`for` Loop

The for loop iterates over a numeric range. It supports ascending (to) and descending (downto) sequences with an optional step value.

Ascending

for i in 0 to 5 do              # 0, 1, 2, 3, 4
    # body
end

for i in 0 to 10 by 2 do        # 0, 2, 4, 6, 8
    # body
end

Descending

for i in 5 downto 0 do          # 5, 4, 3, 2, 1
    # body
end

for i in 10 downto 0 by 2 do    # 10, 8, 6, 4, 2
    # body
end

The step must always be a positive number. For descending (downto) loops, the step is subtracted from the loop variable automatically. Writing by -2 or by 0 causes a runtime error.

Exclusive bounds

Bounds are always exclusive - the end value is never reached. For to, the bound behaves like <. For downto, it behaves like >.

To iterate over an inclusive range, adjust the bound by one:

for i in 1 to 10 + 1 do         # 1 through 10 inclusive
    # body
end

for i in 10 downto 1 - 1 do     # 10 through 1 inclusive
    # body
end

Non-iterable values

Only arrays, strings, and objects are iterable. Attempting to iterate over a number, boolean, or null is an error:

Compile-time - if the collection expression is a literal of a non-iterable type (e.g. foreach x in 42 do), the compiler reports error JM3402.
Runtime - if a variable evaluates to a non-iterable type at runtime, error JM5402 is raised.

Evaluation

The start, end, and step expressions are evaluated once at loop entry. Modifying variables used in these expressions during the loop body has no effect on the iteration range.

Loop variable scoping

The loop variable is scoped to the loop body. It is not accessible before or after the loop.

for i in 0 to 5 do
    # i is available here
end
# i is NOT available here

for Loop