Count similar adjacent items in List<string>

Question

I'm trying to find similar adjacent items in List and count its number, e.g.:

List<string> list = new List<string> {"a", "a", "b", "d", "c", "c"};

Desired Output:

a = 2, c = 2

What I've done is use for loop to iterate over each element of the list and to see whether it has similar adjacent element, but understandably it gives ArgumentOutOfRangeException() because I don't know how to keep track of the position of the iterator so that it doesn't go out of bounds. Here's what I've done:

for (int j = 0; j < list.Count; j++)
{
      if (list[j] == "b")
      {
             if ((list[j + 1] == "b") && (list[j - 1] == "b"))
             {
                     adjacent_found = true;
             }
      }
}

Having said that, if there's another easier way to find similar adjacent elements in a List other than using for loop iteration, please advise. Thanks.

Answer 1

You can do something like this:

static IEnumerable<Tuple<string, int>> FindAdjacentItems(IEnumerable<string> list)
{
    string previous = null;
    int count = 0;
    foreach (string item in list)
    {
        if (previous == item)
        {
            count++;
        }
        else
        {
            if (count > 1)
            {
                yield return Tuple.Create(previous, count);
            }
            count = 1;
        }
        previous = item;
    }

    if (count > 1)
    {
        yield return Tuple.Create(previous, count);
    }
}

Answer 2

for (int i= 0; i < list.Count; i++)
   {
      for (int j = i + 1; j < list.Count; j++)
        {
           if (list[i] == list[j])
             {
                adjacent_found = true;
                count++;
             }
        }
   }

Answer 3

Check this:

Dictionary<char,int> dic=new Dictionary<char,int>();
for(int i=1;i<list.count;i++)
{
          if(list[i]==list[i-1])
                {
                      if(dic.ContainsKey(list[i]))
                                 {
                                   dic[list[i]]+=1;
                             }
                          else
                                 { 
                                    dic.Add(list[i],2)
                                  }
                  }
}

Answer 4

To avoid ArgumentOutOfRangeException use for (int j = 1; j < list.Count - 1; j++). Desired answer can't be achieved this way. Try this:

IEnumerable<Adjacent> CountAdjacents(List<string> source)
{
    var result = new List<Adjacent>();

    for (var i = 0; i < source.Count() - 1; i++)
    {
        if (source[i] == source[i + 1])
        {
            if (result.Any(x => x.Word == source[i]))
            {
                result.Single(x => x.Word == source[i]).Quantity++;
            }
            else
                result.Add(new Adjacent
                {
                    Word = source[i],
                    Quantity = 2
                });
        }
    }
    return result;
}

class Adjacent
{
    public string Word;
    public int Quantity;
}

Answer 5

Maintain an int array of 256 size, initialized to 1. Run a loop [O(n)] for i=0 to i-2, compare each char with the next char. If same then find the ascii value of the char and increment the corresponding value in array. Hope this helps!