Counting the occurrences frequency of array elements

Question

In Javascript  I m trying to take an initial array of number values and count the elements inside it  Ideally  the result would be two new arrays  the first specifying each unique element  and the second containing the number of times each element occurs  However  I m open to suggestions on the format of the output   For example  if the initial array was   5  5  5  2  2  2  2  2  9  4   Then two new arrays would be created  The first would contain the name of each unique element   5  2  9  4   The second would contain the number of times that element occurred in the initial array   3  5  1  1   Because the number 5 occurs three times in the initial array  the number 2 occurs five times and 9 and 4 both appear once   I ve searched a lot for a solution  but nothing seems to work  and everything I ve tried myself has wound up being ridiculously complex  Any help would be appreciated   Thanks

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One line ES6 solution  So many answers using object as a map but I can t see anyone using an actual Map  const map   arr reduce  acc  e    gt  acc set e   acc get e     0    1   new Map       Use map keys   to get unique elements  Use map values   to get the occurrences  Use map entries   to get the pairs  element  frequency    x000D   x000D  var arr    5  5  5  2  2  2  2  2  9  4  x000D   x000D  const map   arr reduce  acc  e    gt  acc set e   acc get e     0    1   new Map     x000D   x000D  console info     map keys     x000D  console info     map values     x000D  console info     map entries     x000D   x000D   x000D

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If you are using underscore you can go the functional route  a     foo    foo    bar     var results     reduce a function counts key   counts key     return counts                        object    map    uniq a   function key    return  key  0         so your first array is    keys results    and the second array is    values results    most of this will default to native javascript functions if they are available   demo   http   jsfiddle net dAaUU

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I know this question is old but I realized there are too few solutions where you get the count array as asked with a minimal code so here is mine    The initial array we want to count occurences var initial    5  5  5  2  2  2  2  2  9  4         The count array asked for var count   Array from new Set initial   map val   gt  initial filter v   gt  v     val  length         Outputs   3  5  1  1    Beside you can get the set from that initial array with var set   Array from new Set initial         set    5  2  9  4

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ES6 version should be much simplifier  another one line solution   let arr    5  5  5  2  2  2  2  2  9  4   let acc   arr reduce  acc  val    gt  acc set val  1    acc get val     0    new Map      console log acc      output  Map   5   gt  3  2   gt  5  9   gt  1  4   gt  1     A Map instead of plain Object helping us to distinguish different type of elements  or else all counting are base on strings

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Here is a way to count occurrences inside an array of objects  It also places the first array s contents inside a new array to sort the values so that the order in the original array is not disrupted  Then a recursive function is used to go through each element and count the quantity property of each object inside the array   var big array         name   Pineapples   quantity  3        name   Pineapples   quantity  1        name   Bananas   quantity  1        name   Limes   quantity  1        name   Bananas   quantity  1        name   Pineapples   quantity  2        name   Pineapples   quantity  1        name   Bananas   quantity  1        name   Bananas   quantity  1        name   Bananas   quantity  5        name   Coconuts   quantity  1        name   Lemons   quantity  2        name   Oranges   quantity  1        name   Lemons   quantity  1        name   Limes   quantity  1        name   Grapefruit   quantity  1        name   Coconuts   quantity  5        name   Oranges   quantity  6       function countThem       var names array         for  var i   0  i  lt  big array length  i          names array push  Object assign     big array i             function outerHolder item array        if  item array length  gt  0          var occurrences             var counter   0        var bgarlen   item array length        item array sort function a  b    return  a name  gt  b name    1     b name  gt  a name    -1   0              function recursiveCounter             occurrences push item array 0            item array splice 0  1           var last occurrence element   occurrences length - 1          var last occurrence entry   occurrences last occurrence element  name          var occur counter   0          var quantity counter   0          for  var i   0  i  lt  occurrences length  i                if  occurrences i  name     last occurrence entry                occur counter   occur counter   1              if  occur counter     1                  quantity counter   occurrences i  quantity                else                 quantity counter   quantity counter   occurrences i  quantity                                               if  occur counter  gt  1              var current match   occurrences length - 2            occurrences current match  quantity   quantity counter            occurrences splice last occurrence element  1                      counter   counter   1           if  counter  lt  bgarlen              recursiveCounter                             recursiveCounter           return occurrences              alert JSON stringify outerHolder names array

