• The probabilistic function, P
k
ij(t) , is designed to favour the selection of a
road that has a high pheromone value, tÝ , and high visibility value, ?Ý , which is given
by, l/d
ij, where d
ij is the distance to the city. The pheromone scaling factor, a“ ,
and visibility scaling factor, ßG , are parameters used to tune the relative
importance of pheromone and road length in selecting the next city.
Apply Local Search
• Not used in Ant System, but is used in several variations of the TSP problem
where 2-opt or 3-opt local optimizers are used.
Best Tour check
• For each ant, calculate the length of the ant’s tour and compare to the best
tour’s length. If there is an improvement, update it.
Update Trails
• Evaporate a fixed proportion of the pheromone on each road.
• For each ant perform the “ant-cycle” pheromone update.
• Reinforce the best tour with a set number of “elitist ants” performing the “antcycle”
pheromone update.