How do I create an average from a Ruby array

Question

How would get find an average from an array   If I have the array    0 4 8 2 5 0 2 6    Averaging would give me 3 375

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You could try something like the following   a    1 2 3 4 5      gt   1  2  3  4  5   a sum a length  to f     gt  3 0

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1 2  tap    a   asize   a size   inject     to f  asize   Short but using instance variable

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For public amusement  yet another solution   a   0  4  8  2  5  0  2  6 a reduce   0 0  0   do   s  c   e    s   e  c   1   end reduce       gt  3 375

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Let me bring something into competition which solves the division by zero problem   a    1 2 3 4 5 6 7 8  a reduce     try  to f  try    a size      gt  4 5  a      a reduce     try  to f  try    a size      gt  nil   I must admit  however  that  try  is a Rails helper  But you can easily solve this   class Object def try  options  self amp  amp send  options  end end class Array def avg reduce     try  to f  try    size  end end   BTW  I think it is correct that the average of an empty list is nil  The average of nothing is nothing  not 0  So that is expected behavior  However  if you change to   class Array def avg reduce 0 0     try    size  end end   the result for empty Arrays won t be an exception as I had expected but instead it returns NaN    I ve never seen that before in Ruby   -  Seems to be a special behavior of the Float class     0 0 0     gt  NaN 0 1 0     gt  Infinity 0 0 class     gt  Float

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a    0 4 8 2 5 0 2 6  a empty    nil   a reduce     a size to f   gt  3 375   Solves divide by zero  integer division and is easy to read  Can be easily modified if you choose to have an empty array return 0   I like this variant too  but it s a little more wordy   a    0 4 8 2 5 0 2 6  a empty    nil    a reduce      a size to f  reduce       gt  3 375

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Try this   arr    5  6  7  8  arr inject   sum  el  sum   el   to f   arr size   gt  6 5   Note the  to f  which you ll want for avoiding any problems from integer division  You can also do   arr    5  6  7  8  arr inject 0 0     sum  el  sum   el     arr size   gt  6 5   You can define it as part of Array as another commenter has suggested  but you need to avoid integer division or your results will be wrong  Also  this isn t generally applicable to every possible element type  obviously  an average only makes sense for things that can be averaged   But if you want to go that route  use this   class Array   def sum     inject 0 0     result  el  result   el     end    def mean      sum   size   end end   If you haven t seen inject before  it s not as magical as it might appear  It iterates over each element and then applies an accumulator value to it  The accumulator is then handed to the next element  In this case  our accumulator is simply an integer that reflects the sum of all the previous elements   Edit  Commenter Dave Ray proposed a nice improvement   Edit  Commenter Glenn Jackman s proposal  using arr inject     to f  is nice too but perhaps a bit too clever if you don t know what s going on  The    is a symbol  when passed to inject  it applies the method named by the symbol  in this case  the addition operation  to each element against the accumulator value

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Don t have ruby on this pc  but something to this extent should work   values    0 4 8 2 5 0 2 6  total   0 0 values each do  val   total    val end  average   total values size

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Ruby versions    2 4 has an Enumerable sum method    And to get floating point average  you can use Integer fdiv  arr    0 4 8 2 5 0 2 6   arr sum fdiv arr size      gt  3 375   For older versions   arr reduce     fdiv arr size      gt  3 375

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Some benchmarking of top solutions  in order of most efficient    Large Array   array    1  10 000 000  to a  Benchmark bm do  bm    bm report   array instance eval   reduce       size to f       bm report   array sum fdiv array size      bm report   array sum   array size to f     bm report   array reduce     to f   array size     bm report   array reduce     try  to f  try     array size      bm report   array inject 0 0     sum  el  sum   el   to f   array size     bm report   array reduce   0 0  0        s  c   e    s   e  c   1     reduce       end       user     system      total        real 0 480000   0 000000   0 480000    0 473920  0 500000   0 000000   0 500000    0 502158  0 500000   0 000000   0 500000    0 508075  0 510000   0 000000   0 510000    0 512600  0 520000   0 000000   0 520000    0 516096  0 760000   0 000000   0 760000    0 767743  1 530000   0 000000   1 530000    1 534404    Small Arrays   array   Array new 10    rand 0 5  2 0     Benchmark bm do  bm    bm report   1 000 000 times   array reduce     to f   array size       bm report   1 000 000 times   array sum   array size to f       bm report   1 000 000 times   array sum fdiv array size        bm report   1 000 000 times   array inject 0 0     sum  el  sum   el   to f   array size       bm report   1 000 000 times   array instance eval   reduce       size to f         bm report   1 000 000 times   array reduce     try  to f  try     array size        bm report   1 000 000 times   array reduce   0 0  0        s  c   e    s   e  c   1     reduce         end       user     system      total        real 0 760000   0 000000   0 760000    0 760353  0 870000   0 000000   0 870000    0 876087  0 900000   0 000000   0 900000    0 901102  0 920000   0 000000   0 920000    0 920888  0 950000   0 000000   0 950000    0 952842  1 690000   0 000000   1 690000    1 694117  1 840000   0 010000   1 850000    1 845623

