Enumerables

Map and collect

map builds a new array by running a block once per element. collect is the same method under a different name for readers who prefer it.

Objects like arrays carry these methods, as you saw with custom classes in Inheritance.

names = ["ada", "lin"] loud = names.map { |n| n.upcase } puts loud.inspect

Select and reject

select keeps elements where the block returns true. reject does the opposite and keeps elements where the block returns false.

Both return new arrays, so the original stays untouched for later steps.

nums = [1, 2, 3, 4] even = nums.select { |n| n.even? } puts even.inspect

Reduce into one value

reduce folds the whole collection into a single value: a total, a string, or a hash. The block receives the running result and the next element.

Pass the starting value in parentheses. Without it, Ruby uses the first element as the start, which surprises readers for empty arrays.

nums = [1, 2, 3, 4] total = nums.reduce(0) { |sum, n| sum + n } puts total

Find and sort

find returns the first element that matches the block, or nil when nothing matches. Use it instead of select when you need only one item.

sort returns a new ordered array. Add reverse for descending order, or sort_by when the order key differs from the item itself.

nums = [3, 1, 2] puts nums.find { |n| n > 1 } puts nums.sort.inspect

Next: Duck typing and core idioms

Article author: Arthur Isaev