Simple way to count character occurrences in a string

Question

Is there a simple way  instead of traversing manually all the string  or loop for indexOf  in order to find how many times  a character appears in a string   Say we have  abdsd3 asda asasdd sadas  and we want that   appears 3 times

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you can also use a for each loop  I think it is simpler to read      int occurrences   0  for char c   yourString toCharArray        if c                occurrences

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public static int countChars String input char find                     if input indexOf find     -1                         return  countChars input substring 0  input indexOf find    find                    countChars input substring input indexOf find  1  find    1                            else                   return 0

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You can use Apache Commons  StringUtils countMatches String string  String subStringToCount

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There is another way to count the number of characters in each string  Assuming we have a String as String str    abfdvdvdfv    We can then count the number of times each character appears by traversing only once as  for  int i   0  i  lt  str length    i           if null  map get str charAt i                     map put str charAt i      new Integer 1              else              Integer count   map get str charAt i              map put str charAt i      count 1             We can then check the output by traversing the Map as  for  Map Entry lt String  Integer gt  entry map entrySet           System out println entry getKey     count is     entry getValue

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A character frequency count is a common task for some applications  such as education  but not general enough to warrant inclusion with the core Java APIs   As such  you ll probably need to write your own function

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Since you re scanning the whole string anyway you can build a full character count and do any number of lookups  all for the same big-Oh cost  n    public static Map lt Character Integer gt  getCharFreq String s      Map lt Character Integer gt  charFreq   new HashMap lt Character Integer gt       if  s    null        for  Character c   s toCharArray            Integer count   charFreq get c         int newCount    count  null   1   count 1         charFreq put c  newCount               return charFreq            String s    abdsd3 asda asasdd sadas   Map counts   getCharFreq s   counts get            gt  3 counts get  a         gt  7 counts get  s         gt  6

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Something a bit more functional  without Regex   public static int count String s  char c        return s length    0   0    s charAt 0   c   1   0    count s substring 1  c       It s no tail recursive  for the sake of clarity

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I believe the  one liner  that you expected to get is this    abdsd3 asda asasdd sadas  replaceAll                 1    length      Remember that the requirements are       instead of traversing manually all the string  or loop for indexOf    and let me add  that at the heart of this question it sounds like  any loop  is not wanted and there is no requirement for speed  I believe the subtext of this question is coolness factor

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Not optimal  but simple way to count occurrences   String s          int counter   s split        -1  length - 1    Note    Dollar sign is a special Regular Expression symbol  so it must be escaped with a backslash  A backslash is a special symbol for escape characters such as newlines  so it must be escaped with a backslash  The second argument of split prevents empty trailing strings from being removed

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Traversing the string is probably the most efficient  though using Regex to do this might yield cleaner looking code  though you can always hide your traverse code in a function

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public int countChar String str  char c        int count   0       for int i 0  i  lt  str length    i             if str charAt i     c              count               return count      This is definitely the fastest way  Regexes are much much slower here  and possible harder to understand

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You can look at sorting the string -- treat it as a char array -- and then do a modified binary search which counts occurrences  But I agree with  tofutim that traversing it is the most efficient -- O N  versus O N   logN    O logN

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Functional style  Java 8  just for fun    str chars   filter num - gt  num         count

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Well there are a bunch of different utilities for this  e g  Apache Commons Lang String Utils  but in the end  it has to loop over the string to count the occurrences one way or another   Note also that the countMatches method above has the following signature so will work for substrings as well   public static int countMatches String str  String sub   The source for this is  from here    public static int countMatches String str  String sub        if  isEmpty str     isEmpty sub             return 0            int count   0      int idx   0      while   idx   str indexOf sub  idx      -1            count            idx    sub length              return count      I was curious if they were iterating over the string or using Regex

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This is simple code  but of course a little bit slower   String s        int countDollar   s length  -s replaceAll           length    int counta   s length  -s replaceAll  a      length      An even better answer is here in a duplicate question

[java] Simple way to count character occurrences in a string

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