How would you count occurrences of a string actually a char within a string

Question

I am doing something where I realised I wanted to count how many  s I could find in a string  and then it struck me  that there were several ways to do it  but couldn t decide on what the best  or easiest  was   At the moment I m going with something like   string source     once upon a time    int count   source Length - source Replace          Length    But I don t like it at all  any takers   I don t really want to dig out RegEx for this  do I   I know my string is going to have the term I m searching for  so you can assume that     Of course for strings where length   1   string haystack     once upon a time   string needle        int needleCount     haystack Length - haystack Replace needle     Length     needle Length

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string source     once upon a time    int count   0  n   0  while   n   source IndexOf      n    1     0  count      A variation on Richard Watson s answer  slightly faster with improving efficiency the more times the char occurs in the string  and less code   Though I must say  without extensively testing every scenario  I did see a very significant speed improvement by using   int count   0  for  int n   0  n  lt  source Length  n    if  source n          count

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For the case of a string delimiter  not for the char case  as the subject says   string source       once   upon   a   time      int count   source Split new              StringSplitOptions RemoveEmptyEntries  Length - 1   The poster s original source value s    once upon a time    natural delimiter is a char     and responses do explain source Split char    option though

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For anyone wanting a ready to use String extension method   here is what I use which was based on the best of the posted answers   public static class StringExtension                lt summary gt  Returns the number of occurences of a string within a string  optional comparison allows case and culture control   lt  summary gt      public static int Occurrences this System String input  string value  StringComparison stringComparisonType   StringComparison Ordinal                if  String IsNullOrEmpty value   return 0           int count      0          int position   0           while   position   input IndexOf value  position  stringComparisonType      -1                        position    value Length              count       1                     return count                  lt summary gt  Returns the number of occurences of a single character within a string   lt  summary gt      public static int Occurrences this System String input  char value                int count   0          foreach  char c in input  if  c    value  count    1          return count

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public static int GetNumSubstringOccurrences string text  string search        int num   0      int pos   0       if   string IsNullOrEmpty text   amp  amp   string IsNullOrEmpty search                 while   pos   text IndexOf search  pos    gt  -1                        num                 pos    search Length                      return num

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String in string   Find  etc  in      JD JD JD JD etc  and etc  JDJDJDJDJDJDJDJD and etc    var strOrigin        JD JD JD JD etc  and etc  JDJDJDJDJDJDJDJD and etc    var searchStr    etc   int count    strOrigin Length - strOrigin Replace searchStr      Length  searchStr Length    Check performance before discarding this one as unsound clumsy

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My initial take gave me something like   public static int CountOccurrences string original  string substring        if  string IsNullOrEmpty substring           return 0      if  substring Length    1          return CountOccurrences original  substring 0        if  string IsNullOrEmpty original             substring Length  gt  original Length          return 0      int substringCount   0      for  int charIndex   0  charIndex  lt  original Length  charIndex                  for  int subCharIndex   0  secondaryCharIndex   charIndex  subCharIndex  lt  substring Length  amp  amp  secondaryCharIndex  lt  original Length  subCharIndex    secondaryCharIndex                          if  substring subCharIndex     original secondaryCharIndex                   goto continueOuter                    if  charIndex   substring Length  gt  original Length              break          charIndex    substring Length - 1          substringCount        continueOuter                      return substringCount     public static int CountOccurrences string original  char  char        if  string IsNullOrEmpty original           return 0      int substringCount   0      for  int charIndex   0  charIndex  lt  original Length  charIndex            if   char    original charIndex               substringCount        return substringCount      The needle in a haystack approach using replace and division yields 21  seconds whereas this takes about 15 2   Edit after adding a bit which would add substring Length - 1 to the charIndex  like it should   it s at 11 6 seconds   Edit 2  I used a string which had 26 two-character strings  here are the times updated to the same sample texts   Needle in a haystack  OP s version   7 8 Seconds  Suggested mechanism  4 6 seconds   Edit 3  Adding the single character corner-case  it went to 1 2 seconds   Edit 4  For context  50 million iterations were used

