How to search in a list whether a list of given items exists in the same order

Question

I need to check if a list contains all the items and in the same sequence using LINQ and if match found return the last matching element.

var allWords = new List<string> { "A", "B", "C", "D", "E", "F"};
var words = new List<string>() { "D", "E" };

Should return element "E" (because there is a match)

Can you provide some sample inputs and outputs? Ideally provide more than 1 set, and enough to demonstrate a few different cases. — ProgrammingLlama, Dec 22 '18 at 15:59

score 3 · Accepted Answer · answered Dec 22 '18 at 16:13

You could do it with Linq.

var allWords = new List<string> { "A", "B", "C", "D", "E", "F","D", "E", "E"};
var words = new List<string>() {"D","E"  };
var indexOfLastOccuranceOfSequence = Enumerable.Range(0, allWords.Count - words.Count + 1)
        .Select(a => allWords.Skip(a).Take(words.Count)).ToList()
        .FindLastIndex(a => a.SequenceEqual(words));
var indexOfLastElement = indexOfLastOccuranceOfSequence+ words.Count() - 1;

Beautiful and compact. Thanks – M.Armoun Dec 22 '18 at 16:16 — M.Armoun, Dec 22 '18 at 16:16

OmG · Answer 2 · 2018-12-22T16:06:47.963

You can find the first word in the list an iterate over them:

int index = allWords.Select((item, i) => new { Item = item, Index = i })
            .First(x => x.Item == words[0]).Index;

foreach(var word in words)
{
     if(allWords[index] != word)
          return(null);
     index += 1;
}
return(index + words.Length - 1);

The above program returns the index of the last match in the allWords. To finding the first occurrence of a word in a list you can see this post.

score 0 · Answer 3 · answered Dec 22 '18 at 16:13

You can find the existence by :

List<int> indexes = allWords.Where(x => words.Contains(x)).Select(x => allWords.IndexOf(x)).ToList();
if(indexes.Count() == words.Count())
{
    // then check indexes if SORTED, if so then they are in the same sequence
    return words.Last();  

}

How to search in a list whether a list of given items exists in the same order

3 Answers3