TomSym Planning a Flight Tour

This page is part of the TomSym Manual. See TomSym Manual.

Problem description

A country in south-east Asia is experiencing widespread flooding. The government, with international help, decides to establish a system of supply by air. Unfortunately, only seven runways are still in a usable state, among which is the one in the capital.

The government decides to make the planes leave from the capital, have them visit all the other six airports and then come back to the capital. The following table lists the distances between the airports. Airport A1 is the one in the capital. In which order should the airports be visited to minimize the total distance covered?

Distance matrix between airports (in km)

+--+---+---+---+---+---+---+
|  | A2| A3| A4| A5| A6| A7|
+--+---+---+---+---+---+---+
|A1|786|549|657|331|559|250|
|A2|668|979|593|224|905|   |
|A3|316|607|472|467|   |   |
|A4|890|769|400|   |   |   |
|A5|386|559|   |   |   |   |
|A6|681|   |   |   |   |   |
+--+---+---+---+---+---+---+

Variables

distances                   The distance matrix

Reference

Applications of optimization... Gueret, Prins, Seveaux

% Marcus Edvall, Tomlab Optimization Inc, E-mail: tomlab@tomopt.com
% Copyright (c) 2005-2009 by Tomlab Optimization Inc., $Release: 7.2.0$
% Written Oct 7, 2005.   Last modified Apr 8, 2009.

Problem setup

distances = [   0   786   549    657    331    559   250;...
    786     0   668    979    593    224   905;...
    549   668     0    316    607    472   467;...
    657   979   316      0    890    769   400;...
    331   593   607    890      0    386   559;...
    559   224   472    769    386      0   681;...
    250   905   467    400    559    681     0];

n = size(distances,1);
fly = tom('fly',n,n,'int');

% All variables are binary.
bnds = {0 <= fly <= 1, fly(1:n+1:n^2) == 0};

% Cycle constraints
con = {sum(fly,1) == 1; sum(fly,2) == 1};

% Objective
objective = sum(sum(distances.*fly));

constraints = {bnds, con};
options = struct;
options.solver = 'cplex';
options.name   = 'Planning a Flight Tour';

stop = 0;

while stop<100
    sol = ezsolve(objective,constraints,[],options);
    % Find a subtour
    idx = zeros(n,n);
    idx(1,1:n) = 1:7;
    for i=1:n
        for j=2:n
            idx(j,i) = find(sol.fly(:,idx(j-1,i)) ~= 0);
            if idx(j,i) == i
                break
            end
        end
    end
    shortmat = spones(idx);
    len = full(sum(shortmat,1));
    [val, idx2] = min(len);
    idx3 = idx(1:val, idx2);

    idx4 = zeros(val-1,1);
    for k=1:val-1
        idx4(k) = idx3(k)+n*(idx3(k+1)-1);
    end
    constraints = {constraints; sum(fly(idx4)) <= 1};

    if val == n
        break
    end
    stop = stop + 1;
end

PriLev = 1;
if PriLev > 0
    temp           = sol.fly;                    % reshape results
    not_home_again = true;                       % help to loop
    current_pos    = 1;                          % we start in 1
    route          = [];                         % initially empty
    route          = [route current_pos];        % add position
    while not_home_again,                        % loop if not home
        current_pos = find(temp(current_pos,:)); % go to next pos
        route = [route current_pos];             % add to route
        if current_pos == 1                      % if home
            not_home_again = false;              % stop loop
        end
    end
    disp(['Shortest route is: ' num2str(route)])
    disp(['               or: ' num2str(route(end:-1:1))])
end

% MODIFICATION LOG
%
% 051107 med   Created.
% 060116 per   Added documentation.
% 060125 per   Moved disp to end
% 090308 med   Converted to tomSym

Problem type appears to be: mip
Time for symbolic processing: 0.018942 seconds
Starting numeric solver
===== * * * =================================================================== * * *
TOMLAB - TOMLAB Development license  999007. Valid to 2011-12-31
=====================================================================================
Problem: ---  1: Planning a Flight Tour         f_k    2220.000000000000000000
                                              f(x_0)      0.000000000000000000

Solver: CPLEX.  EXIT=0.  INFORM=101.
CPLEX Branch-and-Cut MIP solver
Optimal integer solution found

FuncEv   12 
Elapsed time: 0.004000 sec. 
Problem type appears to be: mip
Time for symbolic processing: 0.023414 seconds
Starting numeric solver
===== * * * =================================================================== * * *
TOMLAB - TOMLAB Development license  999007. Valid to 2011-12-31
=====================================================================================
Problem: ---  1: Planning a Flight Tour         f_k    2335.000000000000000000
                                              f(x_0)      0.000000000000000000

Solver: CPLEX.  EXIT=0.  INFORM=101.
CPLEX Branch-and-Cut MIP solver
Optimal integer solution found

FuncEv   11 
Elapsed time: 0.003000 sec. 
Problem type appears to be: mip
Time for symbolic processing: 0.026752 seconds
Starting numeric solver
===== * * * =================================================================== * * *
TOMLAB - TOMLAB Development license  999007. Valid to 2011-12-31
=====================================================================================
Problem: ---  1: Planning a Flight Tour         f_k    2575.000000000000000000
                                              f(x_0)      0.000000000000000000

Solver: CPLEX.  EXIT=0.  INFORM=101.
CPLEX Branch-and-Cut MIP solver
Optimal integer solution found

FuncEv   15 
Elapsed time: 0.003000 sec. 
Shortest route is: 1  7  4  3  2  6  5  1
               or: 1  5  6  2  3  4  7  1