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Ruby's redo, retry and next keywords

Thijs Cadier on

We’ve talked about the retry keyword while discussing retrying after exceptions. Its little-known counterpart redo works similarly, but re-runs loop iterations instead of whole blocks.

The redo keyword

As we learned in the previous Academy article, retry lets you retry a piece of code in a block:

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begin
  puts "Iteration"
  raise
rescue
  retry
end

This example prints the word “Iteration” to the console before raising an exception. The rescue blocks executes, which calls retry and starts the block again from the beginning. This results in our program endlessly printing Iteration. The redo keyword lets you achieve a similar effect when using loops. This is useful in situations where you need to retry while iterating for example.

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10.times do |i|
  puts "Iteration #{i}"
  redo if i > 2
end

This will print:

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$ ruby redo.rb
Iteration 0
Iteration 1
Iteration 2
Iteration 3
Iteration 3
Iteration 3
...

Notice how the iteration count stays the same? It will step back execution to the start of the loop. This variant of the code using retry will print the exact same output:

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10.times do |i|
  begin
    puts "Iteration #{i}"
    raise if i > 2
  rescue
    retry
  end
end

You can use redo to implement retrying in a loop. In the next example we have a queue of jobs. They either return :success or :failure. We keep re-running the same iteration of the loop until the job succeeds.

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[job_1, job_2, job_3, job_4].each do |job|
  redo unless job.call == :success
end

Ruby 1.8

The behavior of retry and redo changed between Ruby 1.8 and 1.9. They used to restart the loop’s iteration, but both in a different way. From 1.9, retry only works with a begin/rescue block and redo only works within loops.

The next keyword

If you want to move to the next iteration of the loop, as opposed to moving back to the start of the current one, you can use next.

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10.times do |i|
  puts "Iteration #{i}"
  next if i > 2
  puts "Iteration done"
end

This will print:

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$ ruby next.rb
Iteration 0
Iteration done
Iteration 1
Iteration done
Iteration 2
Iteration done
Iteration 3
Iteration 4
...

See how the iteration counter keeps incrementing? In most cases using next is what you want. Look at redo if you need a loop that runs an exact number of times or needs error handling when iterating over an array.

We hope you learned something new about redoing iterations in loops and would love to know what you thought of this article (or any of the other ones in the AppSignal Academy series). Please don’t hesitate to let us know what you think, or if you have any Ruby subjects you’d like to learn more about.

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