How to find whether the List<string> has duplicate values or not ?
I tried with below code. Is there any best way to achieve ?
var lstNames = new List<string> { "A", "B", "A" };
if (lstNames.Distinct().Count() != lstNames.Count())
{
Console.WriteLine("List contains duplicate values.");
}
Try to use GroupBy and Any like;
lstNames.GroupBy(n => n).Any(c => c.Count() > 1);
GroupBy method;
Groups the elements of a sequence according to a specified key selector function and projects the elements for each group by using a specified function.
Any method, it returns boolean;
Determines whether any element of a sequence exists or satisfies a condition.
c.Count() > 1 with c.Skip(1).Any()?If you're looking for the most efficient way of doing this,
var lstNames = new List<string> { "A", "B", "A" };
var hashset = new HashSet<string>();
foreach(var name in lstNames)
{
if (!hashset.Add(name))
{
Console.WriteLine("List contains duplicate values.");
break;
}
}
will stop as soon as it finds the first duplicate. You can wrap this up in a method (or extension method) if you'll be using it in several places.
GroupByGroupBy and Distinct take the same time regardless of the underlying data.
A generalized and compact extension version of the answer based on hash technique:
public static bool AreAnyDuplicates<T>(this IEnumerable<T> list)
{
var hashset = new HashSet<T>();
return list.Any(e => !hashset.Add(e));
}
GroupBy is implemented using some kind of hashed structure internally, so basically it should have about the same performance. According to my best knowledge adding elements to a HashSet is "cheap" in terms of computation and uses at most the same amount of memory as the original list. Also, I'm not sure but having GroupBy and Any after each other might not be very lazy while it's obvious that this solution will stop on first duplicate item. Could you please clarify why you think it has poor performance?
HashSet<T> does accept a custom comparer, I think @Eluvatar meant to expose that as a parameter of this extension.var duplicateExists = lstNames.GroupBy(n => n).Any(g => g.Count() > 1);
class Program
{
static void Main(string[] args)
{
var listFruits = new List<string> { "Apple", "Banana", "Apple", "Mango" };
if (FindDuplicates(listFruits)) { WriteLine($"Yes we find duplicate"); };
ReadLine();
}
public static bool FindDuplicates(List<string> array)
{
var dict = new Dictionary<string, int>();
foreach (var value in array)
{
if (dict.ContainsKey(value))
dict[value]++;
else
dict[value] = 1;
}
foreach (var pair in dict)
{
if (pair.Value > 1)
return true;
else
return false;
}
return false;
}
}
