Q:

Retrieve List of picklist values in a Set

I got the following variables(Custom Labels referring to picklist values), and a get method to retrieve List of picklist values of a field.
I need to retrieve those List of picklist values in a Set. How can I do this ?
  public String dailyValue {
      get { return System.Label.Daily.toLowerCase().trim(); } 
      private set;
  }
  public String weeklyValue {
      get { return System.Label.Weekly.toLowerCase().trim(); }
      private set;
  }
  public String monthlyValue {
      get { return System.Label.Monthly.toLowerCase().trim(); }
      private set;
  }
  public String quarterlyValue {
      get { return System.Label.Quarterly.toLowerCase().trim(); }
      private set;
  }
  public String biannuallyValue {
      get { return System.Label.Bi_Annually.toLowerCase().trim(); }
      private set;
  }
  public String annuallyValue {
      get { return System.Label.Annually.toLowerCase().trim(); } 
      private set;
  }

  //values from the frequency picklist
  public List<SelectOption> getMonitoringFrequency(){

       List<SelectOption> frequencyTypes = new List<SelectOption>();
       Schema.DescribeFieldResult fieldResult = Metric__c.Monitoring_Frequency__c.getDescribe();
       List<Schema.PicklistEntry> ple = fieldResult.getPicklistValues();
       for( Schema.PicklistEntry f : ple){
            frequencyTypes.add(new SelectOption(
                 f.getValue().toLowerCase(),f.getLabel().toLowerCase()
            ));
       }       
       return frequencyTypes;
   }

A:

Depending on whether you want the label or the value in your set, create a set:
set<String> picklistValues = new set<String>();
set<String> picklistLabels = new set<String>():

and then in your for-loop, add the value or label, as appropriate:
for( Schema.PicklistEntry f : ple){
    frequencyTypes.add(new 
        SelectOption(f.getValue().toLowerCase(),f.getLabel().toLowerCase()));
    picklistValues.add(f.getValue());
    picklistLabels.add(f.getLabel());
}

