Introduction to Evolutionary Algorithms

8. The Parent Selector

The parent selector is used to select a single parent for use in breeding. The TParentSelector class defines a basic interface - see listing 8. The "SelectParent" function returns a reference to the individual in the population which has been selected to be the parent.

  TParentSelector = class(TComponent)
    // This function returns a reference to a single parent, selected from Pop
    function SelectParent(Population : TPopulation) : TIndividual; virtual; abstract;
  end;

The TParentSelectorTournament class implements a simple parent selection scheme...

• Select two different individuals at random from the population
• Compare their fitness levels (the tournament)
• Select the fittest to be the parent

This parent selection scheme is close to what happens in many natural species where breeding partners are chosen based on displays of strength or healthiness. Code for the TParentSelectorTournament class and the "SelectParent" function is shown in listing 9.

interface

type
  TTSPParentSelectorTournament = class(TTSPParentSelector)
  public
    function SelectParent(Population : TPopulation) : TIndividual; override;
  end;


implementation

function TTSPParentSelectorTournament.SelectParent(Population: TPopulation): TIndividual;
var
  i1, i2  : Integer;
begin
  // Select a random individual
  i1 := Random(Population.Count);

  // Select a *different* random individual
  repeat
    i2 := Random(Population.Count);
  until i1 <> i2;

  // Hold the tournament and return the fittest of the two
  if Population.FitnessOf(i1) < Population.FitnessOf(i2) then
    Result := Population[i1]
  else
    Result := Population[i2]
end;

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