While

While loops will run as long as the condition is truthy.

Parentheses around conditions are optional.

Print numbers from 0 to 3:

🚀 > a = 0
🚀 > while a != 4
  print(a)
  a = a + 1
end

# which prints
0
1
2
3
=> nil

It is possible to use next or break inside a while loop.

i = 0
while i < 10
  if i < 3
    i = i + 1
    next
  end
  print(i)
  if i == 6
    break
  end
  i = i + 1
end

# which prints
3
4
5
6

Return Value

Loops evaluate to nil, whether they run to completion or exit early through break.

def iterate(items)
  for item in items
    print(item)
  end
end

a = [1, 2, 3, 4, 5]

b = iterate(a)

# b is nil

Build up a value explicitly if you need one out of a loop:

def doubled(items)
  result = []
  for item in items
    result.append!(item * 2)
  end
  return result
end

Until 0.24, foreach (as for was then called) returned the value it was iterating, so b above was [1, 2, 3, 4, 5] — the same array that went in. That did not hold once the loop hit a break, which produced nil instead, and while never returned anything but nil.