Manual approach

Suppose we have an existing model and we wish to do a comparative analysis altering commodity prices.  In this case we will use as a base a farm profit-maximizing model that is called farmcomp.gms.  Within that model we have a vector of commodity prices

PARAMETER price(primary)  prices for products

         /corn     2.20

         soybeans 5.00

         beef     0.50/

and we wish to run a base case and two cases with alternative prices – a case with the beef price at $0.70 and one with the corn price at $2.70.  Suppose we also have programmed a report writer using the techniques discussed in the Improving Output via Report Writing chapter and placed the calculations for it in the file farmrep.gms.  When that file is included using the $Include syntax as discussed in Including External Files chapter we get a report on whatever solution was generated by the last solve executed in the GAMS program.  This report consists of a number of tables.  We will only focus on the one below

Set alli   allitems

           /Corn,Soybeans,Beef  ,cattle

            Water,Cropland,Pastureland

            Fertilizer,Seed,Othercost, Veternary, Supplement

           "April Labor","May Labor"

           "Summer Labor","Sept Labor","Oct Labor",

            Cattlefeed

            Total/

Set measures  output measures

     / "Net Income", "Land use", "Dry Cropping", "Irr Cropping",

       "Livestock", "Resource Value","Product Value"/

Parameter summary(alli,measures)   Farm Summary;

which computes the report summary which contains rows for the major commodities as named in the alli set and the columns identified in the measures set.

Now given this code we can accomplish a comparative analysis in the file mancomp.gms as follows

$include Farmcomp.gms

display price;

$include Farmrep.gms

price("beef")=0.70;

SOLVE Firm USING LP MAXIMIZING NETINCOME;

display price;

$include Farmrep.gms

price("corn")=2.70;

SOLVE Firm USING LP MAXIMIZING NETINCOME;

display price;

$include Farmrep.gms

This code

This yields in the LST file a display arising from line 287 which is associated with the Base model gives

----    266 PARAMETER SUMMARY       Farm Summary

             Net Income    Land use  Dry Cropp~  Irr Cropp~   Livestock  Resource ~  Product V~

Corn                                      20.00      200.00                                2.20

Soybeans                                 480.00                                            5.00

Beef                                                                                       0.50

cattle                                                           615.79

Water                                                                         16.83

Cropland                     700.00                                          128.49

Pastureland                  130.00                                           84.26

Cattlefeed                                                                                 4.71

Total         162685.05                  500.00      200.00

while a display arising from line 359 which is associated with the $0.70 beef price gives

----    359 PARAMETER SUMMARY       Farm Summary

             Net Income    Land use  Dry Cropp~  Irr Cropp~   Livestock  Resource ~  Product V~

Corn                                      22.84      160.85                                2.34

Soybeans                                 489.86                                            5.00

Beef                                                                                       0.70

cattle                                                           866.67

Cropland                     673.55

Pastureland                  130.00                                         1456.90

Cattlefeed                                                                                 4.89

Total         373686.10                  512.70      160.85

and the display arising from line 431 which is associated with the $2.70 corn price gives

----    431 PARAMETER SUMMARY       Farm Summary

             Net Income    Land use  Dry Cropp~  Irr Cropp~   Livestock  Resource ~  Product V~

Corn                                      31.98      200.00                                2.70

Soybeans                                 410.24                                            5.00

Beef                                                                                       0.70

cattle                                                           866.67

Water                                                                         15.99

Cropland                     642.22

Pastureland                  130.00                                         1316.09

Cattlefeed                                                                                 5.36

Total         375839.30                  442.22      200.00

Now let's appraise how good this is.  In my judgment it is not so good for four reasons

Lets fix these up.