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If you check out this webpage  15 different ways of doing this are benchmarked  including using parallel loops   The fastest way appears to be using either a single threaded for-loop  if you have  Net version  lt  4 0  or a parallel for loop  if using  Net   4 0 with thousands of checks    Assuming  ss  is your Search String   ch  is your character array  if you have more than one char you re looking for   here s the basic gist of the code that had the fastest run time single threaded   for  int x   0  x  lt  ss Length  x          for  int y   0  y  lt  ch Length  y                  for  int a   0  a  lt  ss x  Length  a                       if  ss x  a     ch y                 it s found  DO what you need to here                      The benchmark source code is provided too so you can run your own tests

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str  aaabbbbjjja   int count   0  int size   str Length   string   strarray   new string size   for  int i   0  i  lt  str Length  i          strarray i    str Substring i  1     Array Sort strarray   str       for  int i   0  i  lt  strarray Length - 1  i           if  strarray i     strarray i   1                  count              else               count            str   str   strarray i    count          count   0           count    str   str   strarray strarray Length - 1    count    This is for counting the character occurance  For this example output will be  a4b4j3

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string source     once upon a time    int count   0  int n   0   while   n   source IndexOf      n      -1       n       count        On my computer it s about 2 seconds faster than the for-every-character solution for 50 million iterations   2013 revision   Change the string to a char   and iterate through that  Cuts a further second or two off the total time for 50m iterations   char   testchars   source ToCharArray    foreach  char c in testchars         if  c                  count        This is quicker still   char   testchars   source ToCharArray    int length   testchars Length  for  int n   0  n  lt  length  n          if  testchars n                  count        For good measure  iterating from the end of the array to 0 seems to be the fastest  by about 5    int length   testchars Length  for  int n   length-1  n  gt   0  n--        if  testchars n                  count        I was wondering why this could be and was Googling around  I recall something about reverse iterating being quicker   and came upon this SO question which annoyingly uses the string to char   technique already  I think the reversal trick is new in this context  though   What is the fastest way to iterate through individual characters in a string in C

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string search     string   var occurrences    regex Match search          Count    This will count each time the program finds   s  exactly  case sensitive  and the number of occurrences of this will be stored in the variable  occurrences

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int count   new Regex Regex Escape needle   Matches haystack  Count

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var conditionalStatement   conditionSetting Value                 order of replace matters  remove    before    incase of                 conditionalStatement   conditionalStatement Replace            Replace            Replace           Replace           Replace   gt         Replace   lt         Replace   gt          Replace   lt                        var listOfValidConditions   new List lt string gt                    gt      lt      gt       lt                    if  conditionalStatement Count x   gt  x            1                                result InvalidFieldList Add new KeyFieldData batch DECurrentField   The IsDoubleKeyCondition does not contain a supported conditional statement  Contact System Administrator                      result Status   ValidatorStatus Fail                  return result                  Needed to do something similar to test conditional statements from a string    Replaced what i was looking for with a single character and counted the instances of the single character    Obviously the single character you re using will need to be checked to not exist in the string before this happens to avoid incorrect counts

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string source     once upon a time    int count   0  foreach  char c in source     if  c         count      Has to be faster than the source Replace   by itself

