The yield Keyword
Part of the Logic & Flow section of Coddy's Ruby journey — lesson 17 of 56.
Methods you define yourself can take a block, just like the built-in ones. Inside the method, the keyword yield runs the block the caller passed in.
def greet
puts "Before block"
yield
puts "After block"
end
greet do
puts "Hi from the block!"
end
# Before block
# Hi from the block!
# After blockYou can pass arguments to the block by giving them to yield:
def with_each(items)
items.each { |item| yield item }
end
with_each([10, 20, 30]) { |n| puts n }
# 10
# 20
# 30If a method calls yield but the caller didn't pass a block, Ruby raises an error. block_given? lets you check first:
def maybe
yield if block_given?
end
maybe # nothing happens
maybe { puts "called!" } # called!
Challenge
EasyDefine a method named repeat that takes one argument n (an integer) and yields the values 1, 2, ..., n to its block, one at a time.
Then call your method with n = 4 and a block that prints "Got <value>" for each yielded value.
The output is:
Got 1
Got 2
Got 3
Got 4Cheat sheet
Use yield inside a method to run the block passed by the caller:
def greet
yield
end
greet { puts "Hi!" } # Hi!Pass arguments to the block via yield:
def with_each(items)
items.each { |item| yield item }
end
with_each([1, 2]) { |n| puts n }Use block_given? to check if a block was provided before calling yield:
def maybe
yield if block_given?
endTry it yourself
# TODO: define `repeat` that yields 1..n to its block
# TODO: call repeat(4) with a block that prints "Got <value>"
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Logic & Flow
1Strings In Depth
String Methods OverviewString InterpolationIterating Over StringsSplit and JoinRecap - String Weaver4Blocks, Procs & Lambdas
What is a Block?do..end vs BracesThe yield KeywordBlock ParametersProcs and LambdasRecap - Custom Iterator7Hashes Part 2
Hash.new with DefaultsIterating HashesNested HashesMerging and TransformingRecap - Frequency Counter10Project - Student Records
Project OverviewAdd Student
Add GradeAverage GradeList By CourseTop Students5Enumerable Powerhouse
Select and RejectChaining MapReduce / Injectcount, all?, any?, none?group_by and partitionsort_by, min_by, max_byRecap - Data Pipeline8Advanced Decision Making
Case with Classes & RegexMulti-value whenTernary OperatorInline if / unlessRecap - Grade Classifier