Find common characters from group of string

Question

Given the set of below strings

vJrwpWtwJgWrhcsFMMfFFhFp
jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL
PmmdzqPrVvPwwTWBwg

In above strings only lower case letter "r" is common in all of them.

I know we can use Intersectto find common things in 2 string but how to do using 3 strings?

This code below i did for 2 strings

var commonString = firstPartOfstring
  .Intersect(secondPartOfString)
  .Select(x => x)
  .FirstOrDefault()
  .ToString();

But cant find how to do for 3 strings

Use [`Aggregate`](https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.aggregate?view=net-7.0) — Konrad Rudolph, Dec 06 '22 at 13:14
https://stackoverflow.com/questions/1674742/intersection-of-multiple-lists-with-ienumerable-intersect — Roman Ryzhiy, Dec 06 '22 at 13:17
@Hogan Not sure what part of that is unclear. `Aggregate` generalises an operation of two operands to a set of inputs. Your solution works with exactly 3 inputs, but no longer works for an arbitrary number of inputs. That's where `Aggregate` comes in. — Konrad Rudolph, Dec 06 '22 at 13:18
@KonradRudolph -- yes -- I believe you are saying to use Aggregate on a list of lists. This question is about 3 lists. A general case could be solved with aggregate but to anyone who is asking this question your comment's meaning is a mystery. — Hogan, Dec 06 '22 at 13:20

Dmitry Bychenko · Accepted Answer · 2022-12-06T14:18:42.767

In general case you can query the strings (lines) with a help of Linq Aggregate. Please, note, that to get string from enumeration (IEnumerable<char>) we should use Concat(), not ToString():

using System.Linq;

...

// Organize the strings into a collection, say, an array 
// (Set, List etc. will do as well)
string[] lines = {
  "vJrwpWtwJgWrhcsFMMfFFhFp",
  "jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL",
  "PmmdzqPrVvPwwTWBwg",
};

...

// Query the collection above with a help of Linq:
//   We turn each string into enumeration (IEnumerable<char>)
//   Aggregate all enumerations into one with a help of Intersect
//   Concat characters of the final enumeration to the string
string commonString = string.Concat(lines
  .Select(line => line.AsEnumerable()) // we deal with IEnumerable<char>, not string
  .Aggregate((s, a) => s.Intersect(a))
  .OrderBy(c => c) // In case you want common characters being ordered
);

if null can appear among the strings, change .Select(...) into

.Select(line => line?.AsEnumerable() ?? Array.Empty<char>())

Please, fiddle youself.

If you want just to intersect three strings you can do it as follow:

string first = "vJrwpWtwJgWrhcsFMMfFFhFp";
string second = "jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL";
string third = "PmmdzqPrVvPwwTWBwg";

string commonString = string.Concat(first
  .Intersect(second)
  .Intersect(third));

score 0 · Answer 2 · answered Dec 06 '22 at 13:17

Intersect is transitive -- that means you can string them together in sequence.

var commonString = firstPartOfstring
                  .Intersect(secondPartOfString)
                  .Intersect(thirdPartOfString)
                  .Select(x => x).FirstOrDefault().ToString();

(nb - you can sequence them in any order - the other attribute of transitive)

Find common characters from group of string

2 Answers2