Find Nth occurrence of a character in a string

Question

I need help with creating a C  method that returns the index of the Nth occurrence of a character in a string   For instance  the 3rd occurrence of the character  t  in the string  dtststxtu  is 5   Note that the string has 4 ts

User · Answer

Here is a recursive implementation - as an extension method, mimicing the format of the framework method(s):

public static int IndexOfNth(
    this string input, string value, int startIndex, int nth)
{
    if (nth < 1)
        throw new NotSupportedException("Param 'nth' must be greater than 0!");
    if (nth == 1)
        return input.IndexOf(value, startIndex);

    return input.IndexOfNth(value, input.IndexOf(value, startIndex) + 1, --nth);
}

Also, here are some (MBUnit) unit tests that might help you (to prove it is correct):

[Test]
public void TestIndexOfNthWorksForNth1()
{
    const string input = "foo<br />bar<br />baz<br />";
    Assert.AreEqual(3, input.IndexOfNth("<br />", 0, 1));
}

[Test]
public void TestIndexOfNthWorksForNth2()
{
    const string input = "foo<br />whatthedeuce<br />kthxbai<br />";
    Assert.AreEqual(21, input.IndexOfNth("<br />", 0, 2));
}

[Test]
public void TestIndexOfNthWorksForNth3()
{
    const string input = "foo<br />whatthedeuce<br />kthxbai<br />";
    Assert.AreEqual(34, input.IndexOfNth("<br />", 0, 3));
}

User · Answer

Since the built-in IndexOf function is already optimized for searching a character within a string  an even faster version would be  as extension method    public static int NthIndexOf this string input  char value  int n        if  n  lt   0  throw new ArgumentOutOfRangeException  n   n   n is less than zero          int i   -1      do               i   input IndexOf value  i   1           n--            while  i    -1  amp  amp  n  gt  0        return i      Or to search from the end of the string using LastIndexOf   public static int NthLastIndexOf this string input  char value  int n        if  n  lt   0  throw new ArgumentOutOfRangeException  n   n   n is less than zero          int i   input Length      do               i   input LastIndexOf value  i - 1           n--            while  i    -1  amp  amp  n  gt  0        return i      Searching for a string instead of a character is as simple as changing the parameter type from char to string and optionally add an overload to specify the StringComparison

User · Answer

There is a minor bug in previous solution   Here is some updated code   s TakeWhile c   gt   n -   c    t   1   0    gt  0  Count

User · Answer

public int GetNthOccurrenceOfChar string s  char c  int occ        return String Join c ToString    s Split new char     c    StringSplitOptions None  Take occ   Length

User · Answer

public static int IndexOfAny this string str  string   values  int startIndex  out string selectedItem                int first   -1          selectedItem   null          foreach  string item in values                        int i   str IndexOf item  startIndex  StringComparison OrdinalIgnoreCase               if  i  gt   0                                if  first  gt  0                                        if  i  lt  first                                                first   i                          selectedItem   item                                                          else                                       first   i                      selectedItem   item                                                    return first

User · Answer

you can do this work with Regular Expressions           string input    dtststx           char searching char    t           int output   Regex Matches input       searching char       2  Index    best regard

User · Answer

Here s another  maybe simpler implementation of string IndexOfNth   with strings implementation   Here s the string match version   public static int IndexOfNth this string source  string matchString                                int charInstance                                StringComparison stringComparison   StringComparison CurrentCulture        if  string IsNullOrEmpty source           return -1       int lastPos   0      int count   0       while  count  lt  charInstance                 var len   source Length - lastPos          lastPos   source IndexOf matchString  lastPos len stringComparison           if  lastPos    -1              break           count            if  count    charInstance              return lastPos           lastPos    matchString Length            return -1      and the char match version   public static int IndexOfNth string source  char matchChar  int charInstance                if  string IsNullOrEmpty source           return -1       if  charInstance  lt  1          return -1       int count   0      for  int i   0  i  lt  source Length  i                  if  source i     matchChar                        count                if  count    charInstance                                   return i                                       return -1      I think for such a low level implementation you d want to stay away from using LINQ  RegEx or recursion to reduce overhead

