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ant-colony

ACO Pheromone update

I'm working on ACO and a little confused about the probability of choosing the next city. I have read some papers and books but still the idea of choosing is unclear. I am looking for a simple explanation how this path building works. Also how does the heuristics and pheromone come into this decision making? Because we have same pheromone values at every edge in the beginning and the heuristics (closeness) values remains constant, so how will different ants make different decisions based on these values?

Solution

  • Maybe is too late to answer to question but lately I have been working with ACO and it was also a bit confusing for me. I didn't find much information on stackoverflow about ACO when I needed it, so I have decided to answer this question since maybe this info is usefull for people working on ACO now or in the future.

    Swarm Intelligence Algorithms are a set of techniques based on emerging social and cooperative behavior of organisms grouped in colonies, swarms, etc.

    The collective intelligence algorithm Ant Colony Optimization (ACO) is an optimization algorithm inspired by ant colonies. In nature, ants of some species initially wander randomly until they find a food source and return to their colony laying down a pheromone trail. If other ants find this trail, they are more likely not to keep travelling at random, but instead to follow the trail, and reinforcing it if they eventually find food.

    In the Ant Colony Optimization algorithm the agents (ants) are placed on different nodes (usually it is used a number of ants equal to the number of nodes). And, the probability of choosing the next node (city) is based on agents chose the next node using the equation known as the transition rule, that represents the probability for ant π‘˜ to go from city 𝑖 to city 𝑗 on the 𝑑th tour.

    enter image description here

    In the equation, πœπ‘–π‘— represents the pheromone trail and πœ‚π‘–π‘— the visibility between the two cities, while 𝛼 and 𝛽 are adjustable parameters that control the relative weight of trail intensity and visibility.

    At the beginning there is the same value of pheromone in all the edges. Based on the transition rule, which, in turn, is based on the pheromone and the visibility (distance between nodes), some paths will be more likely to be chosen than others.

    When the algorithm starts to run each agent (ant) performs a tour (visits each node), the best tour found until the moment will be updated with a new quantity of pheromone, which will make that tour more probably to be chosen next time by the ants.

    You can found more information about ACO in the following links:

    http://dataworldblog.blogspot.com.es/2017/06/ant-colony-optimization-part-1.html

    http://dataworldblog.blogspot.com.es/2017/06/graph-optimization-using-ant-colony.html

    http://hdl.handle.net/10609/64285

    Regards,

    Ester