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Check out the code below    lt html gt   lt head gt   lt script gt     array with values var ar    5  5  5  2  2  2  2  2  9  4    var Unique          we ll store a list of unique values in here var Counts          we ll store the number of occurances in here  for var i in ar        var Index   ar i       Unique Index    ar i       if typeof Counts Index     undefined             Counts Index  1      else         Counts Index           remove empty items Unique   Unique filter function    return true    Counts   Counts filter function    return true     alert ar join        alert Unique join        alert Counts join         var a      for var i 0  i lt Unique length  i          a push Unique i          Counts i     x      alert a join           lt  script gt   lt  head gt   lt body gt    lt  body gt   lt  html gt

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You can simplify this a bit by extending your arrays with a count function  It works similar to Ruby   s Array count  if you   re familiar with it   Array prototype count   function obj     var count   this length    if typeof obj       undefined        var array   this slice 0   count   0     clone array and reset count     for i   0  i  lt  array length  i           if array i     obj   count                 return count      Usage   let array     a    b    d    a    c    array count  a         gt  2 array count  b         gt  1 array count  e         gt  0 array count         gt  5   Gist    Edit  You can then get your first array  with each occurred item  using Array filter   let occurred       array filter function item      if   occurred includes item         occurred push item       return true               gt    a    b    d    c     And your second array  with the number of occurrences  using Array count into Array map   occurred map array count bind array         gt   2  1  1  1    Alternatively  if order is irrelevant  you can just return it as a key-value pair   let occurrences      occurred forEach function item    occurrences item    array count item      occurences       gt   2  5  4  1  5  3  9  1

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Here s a classic old school method for counting arrays   var arr    5  5  5  2  2  2  2  2  9  4   var counted       count       var i   0  j   0  k   0  while  k  lt  arr length        if  counted indexOf arr k    lt  0            counted i    arr k           count i    0          for  j   0  j  lt  arr length  j                  if  counted i     arr j                     count i                                     i          else           k              You can sort it first if you want an alphabetical result  but if you want to preserve the order in which the data was entered then give this a try  Nested loops may be a bit slower than some of the other methods on this page

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var aa    1 3 5 7 3 2 4 6 8 1 3 5 5 2 0 6 5 9 6 3 5 2 5 6 8   var newArray       for var element of aa     if typeof newArray element       undefined     newArray element      null       newArray element    1     else      newArray element    1         for   var element in newArray     console log  element    - gt     newArray element

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The more elegant way is using Logical nullish assignment       The main idea is still using Array reduce   to aggregate with output as object to get the highest performance  both time and space complexity  in terms of searching  amp  construct bunches of intermediate arrays like other answers   x000D   x000D  const arr    2  2  2  2  2  4  5  5  5  9   const result   arr reduce  acc  curr    gt      acc curr        curr   0     acc curr  curr          return acc           console log Object values result    x000D   x000D   x000D

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This question is more than 8 years old and many  many answers do not really take ES6 and its numerous advantages into account    Perhaps is even more important to think about the consequences of our code for garbage collection memory management whenever we create additional arrays  make double or triple copies of arrays or even convert arrays into objects  These are trivial observations for small applications but if scale is a long term objective then think about these  thoroughly    If you just need a  counter  for specific data types and the starting point is an array  I assume you want therefore an ordered list and take advantage of the many properties and methods arrays offer   you can just simply iterate through array1 and populate array2 with the values and number of occurrences for these values found in array1    As simple as that    Example of simple class SimpleCounter  ES6  for Object Oriented Programming and Object Oriented Design  class SimpleCounter         constructor rawList      input array type         this rawList   rawList          this finalList                  mapValues       returns a new array          this rawList forEach value   gt                this finalList value    this finalList value      this finalList value    1                       this rawList   null     remove array1 for garbage collection          return this finalList             module exports   SimpleCounter

