AMPL Modeling Tips #3: Alldifferent

Consider the N-Queens problem: How can N queens be placed on an NxN chessboard so that no two of them attack each other? [ Solve it on Google Colab! ]

  • Constraint alldiff enforces a set of integer variables to take distinct values.
s.t. OneJobPerMachine: 
   alldiff {j in JOBS} MachineForJob[j];
  • alldiff can be used conditionally:
s.t. VisitOnce {j in GUESTS}:
   IsHost[j] = 0 ==> alldiff {t in TIMES} Visit[j,t];
  • Older MIP drivers need a manual reformulation of alldiff with binary variables:
s.t. OneMachinePerJob {j in JOBS}:
   sum {k in MACHINES} Assign[j,k] = 1;

s.t. OneJobPerMachine {k in MACHINES}:
   sum {j in JOBS} Assign[j,k] = 1;

Back to the N-Queens problem

New MP Library-based, as well as Constraint Programming drivers, accept alldiff directly:

param n integer > 0;             # N queens
var Row {1..n} integer >= 1 <= n;
s.t. row_attacks: alldiff ({j in 1..n} Row[j]);
s.t. diag_attacks: alldiff ({j in 1..n} Row[j]+j);
s.t. rdiag_attacks: alldiff ({j in 1..n} Row[j]-j);

A reformulated model for older MIP drivers:

param n integer > 0;
var X{1..n, 1..n} binary; 
   # X[i,j] is one if there is a queen at (i,j); else zero

s.t. column_attacks {j in 1..n}:
	sum {i in 1..n} X[i,j] = 1;

s.t. row_attacks {i in 1..n}:
	sum {j in 1..n} X[i,j] = 1;

s.t. diagonal1_attacks {k in 2..2*n}:
	sum {i in 1..n, j in 1..n: i+j=k} X[i,j] <= 1;

s.t. diagonal2_attacks {k in -(n-1)..(n-1)}:
	sum {i in 1..n, j in 1..n: i-j=k} X[i,j] <= 1;

Running both models with HiGHS:

ampl: include nqueens.mod;
ampl: let n := 8;
ampl: option solver highs;
ampl: solve;
HiGHS 1.4.0: optimal solution
0 simplex iterations
1 branching nodes
Objective = find a feasible point.

Another problem can be modeled with alldiff: Sudoku. A GUI-based Colab Notebook.

MP Documentation on the alldiff operator can be found at

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