Loops: for and while

for handles counted iteration with times:

# Perform this operation 10 times.
for (10 times) { ... }

...and iteration over ranges:

# A non-end-inclusive range.
for (x in 1 to 10) { ... }

# An end-inclusive range.
for (x in 1 thru 10) { ... }

...and iteration over collections:

let values = list(1, 2, 3, 4, 5)
for (var value : values) { ... }

A bare for { ... } loops forever until you break:

for {
  if (timeUsedSoFar() > 100.seconds): break
  ...
}

continue skips to the next iteration:

for (operation in tasks) {
  if (operation is NoOpOperation): continue
}

You can destructure the index and value together:

for (index, value in items.withIndex()):
  println("$index $value")

while and do...while

while repeats a block as long as a condition holds:

var value = 10
while (value != 0) {
  print(value--)
}

do...while guarantees the body runs at least once before checking the condition:

var value = 0
do {
  println(value++)
} while (value < 10)