Count, size, length…too many choices in Ruby?


There is a crucial difference for applications which make use of database connections.

When you are using many ORMs (ActiveRecord, DataMapper, etc.) the general understanding is that .size will generate a query that requests all of the items from the database ('select * from mytable') and then give you the number of items resulting, whereas .count will generate a single query ('select count(*) from mytable') which is considerably faster.

Because these ORMs are so prevalent I following the principle of least astonishment. In general if I have something in memory already, then I use .size, and if my code will generate a request to a database (or external service via an API) I use .count.


I can't seem to find a definitive answer on this and I want to make sure I understand this to the "n'th level" :-)

    a = { "a" => "Hello", "b" => "World" }
    a.count  # 2
    a.size   # 2
    a.length # 2

    a = [ 10, 20 ]
    a.count  # 2
    a.size   # 2
    a.length # 2

So which to use? If I want to know if a has more than one element then it doesn't seem to matter but I want to make sure I understand the real difference. This applies to arrays too. I get the same results.

Also, I realize that count/size/length have different meanings with ActiveRecord. I'm mostly interested in pure Ruby (1.92) right now but if anyone wants to chime in on the difference AR makes that would be appreciated as well.


I found a good answare at

In ActiveRecord, there are several ways to find out how many records are in an association, and there are some subtle differences in how they work.

post.comments.count - Determine the number of elements with an SQL COUNT query. You can also specify conditions to count only a subset of the associated elements (e.g. :conditions => {:author_name => "josh"}). If you set up a counter cache on the association, #count will return that cached value instead of executing a new query.

post.comments.length - This always loads the contents of the association into memory, then returns the number of elements loaded. Note that this won't force an update if the association had been previously loaded and then new comments were created through another way (e.g. Comment.create(...) instead of post.comments.create(...)).

post.comments.size - This works as a combination of the two previous options. If the collection has already been loaded, it will return its length just like calling #length. If it hasn't been loaded yet, it's like calling #count.

Also I have a personal experience:

<%= h(params.size.to_s) %> # works_like_that !
<%= h(params.count.to_s) %> # does_not_work_like_that !

Adding more to Mark Byers answer. In Ruby the method array.size is an alias to Array#length method. There is no technical difference in using any of these two methods. Possibly you won't see any difference in performance as well. However, the array.count also does the same job but with some extra functionalities Array#count

It can be used to get total no of elements based on some condition. Count can be called in three ways:

Array#count # Returns number of elements in Array

Array#count n # Returns number of elements having value n in Array

Array#count{|i| i.even?} Returns count based on condition invoked on each element array

array = [1,2,3,4,5,6,7,4,3,2,4,5,6,7,1,2,4]

array.size     # => 17
array.length   # => 17
array.count    # => 17

Here all three methods do the same job. However here is where the count gets interesting.

Let us say, I want to find how many array elements does the array contains with value 2

array.count 2    # => 3

The array has a total of three elements with value as 2.

Now, I want to find all the array elements greater than 4

array.count{|i| i > 4}   # =>6

The array has total 6 elements which are > than 4.

I hope it gives some info about count method.