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// 3x3 grid
// based on Littman, Cassandra and Kaelbling
// Learning policies for partially observable environments: Scaling up
// Technical Report CS, Brown University
pomdp
// only the target is observable which is in the south east corner
observables
o
endobservables
module grid
x : [0..2]; // x coordinate
y : [0..2]; // y coordinate
o : [0..2]; // observables
// 0 - initial observation
// 1 - in the grid (not target)
// 2 - observe target
// initially randomly placed within the grid (not at the target)
[] o=0 -> 1/8 : (o'=1) & (x'=0) & (y'=0)
+ 1/8 : (o'=1) & (x'=0) & (y'=1)
+ 1/8 : (o'=1) & (x'=0) & (y'=2)
+ 1/8 : (o'=1) & (x'=1) & (y'=0)
+ 1/8 : (o'=1) & (x'=1) & (y'=1)
+ 1/8 : (o'=1) & (x'=1) & (y'=2)
// + 1/8 : (o'=1) & (x'=2) & (y'=0) the target
+ 1/8 : (o'=1) & (x'=2) & (y'=1)
+ 1/8 : (o'=1) & (x'=2) & (y'=2);
// move around the grid
[east] o=1 & !(x=1 & y=0) -> (x'=min(x+1,2)); // not reached target
[east] o=1 & x=1 & y=0 -> (x'=min(x+1,2)) & (o'=2);
[west] o=1 -> (x'=max(x-1,0)); // not reached target
[north] o=1 -> (y'=min(y+1,2)); // reached target
[south] o=1 & !(x=2 & y=1) -> (y'=max(y-1,0)); // not reached target
[south] o=1 & x=2 & y=1 -> (y'=max(y-1,0)) & (o'=2); // reached target
// reached target
[done] o=2 -> true;
endmodule
// reward structure for number of steps to reach the target
rewards
[east] true : 1;
[west] true : 1;
[north] true : 1;
[south] true : 1;
endrewards
// target observation
label "goal" = o=2;