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Its easy with filter  In this example we simply assign count  the length of the array filtered by the key you re looking for  let array     name   steve   age  22    name   bob   age  30    let count   array filter obj   gt  obj name     obj name  length  console log count    more on JS Filiters here https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Array filter

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Here you go  Live demo  http   jsfiddle net simevidas bnACW   Note This changes the order of the original input array using Array sort   x000D   x000D  var arr    2  2  2  2  2  4  5  5  5  9    function foo arr      var a           b           prev     arr sort      for  var i   0  i  lt  arr length  i          if  arr i      prev          a push arr i          b push 1         else         b b length - 1               prev   arr i          return  a  b      var result   foo arr   console log       result 0              result 1         x000D   x000D   x000D

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I think this is the simplest way how to count occurrences with same value in array   var a    true  false  false  false   a filter function value       return value     false     length

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You can use an object to hold the results    x000D   x000D  var arr    5  5  5  2  2  2  2  2  9  4   x000D  var counts       x000D   x000D  for  var i   0  i  lt  arr length  i      x000D    var num   arr i   x000D    counts num    counts num    counts num    1   1  x000D    x000D   x000D  console log counts 5   counts 2   counts 9   counts 4    x000D   x000D   x000D    So  now your counts object can tell you what the count is for a particular number   console log counts 5       logs  3    If you want to get an array of members  just use the keys   functions  keys counts      returns   5    2    9    4

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If you favour a single liner   arr reduce function countMap  word   countMap word      countMap word     1 return countMap         Edit  6 12 2015   The Explanation from the inside out  countMap is a map that maps a word with its frequency  which we can see the anonymous function  What reduce does is apply the function with arguments as all the array elements and countMap being passed as the return value of the last function call  The last parameter      is the default value of countMap for the first function call

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function countOcurrences arr       return arr reduce  aggregator  value  index  array    gt          if  aggregator value            return aggregator       aggregator   value   1            else          return aggregator       aggregator   value    aggregator value

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Given array x i e x     boy   man   oldman   scout   pilot    number of occurrences of an element  man  is   x length - x toString   split   man    toString   split      length

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Using Lodash  x000D   x000D  const values    5  5  5  2  2  2  2  2  9  4   const frequency     map   groupBy values   val   gt     value  val 0   frequency  val length      console log frequency   x000D   lt script src  https   cdnjs cloudflare com ajax libs lodash js 4 17 15 lodash min js  gt  lt  script gt  x000D   x000D   x000D

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So here s how I d do it with some of the newest javascript features   First  reduce the array to a Map of the counts   let countMap   array reduce     map  value    gt   map set value   map get value     0    1   return map      new Map       By using a Map  your starting array can contain any type of object  and the counts will be correct  Without a Map  some types of objects will give you strange counts  See the Map docs for more info on the differences   This could also be done with an object if all your values are symbols  numbers  or strings   let countObject   array reduce     map  value    gt    map value     map value     0    1  return map             Or slightly fancier in a functional way without mutation  using destructuring and object spread syntax   let countObject   array reduce     value    value   count   0     rest     gt      value   count   1     rest              At this point  you can use the Map or object for your counts  and the map is directly iterable  unlike an object   or convert it to two arrays   For the Map   countMap forEach  count  value    gt  console log  value    value   count    count     let values   countMap keys   let counts   countMap values     Or for the object   Object    entries countObject     convert to array of  key  valueAtKey  pairs    forEach   value  count     gt  console log  value    value   count    count     let values   Object keys countObject  let counts   Object values countObject

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ES6 solution with reduce  fixed     x000D   x000D  const arr    2  2  2  3  2  x000D   x000D  const count   arr reduce  pre  cur    gt   cur     2      pre   pre  0  x000D  console log count     4 x000D   x000D   x000D