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I felt that we were lacking certain kinds of sub string counting  like unsafe byte-by-byte comparisons  I put together the original poster s method and any methods I could think of   These are the string extensions I made   namespace Example       using System      using System Text       public static class StringExtensions               public static int CountSubstr this string str  string substr                        return  str Length - str Replace substr      Length    substr Length                     public static int CountSubstr this string str  char substr                        return  str Length - str Replace substr ToString        Length                      public static int CountSubstr2 this string str  string substr                        int substrlen   substr Length              int lastIndex   str IndexOf substr  0  StringComparison Ordinal               int count   0              while  lastIndex    -1                                  count                  lastIndex   str IndexOf substr  lastIndex   substrlen  StringComparison Ordinal                              return count                     public static int CountSubstr2 this string str  char substr                        int lastIndex   str IndexOf substr  0               int count   0              while  lastIndex    -1                                  count                  lastIndex   str IndexOf substr  lastIndex   1                              return count                     public static int CountChar this string str  char substr                        int length   str Length              int count   0              for  int i   0  i  lt  length    i                  if  str i     substr                        count               return count                     public static int CountChar2 this string str  char substr                        int count   0              foreach  var c in str                  if  c    substr                        count               return count                     public static unsafe int CountChar3 this string str  char substr                        int length   str Length              int count   0              fixed  char  chars   str                                for  int i   0  i  lt  length    i                      if    chars   i     substr                            count                             return count                     public static unsafe int CountChar4 this string str  char substr                        int length   str Length              int count   0              fixed  char  chars   str                                for  int i   length - 1  i  gt   0  --i                      if    chars   i     substr                            count                             return count                     public static unsafe int CountSubstr3 this string str  string substr                        int length   str Length              int substrlen   substr Length              int count   0              fixed  char  strc   str                                fixed  char  substrc   substr                                        int n   0                       for  int i   0  i  lt  length    i                                                if    strc   i       substrc   n                                                           n                              if  n    substrlen                                                                  count                                  n   0                                                                                  else                             n   0                                                                     return count                     public static int CountSubstr3 this string str  char substr                        return CountSubstr3 str  substr ToString                        public static unsafe int CountSubstr4 this string str  string substr                        int length   str Length              int substrLastIndex   substr Length - 1              int count   0              fixed  char  strc   str                                fixed  char  substrc   substr                                        int n   substrLastIndex                       for  int i   length - 1  i  gt   0  --i                                                if    strc   i       substrc   n                                                         if  --n    -1                                                                  count                                  n   substrLastIndex                                                                                  else                             n   substrLastIndex                                                                     return count                     public static int CountSubstr4 this string str  char substr                        return CountSubstr4 str  substr ToString                         Followed by the test code     static void Main         const char matchA            const string matchB    and       const string matchC    