Question

I recently ran into code that looks like this:

 next {
          'foo'         => bar,
          'foobar'      => anotherbar,
      }

At first it looks like a simple hash, but there is no assignment to next. Next in this case looks like a reserved Ruby keyword. What does this code do?

Answer 1

next is similar to the continue keyword in the c family of languages, except in ruby it makes an iterator move to the next iteration. Since blocks always have some sort of return value you can choose to pass one as an argument to next.

next is typically used in cases like iterating through a list of files and taking action (or not) depending on the filename.

next can take a value, which will be the value returned for the current iteration of the block.

  sizes = [0,1,2,3,4].map do |n|
    next("big") if n > 2
    puts "Small number detected!"
    "small"
  end

  p sizes

Output:

  Small number detected!
  Small number detected!
  Small number detected!
  ["small", "small", "small", "big", "big"]

from http://ruby-doc.org/docs/keywords/1.9/