User · Answer

public int GetNthIndex string s  char t  int n        int count   0      for  int i   0  i  lt  s Length  i                  if  s i     t                        count                if  count    n                                return i                                    return -1      That could be made a lot cleaner  and there are no checks on the input

User · Answer

Another RegEx-based solution  untested    int NthIndexOf string s  char t  int n       if n  lt  0    throw new ArgumentException         if n  1    return s IndexOf t        if t        return 0       string et   RegEx Escape t      string pat       lt            Microsoft VisualBasic StrDup n-1  et        n                   et     Match m   RegEx Match s  pat      return m Success   m Index   -1      This should be slightly more optimal than requiring RegEx to create a Matches collection  only to discard all but one match

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string result    i am  bansal vks gmail com       string  int in1   result IndexOf           get the index of first quote  int in2   result IndexOf       in1   1      get the index of second  string quoted text   result Substring in1   1  in2 - in1      get the string between quotes

User · Answer

I add another answer that run pretty fast compared to others methods  private static int IndexOfNth string str  char c  int nth  int startPosition   0        int index   str IndexOf c  startPosition       if  index  gt   0  amp  amp  nth  gt  1                return  IndexOfNth str  c  nth - 1  index   1              return index

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public static int FindOccuranceOf this string str char  char  int occurance               var result   str Select  x  y    gt  new   Letter   x  Index   y                 Where letter   gt  letter Letter     char  ToList           if  occurence  gt  result Count    occurance  lt   0                      throw new IndexOutOfRangeException  occurance                    return result occurance-1  Index

User · Answer

Here is a fun way to do it       int i   0       string s  asdasdasd        int n   3       s Where b   gt   b     d    amp  amp   i      n         return i

User · Answer

Update  Index of Nth occurance one-liner   int NthOccurence string s  char t  int n        s TakeWhile c   gt  n -  c    t  1 0  gt  0  Count          Use these at your own risk   This looks like homework  so I left a few bugs in there for your to find   int CountChars string s  char t       int count   0     foreach  char c in s        if  s Equals t   count        return count         int CountChars string s  char t         return s Length - s Replace t ToString        Length            int CountChars string s  char t        Regex r   new Regex         t             return r Match s  Count

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Here s another LINQ solution   string input    dtststx   char searchChar    t   int occurrencePosition   3     third occurrence of the char var result   input Select  c  i    gt  new   Char   c  Index   i                       Where item   gt  item Char    searchChar                     Skip occurrencePosition - 1                     FirstOrDefault     if  result    null        Console WriteLine  Position  0  of   1   occurs at index   2                            occurrencePosition  searchChar  result Index     else       Console WriteLine  Position  0  of   1   not found                            occurrencePosition  searchChar         Just for fun  here s a Regex solution  I saw some people initially used Regex to count  but when the question changed no updates were made  Here is how it can be done with Regex - again  just for fun  The traditional approach is best for simplicity   string input    dtststx   char searchChar    t   int occurrencePosition   3     third occurrence of the char  Match match   Regex Matches input  Regex Escape searchChar ToString                         Cast lt Match gt                        Skip occurrencePosition - 1                      FirstOrDefault     if  match    null      Console WriteLine  Index      match Index   else     Console WriteLine  Match not found

