Control flow

Every control structure is an expression with a value. Only nil and false are falsy; 0, "" and [] are truthy.

if, elsif, else

if condition [then]
  ...
elsif condition [then]
  ...
else
  ...
end

The value is the last value of the branch that ran, or nil when none did. then is needed only to put a branch on the condition’s line.

gap = 5
fit = if gap < 3
  "tight"
elsif gap < 6
  "snug"
else
  "loose"
end
puts fit # snug
puts(if gap > 1 then "ok" else "no" end) # ok

unless

unless condition runs its body when the condition is falsy, and otherwise goes on to its elsif and else branches as an if does.

holes = 0
label = unless holes > 0
  "solid"
else
  "drilled"
end
puts label # solid

Statement modifiers

if, unless, while and until can follow a statement. The condition is evaluated first: begin ... end while condition checks before the first run too, unlike Ruby’s.

debug = true
puts "building" if debug
k = 0
k += 1 while k < 5
puts k # 5

An assignment under a modifier that doesn’t run leaves the variable unassigned: reading it later is a NameError.

Conditional operator

condition ? a : b evaluates only the branch it picks. See Operators.

case

case value
when a, b [then]
  ...
when c [then]
  ...
else
  ...
end

Each when lists one or more patterns, tried in order; the first branch with a match runs. The value is that branch’s last value, the else branch’s, or nil.

Pattern	Matches when
A class (Number, String, Solid, a class of your own)	value.is_a?(Class)
A Range	The range includes value
Anything else	pattern == value, by content for Arrays, Maps and Ranges

Without a value, case runs the first when whose condition is truthy. There is no pattern matching with in.

size = 5
band = case size
       when 1..3 then "small"
       when 4, 5 then "medium"
       else "large"
       end
puts band # medium

kind = case "M3"
       when Number then "a size"
       when String then "a name"
       end
puts kind # a name

pick = case
       when size > 10 then "big"
       when size > 2 then "mid"
       end
puts pick # mid

while and until

while condition runs its body while the condition is truthy; until condition while it is falsy. The header ends at the line break: do after it is the compile error misplaced-do. The value is nil, or the value given to break.

n = 0
while n < 3
  n += 1
end
m = 10
until m <= 7
  m -= 1
end
puts [n, m].inspect # [3, 7]

for

for name in iterable
  ...
end

for a, b, *rest in iterable
  ...
end

for walks an Array, a Range, a Map or a JsonObject; anything else is a TypeError.

• An Array is walked as it is at each step: elements pushed during the loop are visited too.
• A Range is walked exactly as .each walks it, steps and exclusive ends included.
• A Map (or a JsonObject) gives [key, value] pairs in insertion order: the entries there when the loop began, each with its value when its turn comes.

One variable takes each element whole; several spread it as a multiple assignment does, and one of them may be *rest. The loop opens no scope: its variables, and what the body assigns, remain after it. The value is the iterable, or the value given to break. As with while, do after the header is an error.

for i in 0...3
  cube(2).translate(i * 5, 0, 0)
end
puts i # 2

for x, y in [[0, 0], [10, 5]]
  puts x + y
end

for name, d in { m3: 3, m4: 4 }
  puts "#{name}: #{d}"
end

KupCAD has no loop do ... end; write while true.

break, next and return

Statement	In a loop	In a block or lambda	In a method
break value	Ends the loop; its value is value	Ends the iterating call; the call’s value is value	Error outside a loop or block
next value	Goes on with the next element	Ends this run of the block with value	Error outside a loop or block
return value	Returns from the enclosing method	Ends this run of the block or lambda with value	Returns from the method

break and next outside a loop or block are the compile error outside-loop. A return inside a block leaves only the block, never the method around it; see Blocks and lambdas. A return at the top level ends the script.

found = for v in [1, 2, 3]
  break v * 10 if v == 2
end
puts found # 20

def first_big(xs)
  for x in xs
    return x if x > 2
  end
  nil
end
puts first_big([1, 5, 9]) # 5