Data input from file

With Mosel, there are several different ways of reading and writing data from and to external files. For simplicity's sake we shall limit the discussion here to files in text format. Mosel also provides specific modules to exchange data with spreadsheets and databases, for instance using an ODBC connection. However this is beyond the scope of this document, and for more information see the documentation for these modules (see the Mosel Language Reference Manual and the whitepaper Using ODBC and other database interfaces with Mosel).

We are going to work with a data file folio.dat. Create this by right-clicking in the Project pane, and selecting New > Blank File. The file has the following contents:

! Data file for `folio*.mos'

RET: [("treasury") 5 ("hardware") 17 ("theater") 26 ("telecom") 12
      ("brewery") 8 ("highways") 9 ("cars") 7 ("bank") 6
      ("software") 31 ("electronics") 21 ]

RISK: ["hardware" "theater" "telecom" "software" "electronics"]

NA: ["treasury" "hardware" "theater" "telecom"]

Just as in model files, single-line comments preceded by ! may be used in data files. Every data entry is labeled with the name given to the corresponding entity in the model. Data items may be separated by blanks, tabulations, line breaks, or commas.

Save a copy of the Mosel model from Chapter 3 using the name foliodata.mos, and update this new file as follows:

 declarations
  SHARES: set of string              ! Set of shares
  RISK: set of string                ! Set of high-risk values among shares
  NA: set of string                  ! Set of shares issued in N.-America
  RET: array(SHARES) of real         ! Estimated return in investment
 end-declarations

 initializations from "folio.dat"
  RISK RET NA
 end-initializations

 declarations
  frac: array(SHARES) of mpvar       ! Fraction of capital used per share
 end-declarations

As opposed to the previous model foliolp.mos, all index sets and the data array are now declared without fixing their contents: their size is not known at their creation and they are initialized later with data from the file folio.dat. Optionally, after the initialization from file, we may finalize the sets to make them static. This will make more efficient the handling of any arrays indexed by these sets, and more importantly, this allows Mosel to check for `out of range' errors that cannot be detected if the sets are allowed to grow dynamically. (Note that sets and arrays in Mosel can also be explicitly marker as dynamic in order to prevent them from being finalized/fixed.)

 finalize(SHARES); finalize(RISK); finalize(NA)

Notice that we do not initialize explicitly the set SHARES, it is filled automatically when the array RET is read. Notice further that we only declare the decision variables after initializing the data, and hence when their index set is known.