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if your interested you can also create string extension methods like so        public static int Search this string yourString  string yourMarker  int yourInst   1  bool caseSensitive   true                  returns the placement of a string in another string         int num   0          int currentInst   0            if optional argument  case sensitive is false convert string and marker to lowercase         if   caseSensitive    yourString   yourString ToLower    yourMarker   yourMarker ToLower              int myReturnValue   -1    if nothing is found the returned integer is negative 1         while   num   yourMarker Length   lt   yourString Length                        string testString   yourString Substring num  yourMarker Length                if  testString    yourMarker                                currentInst                    if  currentInst    yourInst                                        myReturnValue   num                      break                                              num                                return myReturnValue            public static int Search this string yourString  char yourMarker  int yourInst   1  bool caseSensitive   true                  returns the placement of a string in another string         int num   0          int currentInst   0          var charArray   yourString ToArray lt char gt             int myReturnValue   -1          if   caseSensitive                        yourString   yourString ToLower                yourMarker   Char ToLower yourMarker                     while  num  lt   charArray Length                                        if  charArray num     yourMarker                                currentInst                    if  currentInst    yourInst                                        myReturnValue   num                      break                                              num                      return myReturnValue

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Marc Cals  LINQ Extended for generic       using System     using System Collections Generic     using System Linq      namespace fNns             public class indexer lt T gt  where T   IEquatable lt T gt                      public T t   get  set               public int index   get  set                    public static class fN                     public static indexer lt T gt  findNth lt T gt  IEnumerable lt T gt  tc  T t                 int occurrencePosition  where T   IEquatable lt T gt                              var result   tc Select  ti  i    gt  new indexer lt T gt    t   ti  index   i                           Where item   gt  item t Equals t                          Skip occurrencePosition - 1                         FirstOrDefault                   return result                          public static indexer lt T gt  findNthReverse lt T gt  IEnumerable lt T gt  tc  T t         int occurrencePosition  where T   IEquatable lt T gt                              var result   tc Reverse lt T gt    Select  ti  i    gt  new indexer lt T gt   t   ti  index   i                           Where item   gt  item t Equals t                          Skip occurrencePosition - 1                         FirstOrDefault                   return result                               Some tests      using System     using System Collections Generic     using NUnit Framework     using Newtonsoft Json     namespace FindNthNamespace Tests              public class fNTests                      TestCase  pass    dtststx    t   3  Result       t     t     index   5                 TestCase  pass   new int     0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5  6  7  8  9            0  2  Result     t   0   index   10                public string fNMethodTest lt T gt  string scenario  IEnumerable lt T gt  tc  T t  int occurrencePosition  where T   IEquatable lt T gt                              Console WriteLine scenario                  return JsonConvert SerializeObject fNns fN findNth lt T gt  tc  t  occurrencePosition   ToString                              TestCase  pass    dtststxx    t   3  Result       t     t     index   6                 TestCase  pass   new int     0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5  6  7  8  9            0  2  Result       t   0   index   19                public string fNMethodTestReverse lt T gt  string scenario  IEnumerable lt T gt  tc  T t  int occurrencePosition  where T   IEquatable lt T gt                              Console WriteLine scenario                  return JsonConvert SerializeObject fNns fN findNthReverse lt T gt  tc  t  occurrencePosition   ToString

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ranomore correctly commented that Joel Coehoorn s one-liner doesn t work   Here is a two-liner that does work  a string extension method that returns the 0-based index of the nth occurrence of a character  or -1 if no nth occurrence exists   public static class StringExtensions       public static int NthIndexOf this string s  char c  int n                var takeCount   s TakeWhile x   gt   n -   x    c   1   0    gt  0  Count            return takeCount    s Length   -1   takeCount

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Hi all i have created two overload methods for finding nth occurrence of char and for text with less complexity without navigating through loop  which increase performance of your application   public static int NthIndexOf string text  char searchChar  int nthindex       int index   -1     try            var takeCount   text TakeWhile x   gt   nthindex -   x    searchChar   1   0    gt  0  Count          if  takeCount  lt  text Length  index   takeCount          catch        return index    public static int NthIndexOf string text  string searchText  int nthindex         int index   -1       try                Match m   Regex Match text         searchText              nthindex                 if  m Success  index   m Groups 2  Captures nthindex - 1  Index              catch          return index

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string theString    The String   int index   theString NthIndexOf  THEVALUE   3  true

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Joel s answer is good  and I upvoted it   Here is a LINQ-based solution   yourString Where c   gt  c     t   Count

[c#] Find Nth occurrence of a character in a string

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