How to count certain elements in array

Question

I have an array    1  2  3  5  2  8  9  2    I would like to know how many 2s are in the array   What is the most elegant way to do it in JavaScript without looping with for loop

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Solution by recursion  function count arr  value       if  arr length     1             return arr 0      value   1   0       else         return  arr shift       value   1   0    count arr  value           count  1 2 2 3 4 5 2   2      3

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Here is an ES2017  way to get the counts for all array items in O N    const arr    1  2  3  5  2  8  9  2   const counts        arr forEach  el    gt      counts el    counts el     counts el     1    1        You can also optionally sort the output   const countsSorted   Object entries counts  sort      a        b     gt  a - b     console log countsSorted  for your example array          2   3         1   1         3   1         5   1         8   1         9   1

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Not using a loop usually means handing the process over to some method that does use a loop   Here is a way our loop hating coder can satisfy his loathing  at a price   var a  1  2  3  5  2  8  9  2    alert String a  replace    2   g     length         returned value   Number  3      You can also repeatedly call indexOf  if it is available as an array method  and move the search pointer each time   This does not create a new array  and the loop is faster than a forEach or filter   It could make a difference if you have a million members to look at   function countItems arr  what       var count  0  i      while  i  arr indexOf what  i     -1             count            i            return count    countItems a 2       returned value   Number  3

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Create a new method for Array class in core level file and use it all over your project      say in app js Array prototype occurrence   function val      return this filter e   gt  e     val  length      Use this anywhere in your project -   1  2  4  5  2  7  2  9  occurrence 2      above line returns 3

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Another approach using RegExp  x000D   x000D  const list    1  2  3  5  2  8  9  2  const d   2  const counter       list join      match new RegExp    d        g           length  console log counter  x000D   x000D   x000D   The Steps follows as below  Join the string using a comma Remember to append     after joining so as not to have incorrect values when value to be matched is at the end of the array Match the number of occurrence of a combination between the digit and comma Get length of matched items

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I believe what you are looking for is functional approach      const arr     a    a    b    g    a    e        const count   arr filter elem   gt  elem      a   length      console log count      Prints 3   elem      a  is the condition  replace it with your own

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Really  why would you need map or filter for this  reduce was  born  for these kind of operations    1  2  3  5  2  8  9  2  reduce   count 2   gt count  item  val   0    that s it   if item  val in each iteration  then 1 will be added to the accumulator count  as true will resolve to 1    As a function    function countInArray arr  val       return arr reduce  count item   gt count  item  val  0      Or  go ahead and extend your arrays   Array prototype count   function val       return this reduce  count item   gt count  item  val  0

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Modern JavaScript  Note that you should always use triple equals     when doing comparison in JavaScript  JS   The triple equals makes sure  that JS comparison behaves like double equals    in other languages  The following solution shows how to solve this the functional way  which will never have out of bounds error     Let has local scope let array    1  2  3  5  2  8  9  2      Functional filter with an Arrow function array filter x   gt  x     2  length     - gt  3  The following anonymous Arrow function  lambda function  in JavaScript   x    gt       const k   2    return k   x    may be simplified to this concise form for a single input  x   gt  2   x  where the return is implied

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this answer is a bit dated  read the edits   Say hello to your friends  map and filter and reduce and forEach and every etc    I only occasionally write for-loops in javascript  because of block-level scoping is missing  so you have to use a function as the body of the loop anyway if you need to capture or clone your iteration index or value  For-loops are more efficient generally  but sometimes you need a closure    The most readable way          filter x   gt  x  2  length    We could have written  filter function x  return x  2   length instead   The following is more space-efficient  O 1  rather than O N    but I m not sure how much of a benefit penalty you might pay in terms of time  not more than a constant factor since you visit each element exactly once           reduce  total x    gt   x  2   total 1   total   0     If you need to optimize this particular piece of code  a for loop might be faster on some browsers    you can test things on jsperf com      You can then be elegant and turn it into a prototype function    1  2  3  5  2  8  9  2  count 2    Like this   Object defineProperties Array prototype        count            value  function value                return this filter x   gt  x  value  length                        You can also stick the regular old for-loop technique  see other answers  inside the above property definition  again  that would likely be much faster      2017 edit   Whoops  this answer has gotten more popular than the correct answer  Actually  just use the accepted answer  While this answer may be cute  the js compilers probably don t  or can t due to spec  optimize such cases  So you should really write a simple for loop   Object defineProperties Array prototype        count            value  function query                                   Counts number of occurrences of query in array  an integer  gt   0                 Uses the javascript    notion of equality                             var count   0              for let i 0  i lt this length  i                    if  this i   query                      count                return count                        You could define a version  countStrictEq      which used the     notion of equality  The notion of equality may be important to what you re doing   for example  1 10 3  10   count 10   2  because numbers like  4   4 in javascript    hence calling it  countEq or  countNonstrict stresses it uses the    operator     Also consider using your own multiset data structure  e g  like python s  collections Counter   to avoid having to do the counting in the first place   class Multiset extends Map       constructor    args            super    args             add elem            if   this has elem               this set elem  1           else             this set elem  this get elem  1             remove elem            var count   this has elem    this get elem    0          if  count gt 1                this set elem  count-1             else if  count  1                this delete elem             else if  count  0              throw  tried to remove element   elem  of type   typeof elem  from Multiset  but does not exist in Multiset  count is 0 and cannot go negative                   alternatively do nothing              Demo    gt  counts   new Multiset    a  1    b  3    Map 2    a    gt  1   b    gt  3    gt  counts add  c    gt  counts Map 3    a    gt  1   b    gt  3   c    gt  1    gt  counts remove  a    gt  counts Map 2    b    gt  3   c    gt  1    gt  counts remove  a   Uncaught tried to remove element a of type string from Multiset  but does not exist in Multiset  count is 0 and cannot go negative    sidenote  Though  if you still wanted the functional-programming way  or a throwaway one-liner without overriding Array prototype   you could write it more tersely nowadays as       filter x   gt  x  2  length  If you care about performance  note that while this is asymptotically the same performance as the for-loop  O N  time   it may require O N  extra memory  instead of O 1  memory  because it will almost certainly generate an intermediate array and then count the elements of that intermediate array

