Find char in a string C#

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if I have this string : "ciaobciaodciao" and I must find the substring "ciao", but if I write:

int index = fileContent.IndexOf("ciao");

index is the first "ciao", but I want to find the index of every "ciao"

What I have tried:

I tried to use the method "IndexOf" like I wrote above, but I need to find every substring

Posted 12-Apr-23 5:04am

Member 14594285

Updated 12-Apr-23 20:55pm

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PIEBALDconsult 12-Apr-23 11:46am

Use a Regular Expression.

3 solutions

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Mike Hankey · Answer 1 · 2023-04-12T08:38:00

Using Regular Expressions is very easy.

C#

namespace Examples
{
    public class Example2
    {
        public static void Main()
        {
            string pattern = "ciao";
            string input = "ciao this is a test of ciao and another ciao to go";
            Match m = Regex.Match(input, pattern, RegexOptions.IgnoreCase);
        }
    }
}

output

Match 1: 0-4	ciao
Match 2: 23-27	ciao
Match 3:40-44	ciao

OriginalGriff · Answer 2 · 2023-04-12T05:29:00

Solution 1

Once you have called IndexOf, you can then use it again via an overload to return each in sequence - just pass it the index of the most recently located occurrence plus the length of the search string: String.IndexOf Method (System) | Microsoft Learn[^]

Posted 12-Apr-23 5:29am

OriginalGriff

George Swan · Answer 3 · 2023-04-12T20:55:00

Just for fun. You can achieve your aim with one line of Linq

C#

string target = "ciao";
string str = "ciaobciaodciao";
var indices = str.Select((c, i) => i <= str.Length - target.Length && str.Substring(i, target.Length) == target ? i : -1)
                 .Where(i => i != -1);
Console.WriteLine($"[{string.Join(", ", indices)}]");//prints [0, 5, 10]

I Benchmarked this method against a Regex equivalent method. The RegexTest was 43% slower and used 38% more memory

C#

[Orderer(SummaryOrderPolicy.FastestToSlowest)]
[MemoryDiagnoser]
public class LinqVRegex
  {
    private string target = "ciao";
    private string testString = "ciaobciaodciao";
    [Benchmark(Baseline = true)]
    public List<int> LinqTest()
     {
      return testString.Select((c, i) => i <= testString.Length - target.Length && testString.Substring(i, target.Length) == target ? i : -1)
                       .Where(i => i != -1).ToList();

     }
     [Benchmark]
     public List<int> RegexTest()
     {
       MatchCollection matches = Regex.Matches(testString, target, RegexOptions.IgnoreCase);
       return matches.Select(m => m.Index).ToList();

     }
   }
//|    Method |     Mean |    Error |   StdDev | Ratio | RatioSD |   Gen0 | Allocated | Alloc Ratio |
//|---------- |---------:|---------:|---------:|------:|--------:|-------:|----------:|------------:|
//|  LinqTest | 542.3 ns | 10.86 ns | 17.22 ns |  1.00 |    0.00 | 0.1545 |     648 B |        1.00 |
//| RegexTest | 774.6 ns | 13.60 ns | 26.53 ns |  1.43 |    0.06 | 0.2136 |     896 B |        1.38 |

Find char in a string C#

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