A Road to Trouble
When programming in Java, you generally have collections
of objects, as previously discussed here. Often, you want to create a variable (or constraint) for element of a collection. A bad approach is illustrated below:
Set<String> stringCollection = makeSomeStrings();//assume this is defined elsewhere
IloCplex cplex = new IloCplex();
String[] stringToIndex = new String[stringCollection.size()];
IloIntVar[] variablesToIndex = cplex.boolVarArray(stringCollection.size());
int i = 0;
for(String s: stringCollection){
stringToIndex[i++] = s;
}
Then given a String from the collection, we could access the corresponding IloIntVar, and with an IloIntVar, we would access the corresponding String, with the following snippets
public IloIntVar getVariableForString(String s, String[] stringToIndex, IloIntVar[] variablesToIndex){
for(int i = 0; i < stringToIndex.length; i++){
if(stringToIndex[i].equals(s)){
return variablesToIndex[i];
}
}
return null;
}
public String getStringForVariable(IloIntVar v, String[] stringToIndex, IloIntVar[] variablesToIndex){
for(int i = 0; i < variablesToIndex.length; i++){
if(variablesToIndex[i] == v){
return stringToIndex[i];
}
}
return null;
}
There are several reasons to be concerned with this.
- For \( n \) variables, access time takes \( O(n) \) .
- We are maintaining two data structures and an index by hand, which leave a lot of room for programmer error
- We cannot add new variables at a later date
We can eliminate any chance of an indexing error and improve our access time to
\( O(1) \)
by replacing our two arrays by two HashMaps, e.g.
Set<String> stringCollection = makeSomeStrings();//assume this is defined elsewhere
IloCplex cplex = new IloCplex();
Map<String,IloIntVar> stringToVariable = new HashMap<String,IloIntVar>();
Map<IloIntVar,String> variableToString = new HashMap<IloIntVar,String>();
for(String s: stringCollection){
IloIntVar v = cplex.boolVar();
stringToVariable.put(s,v);
variableToString.put(v,s);
}
However, we are still maintaining two separate data structures which we need to keep synchronized, which is asking for trouble.
Good Style
Instead, we use Guava's special data structure, the ImmutableBiMap, which will maintain two HashMaps for us (and as a bonus, prevent any accidental modifications once we build the map). The following methods can be found in Util.java in your project:
public static <T> ImmutableBiMap<T,IloIntVar> makeBinaryVariables(IloCplex cplex, Iterable<T> set) throws IloException{
Builder<T,IloIntVar> ans = ImmutableBiMap.builder();
for(T t: set){
ans.put(t, cplex.boolVar());
}
return ans.build();
}
Now the unfortunate code from before can be replaced by
Set<String> stringCollection = makeSomeStrings();//assume this is defined elsewhere
IloCplex cplex = new IloCplex();
ImmutableBiMap<String,IloIntVar> stringVarBiMap = Util.makeBinaryVariables(cplex,stringCollection);
for(String s: stringCollection){
stringVarMap.put(s,cplex.boolVar());
}
public IloIntVar getVariableForString(String s, ImmutableBiMap<String,IloIntVar> stringVarBiMap){
return stringVarBiMap.get(s);
}
public String getStringForVariable(IloIntVar v, ImmutableBiMap<String,IloIntVar> stringVarBiMap){
return stringVarBiMap.inverse().get(v);
}
There are a few additional methods in the class Util for creating IloLinearIntExpr objects and IloLinearNumExpr objects designed to keep your code organized and error free. They use another Guava class, Function, (see here for Javadoc). The idea of a Function<F,T> is simple, they take in any object of type F and produce some object of type T. Functions are a little clunky to make (a weakness of Java), but fortunately you won't have to make many. The follwing static functions are also found in Util
Method Name |
Return Type |
Arguments |
Description |
|---|---|---|---|
integerSum |
IloLinearIntExpr |
IloCplex cplex, BiMap<T,IloIntVar> variables, Iterable<T> set |
For each \( e \in \text{set} \) finds the variable \( x_e \) in variables and returns \( \sum_{e \in \text{set}} x_e \) |
integerSum |
IloLinearIntExpr |
IloCplex cplex, BiMap<T,IloIntVar> variables, Iterable<T> set,Function<? super T,Integer> coefficients |
For each \( e \in \text{set} \) finds the variable \( x_e \) in variables and \( c_e \) by applying coefficients to \( e \) and returns \( \sum_{e \in \text{set}} c_e x_e \) |
Simple Example Revisited
Recall the IP we modeled with CPLEX in the previous section:
\[ \begin{aligned} &\min & x + 2y + 3z\\ &\text{subject to}& x + y + z &\geq 2\\ && x,y,z &\in\{0,1\} \end{aligned} \]Lets design a more scalable implementation using our new methods. Finish the method exercise2() from WarmUps.java, which currently reads
public static void exerciseTwo() throws IloException{
List<String> varNames = Arrays.asList("x","y","z");
Map<String,Integer> weightsMap = new HashMap<String,Integer>();
weightsMap.put("x",1);
weightsMap.put("y",2);
weightsMap.put("z",3);
Function<String,Integer> weights = Functions.forMap(weightsMap);
IloCplex cplex = new IloCplex();
//write code here!
}
The functionality should be the same as exercise1(). Modify the main method to test your code.
Solution