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Most of the posted solutions using array functions such as filter are incomplete because they aren t parameterized   Here goes a solution with which the element to count can be set at run time   function elementsCount elementToFind  total  number       return total    number  elementToFind     var ar    1  2  3  5  2  8  9  2   var elementToFind 2  var result   ar reduce elementsCount bind this  elementToFind   0     The advantage of this approach is that could easily change the function to count for instance the number of elements greater than X   You may also declare the reduce function inline  var ar    1  2  3  5  2  8  9  2   var elementToFind 2  var result   ar reduce function  elementToFind  total  number       return total    number  elementToFind    bind this  elementToFind   0

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You can use length property in JavaScript array   var myarray       var count   myarray length   return 0  myarray    1 2   count   myarray length   return 2

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Here is a one liner in javascript    Use map  Find the matching values  v     2  in the array  returning an array of ones and zeros  Use Reduce  Add all the values of the array for the total number found     1  2  3  5  2  8  9  2     map function v        return v     2   1   0          reduce  a  b    gt  a   b  0     The result is 3

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Very simple   var count   0  for var i   0  i  lt  array length    i       if array i     2          count

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It is better to wrap it into function   let countNumber    array specificNumber    gt        return array filter n   gt  n    specificNumber  length    countNumber  1 2 3 4 5  3     returns 1

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x000D   x000D  var arrayCount    1 2 3 2 5 6 2 8   x000D  var co   0  x000D  function findElement    x000D      arrayCount find function value  index    x000D        if value    2  x000D          co    x000D          x000D      console log   found          co     element with value 2    x000D    x000D   x000D   x000D    I would do something like  that    x000D   x000D  var arrayCount    1 2 3 4 5 6 7 8   x000D   x000D  function countarr    x000D    var dd   0  x000D    arrayCount forEach  function s   x000D      dd    x000D        x000D   x000D    console log dd   x000D    x000D   x000D   x000D

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var x   from z in Model ListOfFaculty                     select z                        foreach  var c in x                         lt div class  quot row quot  gt                   lt div class  quot col-lg-3 quot  gt                       lt label gt FacultyName   c Name  lt  label gt                   lt  div gt                   lt div class  quot col-lg-3 quot  gt                       lt label gt                          Count                                 var b   from v in Model ListOfDepartment                                     where  v Faculty id    c ID                                      select v                                                       b Count                        lt  label gt                   lt  div gt               lt  div gt                     lt  div gt

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I m a begin fan of js array s reduce function    const myArray   1  2  3  5  2  8  9  2   const count   myArray reduce  count  num    gt  num     2   count   1   count  0    In fact if you really want to get fancy you can create a count function on the Array prototype  Then you can reuse it    Array prototype count   function filterMethod      return this reduce  count  item    gt  filterMethod item   count   1   count  0        Then do   const myArray   1  2  3  5  2  8  9  2  const count   myArray count x   gt  x  2

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Depending on how you want to run it  const reduced    array  val    gt       self explanatory     return array filter  element    gt  element     val  length     console log reduced  1  2  3  5  2  8  9  2   2        3  const reducer    array    gt       array to set  gt  set forEach  gt  map set     const count   new Map        const values   new Set array       values forEach  element   gt            count set element  array filter  arrayElement    gt  arrayElement     element  length               return count    console log reducer  1  2  3  5  2  8  9  2         Map 6    1   gt  1  2   gt  3  3   gt  1  5   gt  1  8   gt  1

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Weirdest way I can think of doing this is    a length-     a join           split     n      join      match    g  length  1   Where    a is the array n is the number to count in the array   My suggestion  use a while or for loop  -

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2017  If someone is still interested in the question  my solution is the following      x000D   x000D  const arrayToCount    1  2  3  5  2  8  9  2   x000D  const result   arrayToCount filter i   gt  i     2  length  x000D  console log  number of the found elements      result   x000D   x000D   x000D

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If you are using lodash or underscore the   countBy method will provide an object of aggregate totals keyed by each value in the array  You can turn this into a one-liner if you only need to count one value     countBy   foo    foo    bar     foo       2   This also works fine on arrays of numbers  The one-liner for your example would be     countBy  1  2  3  5  2  8  9  2   2      3

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x000D   x000D  Array prototype count   function  v        var c   0      for  let i   0  i  lt  this length  i              if this i     v               c                        return c     var arr    1  2  3  5  2  8  9  2    console log arr count 2      3 x000D   x000D   x000D

[javascript] How to count certain elements in array?

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