What is a block?

A block is code you hand to a method

Most method calls so far in this series passed plain values as arguments, like access_level("admin", true, false). The hashes course's merge took a block once, without explaining it. A block is a small, unnamed chunk of code passed to a method, which the method can run, even more than once.

3.times { puts "hello" }
# hello
# hello
# hello

{ puts "hello" } is the block, and times calls it three times in a row. puts "hello" didn't need to become a named method first; the block is written where it's used.

Two equivalent syntaxes: {} and do...end

3.times { puts "hi" }

3.times do
  puts "hi"
end

Curly braces and do/end both wrap a block. The convention, not a hard rule, is {} for a block that fits on one line and do/end for one that spans several, as in the two examples above.

Blocks can accept a value

[10, 20, 30].each { |n| puts n * 2 }
# 20
# 40
# 60

The |n| between pipes names a variable the block receives each time it runs. each calls the block once per element and hands that element in as n. It's the block version of a method's parameter list, written between pipes instead of parentheses.

Why this matters: one concept, everywhere

Blocks aren't specific to each; times above used one too. This course's each_with_index and the next courses' map and select are built on the same idea: a method receives a block and runs it, handing in whatever value makes sense for that method. Learning to read { |n| ... } here pays off across dozens of methods.

Key takeaways

• A block is code passed to a method, written with {} or do/end; both mean the same thing.
• Convention: {} for a one-line block, do/end for a multi-line one.
• |n| names a variable the block receives; the calling method decides what to hand in and how many times to run the block.
• A method calling the block it was handed is the pattern behind each, times, map, select and most of Ruby's iteration methods.