These methods are used on Arrays, Hashes or Objects. This tutorial will illustrate difference between count length and size.

1. Array#length

Basically length method used on arrays in ruby returns number of elements in the array for which method is invoked.

Example, suppose we have array a as,

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

Calling Array#length method on array will give result as,

a.length
=> 7

For more reference on how length is calculated, refer source code for the length on apidock for Array#length which shows that it uses RARRAY_LEN(array) method to get length.

2. Array#size

Array#size is just an alias to the Array#length method. It executes same Array#length method internally.
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b = [1,2,3,4]
b.size
=> 4

3. Array#count

Count has some more functionalities than length/size. It can be used for getting number of elements based on some condition. Count can be called in three ways:

Example:

Suppose we have array c as,

c = [1,2,3,4,4,7,7,7,9]

Method 1:

c.count
=> 9

Method 2:

c.count 7
=> 3

As there are 3 elements in the array having value = 3

Method 3:

c.count { |i| i>5 }
=> 4

As there are 4 elements in the array having value greater than 5.

These variants are supported by the count method because it actually iterated through each element in the array and thus allows conditional count of the elements in the array.

You can refer source code on apidock for Array#count method for more information.

Conclusion

The concept of ruby count vs length vs size was clarified in this tutorial. Few notes on when each of these methods should be used while coding:

1. Array#count should be used only for getting number of elements in array based on some condition.

2. Otherwise, for getting number of elements without any condition use Array#length or Array#size.