三至五年级Computation计划中
Repetition and decisions
Repeating steps with loops, making decisions with if–then rules, and algorithms for arithmetic such as column addition, in unplugged activities and simple block-based programs.
math.comp.loops-and-conditions
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前置知识
1 项必需的前置知识（一直追溯到底）
你将学会
- Use a loop to shorten a sequence of repeated instructions, such as drawing a square.
- Write if–then rules for sorting objects or playing a simple game.
- Explain the written algorithm for column addition as a loop over place values with a carry.
定义
- A loop repeats a block of instructions a fixed number of times or until a condition holds.
- A condition is a statement that is true or false, used to decide which instructions to follow.
定理
- 'Repeat 4 times: move forward, turn right 90°' draws a square.
- Column addition works through the digits from right to left, carrying 1 whenever a column sum is 10 or more.
常见误解
- Thinking a loop must run forever or exactly once.
- Writing a stopping condition that can never become true, so that the loop never stops.
通向
可选发现
- Computability Can we always tell whether a program will stop? No algorithm can decide this for every program: the halting problem is undecidable.
延伸阅读
- Tim Bell, Ian H. Witten and Mike Fellows, Computer Science Unplugged