## Finding the Nearest Tube Station with LINQ

I recently wrote a little command line program in C# that can tell you the name of the nearest tube station. I’ve put the program on GitHub in my Coding Experiments repository:

https://github.com/robert-impey/CodingExperiments/tree/master/C%23/NearestTube

The interface is very simple- this more of an experiment than something that I intend for mass consumption! You enter a location via the command line as the latitude and longitude:

The starting point of the program was to calculate the distances between points. To do this I adapted some JavaScript code that John D. Cook wrote:

http://www.johndcook.com/lat_long_distance.html

Mathematical code looks very similar in many languages, and the translation from JavaScript to C# was trivially simple as the languages are very similar in this case. This code is part of the Point class:

        ///
/// Porting JavaScript code from
/// http://www.johndcook.com/lat_long_distance.html
///
///
///
public double Distance(Point that)
{
// Compute spherical coordinates

double rho = 6373;

// convert latitude and longitude to spherical coordinates in radians
// phi = 90 - latitude
double phi1 = (90.0 - this.Latitude) * Math.PI / 180.0;
double phi2 = (90.0 - that.Latitude) * Math.PI / 180.0;
// theta = longitude
double theta1 = this.Longitude * Math.PI / 180.0;
double theta2 = that.Longitude * Math.PI / 180.0;

// compute spherical distance from spherical coordinates
// arc length = \arccos(\sin\phi\sin\phi'\cos(\theta-\theta') + \cos\phi\cos\phi')
// distance = rho times arc length
return rho * Math.Acos(
Math.Sin(phi1) * Math.Sin(phi2) * Math.Cos(theta1 - theta2)
+ Math.Cos(phi1) * Math.Cos(phi2)) * 1000;
}


Once I was able to calculate the distances between points on the map, I needed to be able to sort the tube stations of London by distance from the given point. This is the sort of code that LINQ to objects handles very naturally. In the SequentialTubeStationFinder class, I have the following method:

        public TubeStation FindNearestTubeStation(Point point)
{
return (from tubeStation in tubeStations
orderby tubeStation.Point.Distance(point)
select tubeStation).First();
}


tubeStations is a generic list of TubeStation objects. Each has a Point property that is used to sort the tube stations and find the nearest one to our location.

The declarative style of programming that LINQ uses is much easier to read (and therefore maintain) than the equivalent code written with a for loop.