muchlongerword       const string testStrA     and d e banna i o   pfasd  and d e banna i o   pfasd        const string testStrB    and sdf and ans andeians andano ip and and sdf and ans andeians andano ip and       const string testStrC            muchlongerword amuchlongerworsdfmuchlongerwordsdf jmuchlongerworijv muchlongerword sdmuchlongerword dsmuchlongerword       const int testSize   1000000      Console WriteLine testStrA CountSubstr            Console WriteLine testStrA CountSubstr2            Console WriteLine testStrA CountSubstr3            Console WriteLine testStrA CountSubstr4            Console WriteLine testStrA CountChar            Console WriteLine testStrA CountChar2            Console WriteLine testStrA CountChar3            Console WriteLine testStrA CountChar4            Console WriteLine testStrB CountSubstr  and         Console WriteLine testStrB CountSubstr2  and         Console WriteLine testStrB CountSubstr3  and         Console WriteLine testStrB CountSubstr4  and         Console WriteLine testStrC CountSubstr  muchlongerword         Console WriteLine testStrC CountSubstr2  muchlongerword         Console WriteLine testStrC CountSubstr3  muchlongerword         Console WriteLine testStrC CountSubstr4  muchlongerword         var timer   new Stopwatch        timer Start        for  int i   0  i  lt  testSize    i          testStrA CountSubstr matchA       timer Stop        Console WriteLine  CS1 chr      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrB CountSubstr matchB       timer Stop        Console WriteLine  CS1 and      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrC CountSubstr matchC       timer Stop        Console WriteLine  CS1 mlw      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrA CountSubstr2 matchA       timer Stop        Console WriteLine  CS2 chr      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrB CountSubstr2 matchB       timer Stop        Console WriteLine  CS2 and      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrC CountSubstr2 matchC       timer Stop        Console WriteLine  CS2 mlw      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrA CountSubstr3 matchA       timer Stop        Console WriteLine  CS3 chr      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrB CountSubstr3 matchB       timer Stop        Console WriteLine  CS3 and      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrC CountSubstr3 matchC       timer Stop        Console WriteLine  CS3 mlw      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrA CountSubstr4 matchA       timer Stop        Console WriteLine  CS4 chr      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrB CountSubstr4 matchB       timer Stop        Console WriteLine  CS4 and      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrC CountSubstr4 matchC       timer Stop        Console WriteLine  CS4 mlw      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrA CountChar matchA       timer Stop        Console WriteLine  CC1 chr      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrA CountChar2 matchA       timer Stop        Console WriteLine  CC2 chr      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrA CountChar3 matchA       timer Stop        Console WriteLine  CC3 chr      timer Elapsed TotalMilliseconds    ms         timer Restart        for  int i   0  i  lt  testSize    i          testStrA CountChar4 matchA       timer Stop        Console WriteLine  CC4 chr      timer Elapsed TotalMilliseconds    ms        Results  CSX corresponds with CountSubstrX and CCX corresponds with CountCharX   chr  searches a string for       and  searches a string for  and   and  mlw  searches a string for  muchlongerword   CS1 chr  824 123ms CS1 and  586 1893ms CS1 mlw  486 5414ms CS2 chr  127 8941ms CS2 and  806 3918ms CS2 mlw  497 318ms CS3 chr  201 8896ms CS3 and  124 0675ms CS3 mlw  212 8341ms CS4 chr  81 5183ms CS4 and  92 0615ms CS4 mlw  116 2197ms CC1 chr  66 4078ms CC2 chr  64 0161ms CC3 chr  65 9013ms CC4 chr  65 8206ms   And finally  I had a file with 3 6 million characters  It was  derp adfderdserp dfaerpderp deasderp  repeated 100 000 times  I searched for  derp  inside the file with the above methods 100 times these results   CS1Derp  1501 3444ms CS2Derp  1585 797ms CS3Derp  376 0937ms CS4Derp  271 1663ms   So my 4th method is definitely the winner  but  realistically  if a 3 6 million character file 100 times only took 1586ms as the worse case  then all of this is quite negligible   By the way  I also scanned for the  d  char in the 3 6 million character file with 100 times CountSubstr and CountChar methods  Results     CS1  d   2606 9513ms CS2  d   339 7942ms CS3  d   960 281ms CS4  d   233 3442ms CC1  d   302 4122ms CC2  d   280 7719ms CC3  d   299 1125ms CC4  d   292 9365ms   The original posters method is very bad for single character needles in a large haystack according to this   Note  All values were updated to Release version output  I accidentally forgot to build on Release mode upon the first time I posted this  Some of my statements have been amended