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Edit 2020  this is a pretty old answer  nine years   Extending the native prototype will always generate discussion  Although I think the programmer is free to choose her own programming style  here s a  more modern  approach to the problem without extending Array prototype   x000D   x000D         create array with some pseudo random values  1 - 5    const arr   Array from  length  100        map       gt  Math floor 1   Math random     5          frequencies using a reducer   const arrFrequencies   arr reduce  acc  value    gt               acc   value   acc value    1    1           console log arrFrequencies         console log  Value 4 occurs   arrFrequencies 4   times in arrFrequencies          bonus  restore Array from frequencies   const arrRestored   Object entries arrFrequencies       reduce   acc   key  value     gt  acc concat Array value  fill  key            console log arrRestored join         x000D   as-console-wrapper   top  0  max-height  100   important    x000D   x000D   x000D   The old  2011  answer  you could extend Array prototype  like this   x000D   x000D      Array prototype frequencies   function         var l   this length        result             all                  while  l--          result this l     result this l       result this l     1               all pairs  label  frequencies  to an array of arrays 2      for  var l in result          if  result hasOwnProperty l   amp  amp  l      all             result all push  l  result l                       return result          var freqs    5  5  5  2  2  2  2  2  9  4  frequencies      console log  freqs 2     freqs 2          gt  5         or   var freqs    1 1 2 one one 2 2 22 three four five three three five       split           frequencies           console log  freqs three    freqs three         gt  3       Alternatively you can utilize Array map       Array prototype frequencies   function           var freqs             sum  0                this map function a            if    a in this               this a    1            else             this a     1                    this sum    1          return a           freqs         return freqs          x000D   as-console-wrapper   top  0  max-height  100   important    x000D   x000D   x000D

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Here s just something light and easy for the eyes     function count a i    var result   0   for var o in a    if a o     i     result     return result      Edit  And since you want all the occurences     function count a    var result        for var i in a     if result a i      undefined  result a i     0    result a i          return result

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Based on answer of  adamse and  pmandell  which I upvote   in ES6 you can do it in one line    2017 edit  I use    to reduce code size and make it more readable     x000D   x000D  var a  7 1 7 2 2 7 3 3 3 7  7 7 7   x000D  alert JSON stringify  x000D   x000D  a reduce  r k   gt  r k  1 r k   1 return r      x000D   x000D      x000D   x000D   x000D      It can be used to count characters    x000D   x000D  var s  ABRACADABRA   x000D  alert JSON stringify  x000D   x000D  s split     reduce  a  c   gt  a c    0 a c  1 return a      x000D   x000D      x000D   x000D   x000D

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I was solving a similar problem on codewars and devised the following solution which worked for me   This gives the highest count of an integer in an array and also the integer itself  I think it can be applied to string array as well     To properly sort Strings  remove the function a  b  return a-b  from inside the sort   portion  function mostFrequentItemCount collection        collection sort function a  b  return a-b        var i 0      var ans         var int ans         while i lt collection length                if collection i    collection i 1                         int ans push collection i                      else                       int ans push collection i                ans push int ans               int ans                       i               var high count 0      var high ans       i 0      while i lt ans length                if ans i  length gt high count                        high count ans i  length              high ans ans i  0                     i              return high ans

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Using MAP you can have 2 arrays in the output  One containing the occurrences  amp  the other one is containing the number of occurrences     x000D   x000D  const dataset    2 2 4 2 6 4 7 8 5 6 7 10 10 10 15   x000D  let values       x000D  let keys       x000D   x000D  var mapWithOccurences   dataset reduce  a c    gt    x000D    if a has c   a set c a get c  1   x000D    else a set c 1   x000D    return a  x000D     new Map    x000D   forEach  value  key  map    gt    x000D    keys push key   x000D    values push value   x000D      x000D   x000D   x000D  console log keys  x000D  console log values  x000D   x000D   x000D

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x000D   x000D  const data    5  5  5  2  2  2  2  2  9  4  x000D   x000D  function count arr    x000D    return arr reduce  prev  curr    gt   prev curr      prev curr     1  prev       x000D    x000D   x000D  console log count data   x000D   x000D   x000D

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If using underscore or lodash  this is the simplest thing to do     countBy array     Such that     countBy  5  5  5  2  2  2  2  2  9  4     gt  Object  2  5  4  1  5  3  9  1    As pointed out by others  you can then execute the   keys   and   values   functions on the result to get just the unique numbers  and their occurrences  respectively  But in my experience  the original object is much easier to deal with

