Task: Evolutionary algorithm


C++: #include <string>
#include <cstdlib>
#include <iostream>
#include <cassert>
#include <algorithm>
#include <vector>
#include <ctime>

std::string allowed_chars = " ABCDEFGHIJKLMNOPQRSTUVWXYZ";






class selection
{
public:
  
  
  
  
  static int fitness(std::string candidate)
  {
    assert(target.length() == candidate.length());

    int fitness_so_far = 0;

    for (int i = 0; i < target.length(); ++i)
    {
      int target_pos = allowed_chars.find(target[i]);
      int candidate_pos = allowed_chars.find(candidate[i]);
      int diff = std::abs(target_pos - candidate_pos);
      fitness_so_far -= std::min(diff, int(allowed_chars.length()) - diff);
    }

    return fitness_so_far;
  }

  
  static int target_length() { return target.length(); }
private:
  static std::string target;
};

std::string selection::target = "METHINKS IT IS LIKE A WEASEL";


void move_char(char& c, int distance)
{
  while (distance < 0)
    distance += allowed_chars.length();
  int char_pos = allowed_chars.find(c);
  c = allowed_chars[(char_pos + distance) % allowed_chars.length()];
}



std::string mutate(std::string parent, double mutation_rate)
{
  for (int i = 0; i < parent.length(); ++i)
    if (std::rand()/(RAND_MAX + 1.0) < mutation_rate)
    {
      int distance = std::rand() % 3 + 1;
      if(std::rand()%2 == 0)
        move_char(parent[i], distance);
      else
        move_char(parent[i], -distance);
    }
  return parent;
}



bool less_fit(std::string const& s1, std::string const& s2)
{
  return selection::fitness(s1) < selection::fitness(s2);
}

int main()
{
  int const C = 100;

  std::srand(time(0));

  std::string parent;
  for (int i = 0; i < selection::target_length(); ++i)
  {
    parent += allowed_chars[std::rand() % allowed_chars.length()];
  }

  int const initial_fitness = selection::fitness(parent);

  for(int fitness = initial_fitness;
      fitness < 0;
      fitness = selection::fitness(parent))
  {
    std::cout << parent << ": " << fitness << "\n";
    double const mutation_rate = 0.02 + (0.9*fitness)/initial_fitness;
    std::vector<std::string> childs;
    childs.reserve(C+1);

    childs.push_back(parent);
    for (int i = 0; i < C; ++i)
      childs.push_back(mutate(parent, mutation_rate));

    parent = *std::max_element(childs.begin(), childs.end(), less_fit);
  }
  std::cout << "final string: " << parent << "\n";
}







VB: Option explicit

Function rand(l,u) rand= Int((u-l+1)*Rnd+l): End Function

Function fitness(i)
  Dim d,j
  d=0
  For j=1 To lmod
     If Mid(model,j,1)=Mid(cpy(i),j,1) Then d=d+1
  Next
fitness=d
End Function

Sub mutate(i)
Dim j
 cpy(i)=""
 For j=1 To lmod
 If Rnd<mut Then 
  cpy(i)=cpy(i)& Chr(rand(lb,ub))
 Else
  cpy(i)=cpy(i)& Mid(cpy(0),j,1)
 End If
 Next
End sub 
  
Sub print(s): 
   On Error Resume Next
    WScript.stdout.WriteLine (s)  
   If  err= &h80070006& Then WScript.Echo " Please run this script with CScript": WScript.quit
End Sub 

Dim model,lmod,ub,lb,c,cpy,fit,best,i,d,mut,cnt
model="METHINKS IT IS LIKE A WEASEL"
model=Replace(model," ","@")
lmod=Len(model)
Randomize Timer
ub=Asc("Z")
lb=Asc("@")
c=10
mut=.05
best=0
ReDim cpy(c)


For i=0 To lmod-1
 cpy(best)=cpy(best)& chr(rand(lb,ub))
Next
best=0
cnt=0
do
  cpy(0)=cpy(best)
  fit=0
  For i=1 To c
   mutate(i)  
   d=fitness(i)
   If d>fit Then fit=d:best=i  
  Next
  cnt=cnt+1
  If (fit=lmod) Or ((cnt mod 10)=0)  Then print cnt &" " & fit & " "& Replace (cpy(best),"@"," ") 
Loop While fit <>lmod