Generic method to calcuate Euclidean Distance using expressions

c# expression expression-trees linq

Question

I'd like a method that can calculate the Euclidean distance using expressions and order an IQueryable:

sqrt[(q1 - p1)^2 + (q2 - p2)^2 + ... + (qn - pn)^2]

This is the method signature I've come up with:

public static IOrderedQueryable<T> EuclideanDistanceOrder<T>(
    this IQueryable<T> query, IEnumerable<Expression<Func<T, double>>> expressions)
    {
        var orderedQuery = query.OrderBy(i => Math.Sqrt(expressions.Aggregate((total, item) => total + Math.Pow(item, 2))));
        return orderedQuery;
    }

I'm not sure what to do with item and total (since they are Expression<Func<T, double>>). I've tried this a few different ways, including using Expression.Power and Expression.Add. I've tried defining the expressions to be composed separately:

Expression<Func<double, double>> power = i => Math.Pow(i, 2);
Expression<Func<List<Expression<Func<T, double>>>, double>> dist = (items) => Math.Sqrt(items.Sum(power));

But I still don't know what to do with power.

Can I get some help? Is there a better way to approach this?

Popular Answer

To get it working with EF or LinqToSQL you would have to pass all information as Expressions even property accessors for P and Q. So that is why I have modified your method declaration:

public static class Extension
{
    public static IOrderedQueryable<T> EuclideanDistanceOrder<T>(
        this IQueryable<T> query, 
        IEnumerable<Expression<Func<T, double>>> pExpressions, 
        IEnumerable<Expression<Func<T, double>>> qExpressions)
    {
        var parameter = Expression.Parameter(typeof(T));
        var pBodies = pExpressions
            .Select(x => ReplaceParameter(x.Body, parameter))
            .ToArray();

        var qBodies = qExpressions
            .Select(x => ReplaceParameter(x.Body, parameter))
            .ToArray();

        var distances = pBodies
            .Select((x, i) => CreateDistance(x, qBodies[i]))
            .ToArray();

        var squers = distances
            .Select(x => CreateSquerExpression(x))
            .ToArray();

        var sum = squers.First();
        for (int i = 1; i < squers.Count(); i++)
        {
            sum = Expression.Add(sum, squers[i]);
        }
        var funcExpression = Expression.Lambda<Func<T, double>>(sum, parameter);
        //the sqrt is irrelevant to order of this sequence
        return query.OrderBy(funcExpression);
    }

    private static Expression CreateDistance(Expression p, Expression q)
    {
        return Expression.Subtract(q, p);
    }

    private static Expression CreateSquerExpression(Expression x)
    {
        var method = typeof(Math).GetMethod("Pow", BindingFlags.Static | BindingFlags.Public);
        return Expression.Call(method, x, Expression.Constant(2.0));
    }

    private static Expression ReplaceParameter(Expression expression, ParameterExpression parameter)
    {
        var unaryExpression = expression as UnaryExpression;
        MemberExpression memberExpression;
        if (unaryExpression != null)
        {
            memberExpression = unaryExpression.Operand as MemberExpression;
        }
        else
        {
            memberExpression = expression as MemberExpression;
        }

        if (memberExpression == null)
            throw new NotImplementedException();

        if (!(memberExpression.Expression is ParameterExpression) || !(memberExpression.Member is PropertyInfo))
            throw new NotImplementedException();

        return Expression.Property(parameter, (PropertyInfo)memberExpression.Member);
    }
}

While invoking like this:

    var list = new[]{ new Item
    {
        P1 = 0,
        Q1 = 0,
        P2 = 3,
        Q2 = 1,
    },
    new Item
    {
        P1 = 0,
        Q1 = 0,
        P2 = 2,
        Q2 = 1,
    }
};

var query = list.AsQueryable();

var result = query.EuclideanDistanceOrder(new Expression<Func<Item, double>>[]{
    x => x.P1,
    x => x.P2
},
new Expression<Func<Item, double>>[]{
    x => x.Q1,
    x => x.Q2
}).ToArray();

internal class Item
{
    public double P1 { get; set; }
    public double Q1 { get; set; }
    public double P2 { get; set; }
    public double Q2 { get; set; }
}

It works for liq to objects. I'm only not sure if EF or linqtoSql will map Math.Power method to sql. If not it is easy enough to change to multiplication.



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Is this KB legal? Yes, learn why