Contact Us

Things you can do with a Ruby array in one line

Technologies | August 19, 2010

Just wanna share with you guys some of the common ruby array functions that I use every day.

1. Summing elements: This is a fairly common task. We’ll use Ruby’s inject method to sum all the items in the array and then print out the sum:

my_array.inject(0){|sum,item| sum + item}

2. Double every item: This is a class of problems where we want to operate on every element of the array. Again, this is fairly simple using Ruby’s map method. Think of performing a “mapping” from the first array to the second based on the function in the block. Keep in mind, that this will return a new array and will NOT affect the original array. If we want to do a destructive map (change the initial array) we would use a map! This is a common convention in Ruby:{|item| item*2 }

3. Finding all items that meet your criteria: If you want to collect all the values in the array that meet some criteria, we can do this using the (duh) find_all method. Again, this will return an array. The code below finds all items that are multiple’s of three :

my_array.find_all{|item| item % 3 == 0 }

4. Combine techniques: Let’s now say we want to find the sum of all elements in our array that are multiples of 3. Ruby to the rescue! This is very simple because we can chain methods together gracefully in Ruby. Check it out:

my_array.find_all{|item| item % 3 == 0 }.inject(0){|sum,item| sum + item }

5. Sorting: We can sort items in an array quite easily. Below, I will show the standard sort and then a sort based on the negative value of the number. Both are so simple, that my head just exploded:

my_array.sort_by{|item| item*-1}

6. Randomly pick n elements from an array

my_array.sort_by{ rand }.slice(0…number)

7. Sort by 2 criteria (and more):

my_array.sort{|x,y| [y.field_1, y.field_2] <=> [x.field_1, x.field_2] }

8. group_by

if you are using ruby 1.8.7+ or with active_support (or facets, I think), you can use group_by

products.group_by {|prod| prod.category}

9. Bonus!: As a bonus for all of you lovely readers, I will show you how to filter an array of strings based on a pattern. Let’s say you have a huge array of strings and you want to collect any string that looks like this: 555-555-5555. Assume that the 5′s could be any digit (we are looking for phone numbers). First, we create a regular expression that expresses this phone number jazz: /d{3}-d{3}-d{4}/ (I’m not going into regular expressions here, but you can google them if you want to know more). Now, we simply use the find_all method discussed earlier combined with Ruby’s slick =~ operator. This will tell us if there is our string matches the regular expression. So, without further ado, here is how you would filter an array to find strings that contain phone numbers:

my_array.find_all{|item| item =~ /d{3}-d{3}-d{4}/ }

Don’t be shy, check out everything you can do with Ruby’s Array

Thanks for reading!