gistfile1.cpp

   if (Trigger.isUpdate) {    
        
		//Create a list of old and new Values 
		public List<Sales_Support_Request__c> oldList = new List<Sales_Support_Request__c>();
		public List<Sales_Support_Request__c> newList = new List<Sales_Support_Request__c>();
		
		//populate both the list 
		oldList=Trigger.New;
		newList=Trigger.old;
		
		//lets create two maps to compare 
		Map<Id, Sales_Support_Request__c> oldMap = new Map<Id, Sales_Support__c>();
      	Map<Id, Sales_Support_Request__c> newMap = new Map<Id, Sales_Support__c>();	
		
		//Populate the Map iterating through List
		// use put method to add values to map 
		for(Sales_Support_Request__c rec : oldList)
 			   oldMap.put(rec.id,rec);
		
  		for(Sales_Support_Request__c rec : newList)
 			   oldMap.put(rec.id,rec);
		
		//Now we are loaded with data we needed
		//Now lets say you want to compare the old account with new account 
		//here is how you do it while looping through new map 
		for(Sales_Support_Request__c rec : newList)
 			   {
			       
				   Sales_Support_Request__c  createSSR = new Sales_Support_Request__c();
				   
				   //get old value from old map
				   if(rec.Account__c != oldMap(rec.id).Account__c)
				      {
					       //if the account is changed then set account 
						   createSSR.Account=rec.Account__c; 
					  }
					  
			   }
			   
		}