Expression-Examples.md

Expression Examples

Examples of supported LINQ expressions are listed in this file. All examples utilize System.Drawing.Primitives types and share same initial conditions.

var start = new Point(2, 2);
var goal = new Point(18, 18);
var boundary = new Rectangle(0, 0, 20, 20); // map size
var unit = 1;

SelectMany

The example belows uses SelectMany() clause (nested from loops) to eliminate obstacles from available positions.

var queryable = HeuristicSearch.AStar(start, goal, (step, lv) => step.GetFourDirections(unit));
var obstacles = new[] { new Point(5, 5), new Point(6, 6), new Point(7, 7), new Point(8, 8) };
var solution = from step in queryable
               from obstacle in obstacles
               where step != obstacle
               orderby step.GetManhattanDistance(goal)
               select step;

Except

The example uses Except() to eliminate obstacles from available positions. The result is equivalent to SelectMany() example.

var queryable = HeuristicSearch.AStar(start, goal, (step, lv) => step.GetFourDirections(unit));
var obstacles = new[] { new Point(5, 5), new Point(6, 6), new Point(7, 7), new Point(8, 8) };
var solution = from step in queryable.Except(obstacles)
               where boundary.Contains(step)
               orderby step.GetManhattanDistance(goal)
               select step;

Using Except() is recommended when there are many positions to be eliminated.

Contains

The example uses Contains() to specify limited positions on the map.

var queryable = HeuristicSearch.AStar(start, goal, (step, lv) => step.GetFourDirections(unit));
var solution = from step in queryable.Contains(boundary.GetAvailablePoints(unit))
               orderby step.GetManhattanDistance(goal)
               select step;

Using Contains() is recommended when available nodes are limited such as tree or graph traversal.

Reverse

The example uses Reverse() to reverse solution from start to goal to goal to start.

var queryable = HeuristicSearch.AStar(start, goal, (step, lv) => step.GetFourDirections(unit));
var solution = from step in queryable.Reverse()
               where boundary.Contains(step)
               orderby step.GetManhattanDistance(goal)
               select step;

OrderBy and ThenBy

The example shows two heuristic functions co-existing in same expression. When GetManhattanDistance() method evaluates two positions as same cost, the sequent method GetEuclideanDistance() will be referred to evaluate them.

var queryable = HeuristicSearch.AStar(start, goal, (step, lv) => step.GetFourDirections(unit));
var solution = from step in queryable
               where boundary.Contains(step)
               orderby step.GetManhattanDistance(goal), step.GetEuclideanDistance(goal)
               select step;