GLPK：下面的默認設置表達式必須具有2維而不是1

Question

我有一個AMPL模型文件，我正在將其轉換爲GLPK。它開始：

param n;  # The number of nodes in the graph 
set V := {1 .. n}; # The set of vertices in the graph 
set E within V cross V; # The set of edges in the graph 
set NE within V cross V := {i in V, j in V: i < j} diff E; 
set FIXED within V cross V default {}; # The set of demand pairs with fixed flow 

運行此，我得到以下錯誤：

_test.mod:5: set expression following default must have dimension 2 rather than 1 
Context: : i < j } diff E ; set FIXED within V cross V default { } ; 
MathProg model processing error 

這必須是MathProg和超之間的語法差異，AMPL運行的代碼AMPL完美。如何在MathProg中表達2D空集？

Answer 1

好了，解決的黑客是這樣的：

set FIXED within V cross V default {i in V, j in V: 1 < 0}; 

把一個顯然是錯誤的條件。它會有你想要的維度，並且仍然是空的。