Ruby Examples > Loops 

#---Loops---

Loops are how you run a piece of code repeatedly as long as a condition is true, or for a specific number of times.

Ruby has several ways to loop.

The “while” loop

Runs the supplied code block as long as the condition is true.

 
i = 0
while i <= 3 # do is optional
  i += 1
end
p i # => 4

The “until” loop

Runs the supplied code block as long as the condition is false.

 
i = 100
while i > 11
  i -= 1
end
p i # => 11

Both while and until support this inline form.

 
i = 100
i -= 1 until i <= 42
p i # => 42

The “for..in” loop

Use for..in to loop over an array or a hash.

 
for ele in ['a', 'b', 'c']
  print ele
end # => abc
 
for item in {a: 'aa', b: 'bb'}
  p item
end
## => [:a, "aa"]
## => [:b, "bb"]

The “each” loop

A better way to iterate over an array or a hash is the each method available on them. For every element it calls the block with that element.

 
[11, 22, 33].each { |e| print e }
## => 112233
{a: 'aa', b: 'bb'}.each { |k, v| p "k: #{k}, v: #{v}" }
## => "k: a, v: aa"
## => "k: b, v: bb"

each is available on many more iterable types like Dir, IO, Range, Set etc:

 
Dir.new(Dir.home).each { |e| p e}
## => lists all files and dirs in your home dir.
('a'..'j').each { |e| print e } # => abcdefghij

The Kernel “loop”

You can also create a loop using the Kernel module’s loop method which you can use without including the module.

 
i = 1
loop do
  print i
  break if i >= 9
  i += 1
end
## => 123456789

The “n.times” loop

If you want to do a thing an ‘n’ number of times, then there’s the Integer class’s times method.

 
10.times { p "Life's good!" }

Note: The above 3 - each, loop and times - are all ruby methods, whereas while, until and for are all ruby keywords.

Which one to use?

For enumerable types, prefer the each method.
For others, prefer the loop or the n.times methods.

Ruby folks rarely use for, while and until.

But the most common way of looping is actually the various methods defined in the Enumerable module which is available on all enumerables.
These are specialized loops like map, find_all, any?, zip, group_by, min, max, reduce, grep etc.
These are discussed here.

Official docs

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