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h1. A Road to Trouble

When programming in Java, you generally have [collections|http://docs.oracle.com/javase/tutorial/collections/] of objects, as previously discussed [here|Do you know Java].  Often, you want to create a variable (or constraint) for element of a collection.  A *bad* approach is illustrated below:
{code}
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;
}
{code}
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
{code}
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;
}
{code}
There are several reasons to be concerned with this.
* For {mathinline}n{mathinline} variables, access time takes {mathinline}O(n){mathinline}.
* 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 {mathinline}O(1){mathinline} by replacing our two arrays by two [HashMaps|http://docs.oracle.com/javase/6/docs/api/java/util/HashMap.html], e.g.
{code}
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);
}
{code}
However, we are still maintaining two separate data structures which we need to keep synchronized, which is asking for trouble.  

h1. Good Style

Instead, we use [Guava's|http://code.google.com/p/guava-libraries/wiki/GuavaExplained?tm=6] special data structure, the [ImmutableBiMap|http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/ImmutableBiMap.html], 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:
{code}
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();
}
{code}
Now the unfortunate code from before can be replaced by
{code}
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);
}
{code}
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|http://code.google.com/p/guava-libraries/wiki/FunctionalExplained], (see [here|http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Function.html] 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||
|{anchor:integerSum}integerSum|IloLinearIntExpr|IloCplex cplex, BiMap<T,IloIntVar> variables, Iterable<T> set|For each {mathinline} e \in \text{set}{mathinline} finds the variable {mathinline} x_e {mathinline} in variables and returns {mathinline} \sum_{e \in \text{set}} x_e{mathinline}|
|integerSum|IloLinearIntExpr|IloCplex cplex, BiMap<T,IloIntVar> variables, Iterable<T> set,Function<? super T,Integer> coefficients|For each {mathinline} e \in \text{set}{mathinline} finds the variable {mathinline} x_e {mathinline} in variables and {mathinline}c_e{mathinline} by applying coefficients to {mathinline}e{mathinline} and returns {mathinline} \sum_{e \in \text{set}} c_e x_e{mathinline}|
|{anchor:calcSum}calcSum|double|Set<E> terms, Map<E,Double> coefficients|If for every {mathinline}e \in \text{terms}{mathinline}, we let {mathinline}c_e{mathinline} be zero if coefficients does not contain the key {mathinline}e{mathinline} and the value of {mathinline}e{mathinline} coefficientsComputes otherwise, returns {mathinline} \sum_{e \in \text{terms}} c_e{mathinline}.|

While the final function actually has nothing to do with CPLEX, it will often be useful when using CPLEX.

h1. Simple Example Revisited

Recall the IP we modeled with CPLEX in the previous section:
{mathdisplay} 
\begin{aligned} 
&\min & x + 2y + 3z\\ 
&\text{subject to}& x + y + z &\geq 2\\ 
&& x,y,z &\in\{0,1\} 
\end{aligned} 
{mathdisplay}
Lets design a more scalable implementation using our new methods.  Finish the method {{exercise2()}} from _WarmUps.java_, which currently reads
{code}
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!
}
{code}
The functionality should be the same as {{exercise1()}}.  Modify the {{main}} method to test your code.
{toggle-cloak:id=GoodStyleSolution} _Solution_ 
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{code}
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!
	ImmutableBiMap<String,IloIntVar> varMap = Util.makeBinaryVariables(cplex,varNames);
	cplex.addGe(Util.integerSum(cplex,varMap,varNames),2);
	cplex.addMinimize(Util.integerSum(cplex,varMap,varNames,weights));
	cplex.solve();
	for(String varName: varNames){
		System.out.println(varName +": " + cplex.getValue(varMap.get(varName)));
	}
	System.out.println("obj: " + cplex.getObjValue());
}
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