For

For loops allow to iterate over different sets of data and perform actions based on them.

# read a file with numbers in it (file content will always be represented by strings)
# .lines() splits the lines of the file into an array
input = open("examples/aoc/2021/day-1/input").lines()

# define temporary array
a = []

for i, number in input
  # read each line into temporary array and cast it into an integer
  a.append!(number.trim().to_integer())
end

# assign temporary array to input array
input = a

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.

Scope of the loop variable

The loop variable is created in the loop's own scope, so it does not exist after the loop finishes:

for j in [1, 2, 3]
end

print(j)  # ERROR: identifier not found: j

If a variable of that name already exists outside the loop, though, the loop assigns to that variable rather than creating a new one, and it keeps the last value the loop gave it:

i = 100

for i in [1, 2, 3]
end

print(i)  # 3, not 100

This follows from how assignment works generally: assigning to a name that already exists further out updates it in place instead of shadowing it. Use a loop variable name that is not already taken if you need to keep the outer value.

Using an integer

Count form zero to a given number (excluding):

🚀 > for i in 5
  print(i)
end

0
1
2
3
4
=> nil

Using a string

Iterate over a string:

🚀 > for i in "test"
  print(i)
end

t
e
s
t
=> nil

Using break and next

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

for i in 5
  if (i == 2)
    next
  end
  print(i)
end

for i in 5
  if (i == 2)
    break
  end
  print(i)
end

# Returns
0
1
3
4
0
1
nil

Using range

You can use the so called rocket range operator to create an individual range with optional stepping:

for i in 0 -> 5
  print(i)
end

# outputs
0
1
2
3
4

There is also an inclusive alternative:

for i in 0 => 5
  print(i)
end

# outputs
0
1
2
3
4
5

Stepping

You can specify stepping to change the default of 1

for i in 0 -> 5 ^ 2
  print(i)
end

# outputs
0
2
4

Reverse

Ranges do support going from a higher value to a lower one

for i in 5 -> 0 ^ 2
  print(i)
end

# outputs
5
3
1