The whole If prevValue != currValue thing in a loop

Question

Say we have a list {a, a, a, b, b, c, c }

We want to loop through the list and make some kind of change when the item value changes... for example:

prevEmployer = String.empty;
foreach(Person p in PersonList){
  if(p.Employer != prevEmployer){ 
    doSomething(); 
    prevEmployer = p.Employer;
  }
  ... more code
}

Is there any alternative to this? It just looks cludgy to me.

Edit: made the code more realistic to the problem at hand.

Answer 1

You could just get the distinct values out of the array?

Answer 2

Do you want the distinct values? i.e. will there ever be {a,a,a,b,b,a,c,c,a}? If not, you could use LINQ:

foreach(string s in theList.Distinct()) {
   doSomething(); // with s
}

---

Re your update; perhaps use something like DistinctBy:

foreach(var item in data.DistinctBy(x=>x.Foo)) {
    Console.WriteLine(item.Bar);
}

public static IEnumerable<TSource> DistinctBy<TSource,TValue>(
        this IEnumerable<TSource> source, Func<TSource,TValue> selector) {
    var set = new HashSet<TValue>();
    foreach (var item in source) {
        if (set.Add(selector(item))) {
            yield return item;
        }
    }
}

Answer 3

Looks good to me as long as String.Empty is never an element of your list and your list is sorted. May want to make sure theList isn't mutated by some other thread. Looks like a pretty stock way to handle work on a transition in the list...

Answer 4

This really depends on what you are trying to do with the rest of the code. @Marc Gravell's response is correct if you only need to perform an action for each distinct element in the list, however, if doSomething() just sets up some state based on the fact that list element has changed and you are operating on each element in the list (or need the individual elements for another purpose), then your method seems perfectly reasonable.

Answer 5

I decided to go with the following solution, which is much prettier but results in more db calls.

foreach(Employer employer in EmployerList){
  ProcessEmployees(employer);
}

ProcessEmployees(Employer employer){
  EmployeeList = GetEmployees(employer);
  foreach(Employee employee in EmployeeList){
    doStuff...
  }
}

So instead of starting with {a, a, a, b, b, c, c }, I'll be processing {a, a, a} {b, b} {c, c}