GDX data terminology

Objects in GDX files are termed Symbols, and are of several types:

  • Sets are ordered collections of labels.
  • Parameters contain numerical data.
  • Variables are scalar values.
  • Aliases are alternate names for other Symbols.
  • Equations, not currently supported by PyGDX.

For clarity (e.g., Python has a built-in class python.set), these terms are capitalized throughout this documentation.

Both Sets and Parameters may be declared with one-dimensional Sets for each dimension. An example:

set  s  'Animals'  /
  a  Aardvark
  b  Blue whale
  c  Chicken
  d  Dingo
  e  Elephant
  f  Frog
  g  Grasshopper
  /;

set  t  'Colours'  /
  r  Red
  o  Orange
  y  Yellow
  g  Green
  b  Blue
  i  Indigo
  v  Violet
  /;

set  u  'Countries'  /
  CA  Canada
  US  United States
  CN  China
  JP  Japan
  /;

set v(s,t) 'Valid animal colours'
  / set.s.set.t yes /;

parameter p(s,t,u) 'Counts of nationalistic, colourful animals'
  / set.s.set.t.set.u 5 /;

parameter total(s) 'Total populations of each type of animal';
total(s) = sum((t, u), p(s, t, u));

execute_unload 'example.gdx';

In the resulting file example.gdx:

  • s, t and u are 1-dimensional Sets.

  • v is a 2-dimensional Set, defined over the parent Sets s and t. Any Set defined with reference to others, in this way, may include or exclude each element of the parent set. For instance, the following GAMS code defines a subset of u:

    set na(u)  'North American countries' / CA, US /;

  • p and total are Parameters containing numerical data.

Other concepts

The universal Set, *, contains every element appearing in any Set in the GDX file.

  • In the above example, * would contain: a b c d e f g r o y b i v CA US CN JP.

  • GAMS allows defining Sets and Parameters over the universal set:

    parameter new(*)  'More data';
new('L') = 3;

    This would add L to the universal Set.

The descriptive text provided on declaration of Symbols or Set elements is stored in GDX files along with the data contained in those variables.

  • For Set v, the string "Valid animal colours".
  • For Set element o, the string "Orange".