18

What is the fastest way to compare a string with an array of strings in C#2.0

Pratik
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Greens
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5 Answers5

32

What kind of comparison do you want? Do you want to know if the given string is in the array?

bool targetStringInArray = array.Contains(targetString);

do you want an array of comparison values (positive, negative, zero)?

var comparisons = array.Select(x => targetString.CompareTo(x));

If you're checking for containment (i.e. the first option) and you're going to do this with multiple strings, it would probably be better to build a HashSet<string> from the array:

var stringSet = new HashSet<string>(array);

if (stringSet.Contains(firstString))  ...
if (stringSet.Contains(secondString)) ...
if (stringSet.Contains(thirdString))  ...
if (stringSet.Contains(fourthString)) ...
Jon Skeet
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9

You mean to see if the string is in the array? I can't remember if arrays support the .Contains() method, so if not, create a List< string >, add your array to the list via AddRange(), then call list.Contains({string to compare}). Will return a boolean value indicating whether or not the string is in the array.

James McConnell
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    Correct, in 2.0 you would have to do `(new List(array)).Contains(targetString);` - but from 3.5 you have the IEnumerable.Contains extension method, so it's available for arrays also. – vgru Jun 26 '09 at 21:17
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    In 2.0, you can check whether the array contains a specific value or not with a simple `Array.IndexOf(arr, val) >= 0`. – Mehrdad Afshari Jun 26 '09 at 21:35
5

If you are doing this many times with a single array, you should sort the array and binary search it:

Array.Sort(array);
int index = Array.BinarySearch(array, input);
// if (index < 0) 
//      does not exists, "items > ~index" are larger and "< ~index" are smaller
// otherwise, "items > index" are larger and "< index" are smaller.

Otherwise just check the whole array naively:

bool exists = Array.IndexOf(array, input) >= 0;
Mehrdad Afshari
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    If you're going to do it many times, there are more efficient searches than a binary search (assuming it's reasonably large, anyway). I'd go for something hash-based, e.g. HashSet. – Jon Skeet Jun 26 '09 at 21:08
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    Yep. The hash table/binary search tree debate... Both are better than searching the whole array, nevertheless. HashSet is only available in 3.5 though. – Mehrdad Afshari Jun 26 '09 at 21:39
0

If your requirement to see if one list is a part of another, then you can use Contains().

Lets say

List<string> list1 = new List<string>(){"1", "2"};
List<string> list2 = new List<string>(){"1", "2", "3"};

list2.Contains(list1) //will be True, but not vice versa.

That said, if you want to know not partial match, but exact match, you can do use Except(), and check for remainder.

if(list2.Except(list1).Length == 0) //will return false.
PhD
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-1

//get data in list from source List checklist = Directory.GetFiles(SourcePath, ".", SearchOption.AllDirectories).Where(x => x.ToLower().EndsWith("apk")).ToList();

            //get date from a text file
           List<string>  ls = ReadFile();

            foreach(string file in checklist)
            {
                //get file name
                string filename = Path.GetFileName(file);

                string TargetLocation = Path.Combine(TargetPath, filename);

                //now compare single string to a list 
                //it give in true and false 
                if(ls.Contains(filename)) 
                {
                    //do your task
                    //File.Copy(file, TargetLocation);
                }
            }
kirti kant pareek
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