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A generic function for occurrences of strings   public int getNumberOfOccurencies String inputString  String checkString        if  checkString Length  gt  inputString Length    checkString Equals        return 0        int lengthDifference   inputString Length - checkString Length      int occurencies   0      for  int i   0  i  lt  lengthDifference  i              if  inputString Substring i  checkString Length  Equals checkString     occurencies    i    checkString Length - 1          return occurencies

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Edit   source Split      Length-1

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If you re using  NET 3 5 you can do this in a one-liner with LINQ   int count   source Count f   gt  f            If you don t want to use LINQ you can do it with   int count   source Split      Length - 1      You might be surprised to learn that your original technique seems to be about 30  faster than either of these  I ve just done a quick benchmark with   once upon a time   and the results are as follows      Your original   12s   source Count   19s   source Split   17s   foreach  from bobwienholt s answer    10s     The times are for 50 000 000 iterations so you re unlikely to notice much difference in the real world

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Thought I would throw my extension method into the ring  see comments for more info   I have not done any formal bench marking  but I think it has to be very fast for most scenarios   EDIT  OK - so this SO question got me to wondering how the performance of our current implementation would stack up against some of the solutions presented here  I decided to do a little bench marking and found that our solution was very much in line with the performance of the solution provided by Richard Watson up until you are doing aggressive searching with large strings  100 Kb     large substrings  32 Kb    and many embedded repetitions  10K     At that point our solution was around 2X to 4X slower  Given this and the fact that we really like the solution presented by Richard Watson  we have refactored our solution accordingly  I just wanted to make this available for anyone that might benefit from it    Our original solution            lt summary gt          Counts the number of occurrences of the specified substring within         the current string           lt  summary gt           lt param name  s  gt The current string  lt  param gt           lt param name  substring  gt The substring we are searching for  lt  param gt           lt param name  aggressiveSearch  gt Indicates whether or not the algorithm          should be aggressive in its search behavior  see Remarks   Default          behavior is non-aggressive  lt  param gt           lt remarks gt This algorithm has two search modes - aggressive and          non-aggressive  When in aggressive search mode  aggressiveSearch            true   the algorithm will try to match at every possible starting          character index within the string  When false  all subsequent          character indexes within a substring match will not be evaluated           For example  if the string was  abbbc  and we were searching for          the substring  bb   then aggressive search would find 2 matches          with starting indexes of 1 and 2  Non aggressive search would find          just 1 match with starting index at 1  After the match was made           the non aggressive search would attempt to make it s next match          starting at index 3 instead of 2  lt  remarks gt           lt returns gt The count of occurrences of the substring within the string  lt  returns gt      public static int CountOccurrences this string s  string substring           bool aggressiveSearch   false                   if s or substring is null or empty  substring cannot be found in s         if  string IsNullOrEmpty s     string IsNullOrEmpty substring               return 0              if the length of substring is greater than the length of s             substring cannot be found in s         if  substring Length  gt  s Length              return 0           var sChars   s ToCharArray            var substringChars   substring ToCharArray            var count   0          var sCharsIndex   0              substring cannot start in s beyond following index         var lastStartIndex   sChars Length - substringChars Length           while  sCharsIndex  lt   lastStartIndex                        if  sChars sCharsIndex     substringChars 0                                    potential match checking                 var match   true                  var offset   1                  while  offset  lt  substringChars Length                                        if  sChars sCharsIndex   offset     substringChars offset                                                 match   false                          break                                            offset                                      if  match                                        count                           if aggressive  just advance to next char in s  otherwise                          skip past the match just found in s                     sCharsIndex    aggressiveSearch   1   substringChars Length                                    else                                          no match found  just move to next char in s                     sCharsIndex                                                else                                  no match at current index  move along                 sCharsIndex                                     return count          And here is our revised solution            lt summary gt          Counts the number of occurrences of the specified substring within         the current string           lt  summary gt           lt param name  s  gt The current string  lt  param gt           lt param name  substring  gt The substring we are searching for  lt  param gt           lt param name  aggressiveSearch  gt Indicates whether or not the algorithm          should be aggressive in its search behavior  see Remarks   Default          behavior is non-aggressive  lt  param gt           lt remarks gt This algorithm has two search modes - aggressive and          non-aggressive  When in aggressive search mode  aggressiveSearch            true   the algorithm will try to match at every possible starting          character index within the string  When false  all subsequent          character indexes within a substring match will not be evaluated           For example  if the string was  abbbc  and we were searching for          the substring  bb   then aggressive search would find 2 matches          with starting indexes of 1 and 2  Non aggressive search would find          just 1 match with starting index at 1  After the match was made           the non aggressive search would attempt to make it s next match          starting at index 3 instead of 2  lt  remarks gt           lt returns gt The count of occurrences of the substring within the string  lt  returns gt      public static int CountOccurrences this string s  string substring           bool aggressiveSearch   false                   if s or substring is null or empty  substring cannot be found in s         if  string IsNullOrEmpty s     string IsNullOrEmpty substring               return 0              if the length of substring is greater than the length of s             substring cannot be found in s         if  substring Length  gt  s Length              return 0           int count   0  n   0          while   n   s IndexOf substring  n  StringComparison InvariantCulture      -1                        if  aggressiveSearch                  n                else                 n    substring Length              count                       return count

