🔁 循环：让电脑重复干活🔁 Loop: Let the Computer Repeat

上一片，小电脑有了盒子 total。可主人要数 100 包货，一行行写到天黑。这一片，让电脑自己转圈数。 Last leaf, the little computer got its box total. Now the owner must count 100 parcels — writing 100 lines lasts till dark. This leaf lets the computer run laps itself.

第 1 步：再来一圈Step 1: One more round

100 行代码，能压成几行？Can 100 lines of code shrink to a few?

跑步机圈数器：第 3 圈，再跑一圈 — 循环就是“重复跑”。跑步机上，第 3 圈和第 1 圈动作一样。写一次，让电脑跑 100 次。数 1 到 100，一个循环搞定。 A loop means “run again”. On a treadmill, lap 3 copies lap 1. Write once, let the computer run 100 times. Counting 1 to 100 takes one loop.

电脑怎么知道何时停？How does the computer know when to stop?

第 2 步：说清跑几次Step 2: Say how many runs

闹钟每天 7 点响：循环重复 — 三个词说清：起始、终止、步长。从 0 到 10，步长 5。只经过 0、5、10。闹钟也一样：每天 7 点，响 7 天就是 7 次。 Three words fix it: start, end, step. From 0 to 10, step 5. Only 0, 5, 10 pass by. Alarms match: 7 o'clock daily, 7 days means 7 rings.

停的条件丢了，会怎样？What if the stop rule is lost?

第 3 步：忘了说停Step 3: Forgot to say stop

死循环：忘了说停，停不下来了 — 会转成死循环！小电脑晕头转向，停不下来。写循环先写停的条件。这是新手最常见的坑，检查一遍再跑。 You get an endless loop! The little computer spins dizzy, never stopping. Always write the stop rule first. It is the top rookie trap — check before you run.

🎮 你来当教练（30 秒）🎮 Your turn: be the coach (30 seconds)

道理讲完了。先拖滑块跑几遍，再接 4 关：预测跑几次，揪出死循环。Theory done. First drag the sliders and run a few rounds, then take 4 challenges: predict the count and catch the endless loop.

一句话记住它：循环 = 让电脑重复干活；起始 / 终止 / 步长说清次数；丢了终止条件就是死循环。 Remember it in one line: a loop = make the computer repeat; start / end / step fix the count; losing the stop rule is an endless loop.

一次写完重复跑：数到 100 不用 100 行Write once, repeat: count to 100 without 100 lines 三件套：起始 / 终止 / 步长The trio: start / end / step 先写停的条件，别写死循环Write the stop rule first — no endless loops

内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。 Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.