Pebellslearn by building
0%
RoadmapsMatrixCoursesSandboxSign in
Control Flow  ›  Capstone Project

while, repeat-while, break and continue

while 12 minwhile checks its condition before each pass, repeat-while runs at least once and checks after, continue skips to the next pass and break leaves the loop.

while, repeat-while, break and continue

Sprout asks new users for a daily goal and must keep asking until they type a real number. It also counts a streak by walking back through the days until it meets one that was missed. Neither knows in advance how many passes it needs. That is when for gives way to while.

The rule

while checks first, repeat checks after; continue skips, break leaves.

A while loop tests its condition before every pass. If the condition is false from the start, the body never runs:

var minutes = 0
while minutes < 30 {
    minutes += 10
}
// minutes is 30 after three passes

A repeat-while loop runs the body first and tests afterwards, so it always runs at least once. That fits "ask, then check the answer":

var goal = 0
repeat {
    print("How many minutes a day?")
    goal = Int(readLine() ?? "") ?? 0
} while goal <= 0

Inside any loop:

  • continue ends this pass early and jumps to the next one.
  • break ends the whole loop immediately.
let week = ["done", "rest", "done", "missed", "done"]
var streak = 0
for day in week {
    if day == "rest" { continue }     // rest days do not count or break it
    if day == "missed" { break }      // the streak ends here
    streak += 1
}
// streak is 2

Make sure something in a while body moves it toward stopping. A condition that never becomes false is an infinite loop, and the app freezes.

The shortcut

Pick the loop by one question: must the body run at least once? Yes → repeat-while. No, and the count is unknown → while. Count known → for-in. Then trace passes on paper, marking each continue as "skip to next" and each break as "exit now".

Tracing Sprout's streak loopStep 1 of 4

  1. Pass 1: "done"

    Not rest, not missed. streak becomes 1.

Drills

Answer fast — trust the rule.

quick drill

How many times does the body of while count > 10 { count -= 1 } run when count starts at 5?

quick drill

Which loop always runs its body at least once?

quick drill

Inside a loop, what does continue do?

quick drill

With the week ["done", "done", "missed", "rest", "done"], what streak does the lesson's loop give?

Remember

  • while checks first, repeat checks after; continue skips, break leaves.
  • Must the body run once? repeat. Unknown count? while. Known? for.
  • Every while needs something that moves it toward false.

Practice — level up

0/1 solved

Write your answer, then compare it with a rubric and a model answer.

ExplainChoose the loop

For each Sprout task choose for-in, while or repeat-while and justify it: drawing seven day circles, asking for a goal until the user types a valid number, and adding 5 minutes to a timer until it reaches the goal (which may already be reached).