Question: I've to do a generic method `Classify<T>` that, given a sequence of elements of type `T` and an arbitrary number of predicates, returns an array of `n+1` lists (where `n` is the number of the predicates). More precisely, if the element satisfies the first predicate then it is added to the first list. If not, then if the element satisfies the second predicate it is added to the second list, etc. If none of the predicates can be applied to one element, then that element is added to the last list (the n+1 th).

I managed to do that with the following code. Is there any way to do it better, maybe with Linq?

```
public List<T>[] Classify<T>(IEnumerable<T> sequence, params Predicate<T>[] predicates)
{
    var result = new List<T>[predicates.Length+1];
    var counter = 0;    
    foreach (var elem in sequence)      
    {
        var valid = false;
        foreach (var predicate in predicates) 
        {
            if(result[counter]==null)
                result[counter] = new List<T>();
            if (predicate(elem) && valid==false)
            {
                var partition = result[counter];
                partition.Add(elem);
                valid = true;
            }
            counter++;
        }
        if (!valid)     
        {
            if (result[counter] == null)
                result[counter] = new List<T>();
            result[counter].Add(elem);
        }
        counter=0;
    }
    return result;
}

```

Answer: 
```
public List<T>[] Classify<T>(IEnumerable<T> sequence, params Predicate<T>[] predicates)
{
    var result = new List<T>[predicates.Length + 1];
    // It's clearer to create elements of the array explicitly
    for (int i = 0; i < result.Length; i++)
        result[i] = new List<T>();

    foreach (var elem in sequence)
    {
        var index = predicates.TakeWhile(p => !p(elem))
                              .Count();

        result[index].Add(elem);
    }
}

```

There is a few LINQ methods to get an index, f.e. `Select((i, e) => ...)`. If the list of unsatisfied elements was the first (0th), I would use `Select`. But since it is the last, `TakeWhile` gives the shorter code.
The idea is to group the elements in the sequence by the predicate they satisfy (line 10) and return an array (line 12) of lists containing the elements in each group (line 11). 

This is one way to find the predicate index for the first predicate that is satisfied by an element (line 7): `predicates.TakeWhile(p => !p(elem)).Count()` ("stolen" from @Mark Shevchenko's answer). This has the advantage that it will return `n+1` for elements which do not satisfy any predicate and the ordering is simpler.

After selecting the element and it's coresponding predicate index (lines 3-8), the predicates are ordered by the indexes (line 9).

```
[1]    public List<T>[] Classify<T>(IEnumerable<T> sequence, params Predicate<T>[] predicates)
[2]    {
[3]        return sequence
[4]            .Select(elem => new
[5]            {
[6]                Element = elem,
[7]                PredicateIndex = predicates.TakeWhile(p => !p(elem)).Count()
[8]            })
[9]            .OrderBy(x => x.PredicateIndex)
[10]           .GroupBy(x => x.PredicateIndex)
[11]           .Select(grp => grp.Select(x => x.Element).ToList())
[12]           .ToArray();
[13]   }

```