A Basic Example

Getting started

The aim of this section is to give you an overview of the READ and WRITE statements through a short illustrative example. It shows how to read data from and write data to text files and database tables. It is based on the familiar transport problem with the following input data:

  • the set Cities,

  • the relation Routes from Cities to Cities,

  • the parameters Supply(i) and Demand(i) for each city i, and

  • the parameters Distance(i,j) and TransportCost(i,j) for each route between two cities i and j.

For the sake of simplicity, it is assumed that there is only a single output, the actual Transport(i,j) along each route.

Format of input data

The input data can be conveniently given in the form of tables. One for the identifiers defined over a single city like Supply and Demand, and the other for the identifiers defined over a tuple of cities like Distance and TransportCost. These tables can be provided in the form of text files as in Example data set for the transport model (format explained in Composite Tables). Alternatively, the data can be obtained from particular tables in a database. This example assumes the following database tables exist:

  • CityData for the one-dimensional parameters, and

  • RouteData for the two-dimensional parameters.

Listing 1 Example data set for the transport model
         Cities      Supply  Demand
     !   ----------  ------  ------
         Amsterdam     50
         Rotterdam    100
         Antwerp       75      25
         Berlin               125
         Paris                 75
    i          j          Distance  TransportCost
!   ---------  ---------  --------  -------------
    Amsterdam  Rotterdam       85        1.00
    Amsterdam  Antwerp        170        2.50
    Amsterdam  Berlin         660       10.00
    Amsterdam  Paris          510        8.25
    Rotterdam  Antwerp        100        1.20
    Rotterdam  Berlin         700       10.00
    Rotterdam  Paris          440        7.50
    Antwerp    Berlin         725       11.00
    Antwerp    Paris          340        5.00
    Berlin     Paris         1050       17.50

Simple Data Transfer

Simple data initialization

The simplest use of the READ statement is to initialize data from a fixed name text data file, or a database table. To read all the data from each source, the following groups of statements will suffice

read from file  "transport.inp" ;

read from table CityData;
read from table RouteData;

Such statements are typically found in the body of the predefined procedure MainInitialization.

Reading identifier selections

When a data source also contains data for identifiers that are of no interest to your particular application (but may be to others), AIMMS allows you to restrict the data transfer to a specific selection of identifiers in that data source. For instance, the following READ statement will only read the identifiers Distance and TransportCost, not changing the current contents of the AIMMS identifiers Supply and Demand.

read Distance, TransportCost from file "transport.inp" ;

Similar identifier selections are possible when reading from

a database table.

Writing the solution

After your model has computed the optimal transport, you may want to write the solution Transport(i,j) to an text output file for future reference. You can do this by calling the WRITE statement, which has equivalent syntax to the READ statement. The transfer of Transport(i,j) to the file transport.out is accomplished by the following WRITE statement.

write Transport to file "transport.out" ;

If you omit an identifier selection, AIMMS will write all model data to the file. When writing to a database table, AIMMS can of course only transfer data for those identifiers that are known in the table that you are writing to.

File name need not be explicit

File data transfer is not restricted to files with a fixed name. To choose the name of the data file either during execution or from within the end-user interface, you have several options:

  • replace the filename string in the READ and WRITE statements with a string-valued parameter holding the filename, or

  • use a File identifier (for text files only).

Set Initialization and Domain Checking

Domain restrictions

When you are reading the initial data of the transport model from an external data source several situations can occur:

  • you just want to initialize the set Cities from the data source,

  • the set Cities has already been initialized, and you want to retrieve the parametric data for existing cities only, or

  • the set Cities has already been initialized, but you want to extend it on the basis of the data read from the external data source.

The READ statements

The following statements impose domain restrictions on the READ statement.

read Cities
     from file "transport.inp" ;

read Supply, Demand
     from file "transport.inp"
     filtering i ;

read Supply, Demand
     from file "transport.inp" ;

Initializing sets

The first READ statement is a straightforward initialization of the set Cities. By default, AIMMS reads in replace mode, which implies that any previous contents of the set Cities is overwritten.

Domain checking

The second READ statement assumes that the set Cities has already been initialized. From all entries of the identifiers Supply and Demand it will only read those which correspond to existing elements in the set Cities, and skip over the data from the remaining entries.

Extending domain sets

The third READ statement differs from the second in that the clause FILTERING i has been omitted. As a result, AIMMS will not reject data that does not correspond to an existing label in the set Cities, but will read all available Supply and Demand data, and extend the set Cities accordingly.