The strategic game (.nfg) file format, outcome version

This file format defines a strategic N-player game. In this version, the payoffs are defined by means of outcomes, which may appear more than one place in the game table. This may give a more compact means of representing a game where many different strategy combinations map to the same consequences for the players. For a version of this format in which payoffs are listed explicitly, without identification by outcomes, see the previous section.

A sample file

This is a sample file illustrating the general format of the file. This file defines the same game as the example in the previous section:

NFG 1 R "Selten (IJGT, 75), Figure 2, normal form" { "Player 1" "Player 2" }

{
{ "1" "2" "3" }
{ "1" "2" }
}

{
{ "" 1, 1 }
{ "" 0, 2 }
{ "" 0, 2 }
{ "" 1, 1 }
{ "" 0, 3 }
{ "" 2, 0 }
}
1 2 3 4 5 6

Structure of the prologue

The prologue is constructed as follows. The file begins with the token NFG , identifying it as a strategic gamefile. Next is the digit 1 ; this digit is a version number. Since only version 1 files have been supported for more than a decade, all files have a 1 in this position. Next comes the letter R . The letter R used to distinguish files which had rational numbers for numerical data; this distinction is obsolete, so all new files should have R in this position.

The prologue continues with the title of the game. Following the title is a list of the names of the players defined in the game. This list follows the convention found elsewhere in the file of being surrounded by curly braces and delimited by whitespace (but not commas, semicolons, or any other character). The order of the players is significant; the first entry in the list will be numbered as player 1, the second entry as player 2, and so forth.

Following the list of players is a list of strategies. This is a nested list; each player’s strategies are given as a list of text labels, surrounded by curly braces.

The nested strategy list is followed by an optional text comment field.

The prologue closes with a list of outcomes. This is also a nested list. Each outcome is specified by a text string, followed by a list of numerical payoffs, one for each player defined. The payoffs may optionally be separated by commas, as in the example file. The outcomes are implicitly numbered in the order they appear; the first outcome is given the number 1, the second 2, and so forth.

Structure of the body (list of outcomes)

The body of the file is a list of outcome indices. These are presented in the same lexicographic order as the payoffs in the payoff file format; please see the documentation of that format for the description of the ordering. For each entry in the table, a nonnegative integer is given, corresponding to the outcome number assigned as described in the prologue section. The special outcome number 0 is reserved for the “null” outcome, which is defined as a payoff of zero to all players. The number of entries in this list, then, should be the same as the product of the number of strategies for all players in the game.