Using linq to group/aggregate by string[]

Question

Suppose I have a list with a Tuple.First as a string[] (i.e. my key):

var lst = new List<Tuple<string[], int>> {
     new Tuple<string[], int>(new string[] { "A", "B" }, 5),
     new Tuple<string[], int>(new string[] { "A", "B" }, 10),
     new Tuple<string[], int>(new string[] { "C", "B" }, 10),
}

I would like to aggregate (e.g. Sum) by Tuple.First (i.e. string[]) so expecting an output as follows:

var output = new List<Tuple<string[], int>> {
     new Tuple<string[], int>(new string[] { "A", "B" }, 15),
     new Tuple<string[], int>(new string[] { "C", "B" }, 10),
}

I did it this way but there must be a cleaner way to do it instead of forcing a pipe concatenation:

var output = lst
    .GroupBy(x => string.Join("|", x.First))
    .Select(x => new Tuple<string[], int>( 
             x.Key.Split('|'),   
             Sum(x => x.Second)));

Where does the Pair class come from? How do you access the values that you pass to its constructor? — John Wu
– John Wu, Commented Jan 27, 2021 at 18:56

Denis · Accepted Answer · 2021-01-27 20:58:07Z

2

You need to implement IEqualityComparer for string[]. My example using touples (You don't say what Pair is):

static void Main(string[] args)
{
    var lst = new List<(string[] arr, int num)> {
        (new string[] { "A", "B" }, 5),
        (new string[] { "A", "B" }, 10),
        (new string[] { "C", "B" }, 10),
    };

    var grouped = lst.GroupBy(pair => pair.arr, new EnumerableComparer());
    var sums = grouped.Select(g => (arr: g.Key, num: g.Sum(p => p.num)));
}

class EnumerableComparer<TRecord> : IEqualityComparer<IEnumerable<TRecord>>
    {
        public bool Equals(IEnumerable<TRecord> x, IEnumerable<TRecord> y)
        {
            return ReferenceEquals(x, y) || x != null && x.SequenceEqual(y);
        }

        public int GetHashCode(IEnumerable<TRecord> lst)
        {
            unchecked
            {
                int hash = 387;
                foreach (var elem in lst)
                {
                    hash = hash * 31 + elem.GetHashCode();
                }

                return hash;
            }
        }
    }

edited Jan 27, 2021 at 20:58

Denis

12.1k17 gold badges99 silver badges158 bronze badges

answered Jan 27, 2021 at 19:32

Leszek Mazur

2,5311 gold badge16 silver badges32 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Leszek Mazur Over a year ago

@Denis what was wrong with using native HashCode class?

Denis Over a year ago

It wouldn't resolve with R#. Maybe it needs a high version of .Net?

Collectives™ on Stack Overflow

Using linq to group/aggregate by string[]

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related