Hello Alejandro The comment above these three lines is important to understand what they are meant to do: // Just assume that the circle starts forwards
So this is sort of symmetry breaking here. The "element" constraint here ensures that "p0 is the value whose successor is 0" (i.e. the position/index of the value "0" in the array "succ"). And then they say that the successor of 0 must not be less than this p0. So, the three lines altogether can be summed up as "the successor of 0 must not be smaller then the predecessor of 0". Obviously, every tour (represented by the successor relationship) could be "reversed" and yield the same result (due to symmetry), and the solver shall only consider one out of each pair of such two "equivalent" solutions, which is w.l.o.g. chosen to be the one fulfilling this constraint. Note also that many predefined constraint posting functions in Gecode are overloaded such that where an IntVar object is allowed, you could also just use an int (which is semantically equivalent to having a variable that can only have that one particular value): http://www.gecode.org/doc/5.1.0/reference/group__TaskModelIntElement.html Best Sebastian On 17.02.2018 13:50, Alejandro.Fernández wrote: > Hello group. > > I have been studying the TSP example that is in the gecode.org > <http://gecode.org/> site package and I have the following doubt > regarding the element function: > > IntVar p0 (* this, 0, n - 1); > element (* this, succ, p0, 0); // here (1) > rel (* this, p0, IRT_LE, succ [0]); > > According to 'MPG.pdf' pg. 70 (4.4.12): > > element (home, c, x, y) // (2) > > Says, variable y to be the element of the array c at index x, then how > can it be possible (1) ??? > > regards > Alex > > > _______________________________________________ > Gecode users mailing list > users@gecode.org > https://www.gecode.org/mailman/listinfo/gecode-users > _______________________________________________ Gecode users mailing list users@gecode.org https://www.gecode.org/mailman/listinfo/gecode-users