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var a    5  5  5  2  2  2  2  2  9  4  reduce function  acc  curr      if  typeof acc curr      undefined         acc curr    1      else       acc curr     1         return acc              a     2  5  4  1  5  3  9  1

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Don t use two arrays for the result  use an object   a         5  5  5  2  2  2  2  2  9  4   result        for var i   0  i  lt  a length    i        if  result a i            result a i     0        result a i        Then result will look like         2  5      4  1      5  3      9  1

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Solution using a map with O n  time complexity   var arr    2  2  2  2  2  4  5  5  5  9    const countOccurrences    arr    gt        const map           for   var i   0  i  lt  arr length  i               map arr i       map arr i     1            return map      Demo  http   jsfiddle net simevidas bnACW

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var array    5  5  5  2  2  2  2  2  9  4    function countDuplicates obj  num     obj num       obj num     1     return obj     var answer   array reduce countDuplicates          answer   gt   2 5  4 1  5 3  9 1        If you still want two arrays  then you could use answer like this      var uniqueNums   Object keys answer      uniqueNums   gt    2    4    5    9     var countOfNums   Object keys answer  map key   gt  answer key       countOfNums   gt   5  1  3  1        Or if you want uniqueNums to be numbers   var uniqueNums   Object keys answer  map key   gt   key      uniqueNums   gt   2  4  5  9

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How about an ECMAScript2015 option     const a    5  5  5  2  2  2  2  2  9  4    const aCount   new Map     new Set a   map      x   gt   x  a filter y   gt  y     x  length          aCount get 5      3 aCount get 2      5 aCount get 9      1 aCount get 4      1   This example passes the input array to the Set constructor creating a collection of unique values   The spread syntax then expands these values into a new array so we can call map and translate this into a two-dimensional array of  value  count  pairs - i e  the following structure   Array       5  3       2  5       9  1       4  1      The new array is then passed to the Map constructor resulting in an iterable object   Map       5   gt  3      2   gt  5      9   gt  1      4   gt  1     The great thing about a Map object is that it preserves data-types - that is to say aCount get 5  will return 3 but aCount get  5   will return undefined   It also allows for any value   type to act as a key meaning this solution will also work with an array of objects    x000D   x000D  function frequencies     Array     a   x000D      return new Map     new Set a   map  x000D          x   gt   x  a filter y   gt  y     x  length  x000D          x000D    x000D   x000D  let foo     value   foo     x000D      bar     value   bar     x000D      baz     value   baz     x000D   x000D  let aNumbers    5  5  5  2  2  2  2  2  9  4   x000D      aObjects    foo  bar  foo  foo  baz  bar   x000D   x000D  frequencies aNumbers  forEach  val  key    gt  console log key          val    x000D  frequencies aObjects  forEach  val  key    gt  console log key value          val    x000D   x000D   x000D

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Given the array supplied below  const array      a    b    b    c    c    c      You can use this simple one-liner to generate a hash map which links a key to the number of times it appears in the array  const hash   Object fromEntries      array reduce  map  key    gt  map set key   map get key     0    1   new Map             a  1  b  2  c  3    Expanded  amp  Explained     first  we use reduce to generate a map with values and the amount of times they appear const map   array reduce  map  key    gt  map set key   map get key     0    1   new Map        next  we spread this map into an array const table        map        finally  we use Object fromEntries to generate an object based on this entry table const result   Object fromEntries table    credit to  corashina for the array reduce code

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Try this   Array prototype getItemCount   function item        var counts           for var i   0  i lt  this length  i              var num   this i           counts num    counts num    counts num  1   1            return counts item     0

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There is a much better and easy way that we can do this using ramda js  Code sample here  const ary    5  5  5  2  2  2  2  2  9  4   R countBy r  gt  r  ary   countBy documentation is at documentation

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My solution with ramda   const testArray    5  5  5  2  2  2  2  2  9  4   const counfFrequency   R compose    R map R length     R groupBy R identity      counfFrequency testArray    Link to REPL

[javascript] Counting the occurrences / frequency of array elements

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