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print array sum   array count is how i ve done it

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Another simple solution too arr    0 4 8 2 5 0 2 6  arr sum 0 0    arr size

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class Array   def sum      inject  nil      sum x  sum   sum x   x     end    def mean      sum to f   size to f   end end   0 4 8 2 5 0 2 6  mean

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I was hoping for Math average values   but no such luck   values    0 4 8 2 5 0 2 6  average   values sum   values size to f

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Without having to repeat the array  e g  perfect for one-liners    1  2  3  4  then    a  a sum to f   a size

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This method can be helpful   def avg arr    val   0 0    arr each do  n      val    n   end    len   arr length    val   len  end  p avg  0 4 8 2 5 0 2 6

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You can choose one of the below solutions as you wish  Bruteforce  0 4 8 2 5 0 2 6  sum to f    0 4 8 2 5 0 2 6  size to f    gt  3 375  Method def avg array    array sum to f   array size to f end    avg  0 4 8 2 5 0 2 6     gt  3 375  Monkey Patching class Array   def avg     sum to f   size to f   end end   0 4 8 2 5 0 2 6  avg   gt  3 375  But I don t recommend to monkey patch the Array class  this practice is dangerous and can potentially lead to undesirable effects on your system  For our good  ruby language provides a nice feature to overcome this problem  the Refinements  which is a safe way for monkey patching on ruby  To simplify  with the Refinements you can monkey patch the Array class and the changes will only be available inside the scope of the class that is using the refinement     You can use the refinement inside the class you are working on and you are ready to go  Refinements module ArrayRefinements   refine Array do     def avg       sum to f   size to f     end   end end  class MyClass   using ArrayRefinements    def test array      array avg   end end  MyClass new test  0 4 8 2 5 0 2 6     gt  3 375

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a    0 4 8 2 5 0 2 6  a instance eval   reduce       size to f      gt  3 375   A version of this that does not use instance eval would be   a    0 4 8 2 5 0 2 6  a reduce       a size to f    gt  3 375

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Add Array average  I was doing the same thing quite often so I thought it was prudent to just extend the Array class with a simple average method  It doesn t work for anything besides an Array of numbers like Integers or Floats or Decimals but it s handy when you use it right  I m using Ruby on Rails so I ve placed this in config initializers array rb but you can place it anywhere that s included on boot  etc  config initializers array rb class Array      Will only work for an Array of numbers like Integers  Floats or Decimals          Throws various errors when trying to call it on an Array of other types  like Strings      Returns nil for an empty Array        def average     return nil if self empty       self sum to d   self size   end  end

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I believe the simplest answer is  list reduce     to f   list size

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what I don t like about the accepted solution  arr    5  6  7  8  arr inject   sum  el  sum   el   to f   arr size   gt  6 5   is that it does not really work in a purely functional way  we need a variable arr to compute arr size at the end   to solve this purely functionally we need to keep track of two values  the sum of all elements  and the number of elements    5  6  7  8  inject  0 0 0   do  r ele        r 0  ele  r 1  1   end inject       gt  6 5      Santhosh improved on this solution  instead of the argument r being an array  we could use destructuring to immediatly pick it apart into two variables   5  6  7  8  inject  0 0 0   do   sum  size   ele        sum   ele  size   1   end inject       if you want to see how it works  add some puts    5  6  7  8  inject  0 0 0   do   sum  size   ele      r2     sum   ele  size   1      puts  adding   ele  gives   r2      r2 end inject      adding 5 gives  5 0  1  adding 6 gives  11 0  2  adding 7 gives  18 0  3  adding 8 gives  26 0  4    gt  6 5   We could also use a struct instead of an array to contain the sum and the count  but then we have to declare the struct first   R Struct new  sum   count   5  6  7  8  inject  R new 0 0  0    do  r ele      r sum    ele     r count    1     r end inject

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a    0 4 8 2 5 0 2 6  sum   0 a each    b  sum    b   average   sum   a length

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arr    0 4 8 2 5 0 2 6  average   arr inject  amp     to f   arr size      gt  3 375

[ruby-on-rails] How do I create an average from a Ruby array?

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