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If you want to be able to search for whole strings  and not just characters   src Select  c  i    gt  src Substring i        Count sub   gt  sub StartsWith target     Read as  for each character in the string  take the rest of the string starting from that character as a substring  count it if it starts with the target string

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string s    65 fght 6565 4665 hjk   int count   0  foreach  Match m in Regex Matches s   65      count

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private int CountWords string text  string word        int count    text Length - text Replace word      Length    word Length      return count      Because the original solution  was the fastest for chars  I suppose it will also be for strings  So here is my contribution   For the context  I was looking for words like  failed  and  succeeded  in a log file   Gr  Ben

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In C   a nice String SubString counter is this unexpectedly tricky fellow   public static int CCount String haystack  String needle        return haystack Split new     needle    StringSplitOptions None  Length - 1

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string s    HOWLYH THIS ACTUALLY WORKSH WOWH   int count   0  for  int i   0  i  lt  s Length  i       if  s i      H   count      It just checks every character in the string  if the character is the character you are searching for  add one to count

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I think the easiest way to do this is to use the Regular Expressions  This way you can get the same split count as you could using myVar Split  x   but in a multiple character setting   string myVar    do this to count the number of words in my wording so that I can word it up    int count   Regex Split myVar   word   Length

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I ve made some research and found that Richard Watson s solution is fastest in most cases  That s the table with results of every solution in the post  except those use Regex because it throws exceptions while parsing string like  test test        Name        Short char    Long char   Short short  Long short    Long long       Inspite             134         1853           95         1146          671      LukeH 1             346         4490          N A          N A          N A      LukeH 2             152         1569          197         2425         2171  Bobwienholt             230         3269          N A          N A          N A  Richard Watson           33          298          146          737          543  StefanosKargas          N A          N A          681        11884        12486    You can see that in case of finding number of occurences of short substrings  1-5 characters  in short string 10-50 characters  the original algorithm is preferred   Also  for multicharacter substring you should use the following code  based on Richard Watson s solution   int count   0  n   0   if substring              while   n   source IndexOf substring  n  StringComparison InvariantCulture      -1                n    substring Length            count

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These both only work for single-character search terms     countOccurences  the    the answer is the answer     int countOccurences string needle  string haystack        return  haystack Length - haystack Replace needle     Length    needle Length      may turn out to be better for longer needles     But there has to be a more elegant way

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Regex Matches input   Regex Escape  stringToMatch    Count

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using System Linq   int CountOf     A  B C  D  Split       Length - 1

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LINQ works on all collections  and since strings are just a collection of characters  how about this nice little one-liner   var count   source Count c   gt  c            Make sure you have using System Linq  at the top of your code file  as  Count is an extension method from that namespace

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to count char or string     string st    quot asdfasdfasdfsadfasdf asdfasdfas dfsdfsdafsdfsd fsadfasdf dff quot           int count   0          int location   0                  while  st IndexOf  quot   quot   location   1   gt  0                            count                    location   st IndexOf  quot   quot   location   1                     MessageBox Show count ToString

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string Name    Very good nice one is very good but is very good nice one this is called the term   bool valid true  int count   0  int k 0  int m   0  while  valid        k   Name Substring m Name Length-m  IndexOf  good        if  k    -1                count            m   m   k   4            else         valid   false    Console WriteLine count     Times accures

[c#] How would you count occurrences of a string (actually a char) within a string?

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