ((value.Input - 4) >= 0)
        {
            value.Output += "IV";

            value.Input -= 4;
        }

        while ((value.Input - 3) >= 0)
        {
            value.Output += "III";

            value.Input -= 3;
        }

        while ((value.Input - 2) >= 0)
        {
            value.Output += "II";

            value.Input -= 2;
        }

        while ((value.Input - 1) >= 0)
        {
            value.Output += "I";

            value.Input -= 1;
        }
    }
}

public class RomanTenExpression : Expression
{
    public override void Interpret(RomanContext value)
    {
        while ((value.Input - 90) >= 0)
        {
            value.Output += "XC";

            value.Input -= 90;
        }

        while ((value.Input - 50) >= 0)
        {
            value.Output += "L";

            value.Input -= 50;
        }

        while ((value.Input - 40) >= 0)
        {
            value.Output += "XL";

            value.Input -= 40;
        }

        while ((value.Input - 30) >= 0)
        {
            value.Output += "XXX";

            value.Input -= 30;
        }

        while ((value.Input - 20) >= 0)
        {
            value.Output += "XX";

            value.Input -= 20;
        }

        while ((value.Input - 10) >= 0)
        {
            value.Output += "X";

            value.Input -= 10;
        }
    }
}

In the preceding example one class is responsible for interpreting the numbers 1 to 9 while the second handles 10 to 90. So if we combine them we can parse numbers from 1 to 99 as in the following code snippet:

Expression[] expressions = new Expression[]
{
    new RomanTenExpression(),
    new RomanOneExpression(),
};

var context = new RomanContext(99);

foreach (var expression in expressions)
{
    expression.Interpret(context);
}

context.Output; // "XCIX"

Other practical uses could be parsing source code, or an “engrish sentençe” (did you mean “English sentence”?) and so on.