Algorithms and flowcharts

§Text Passage

1Summary 重點摘要

An algorithm is a clear step-by-step way to solve a problem, like a recipe. A flowchart draws it: oval for start/end, rectangle for actions, diamond for decisions, parallelogram for input/output.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Three requirementsA sound algorithm must meet three requirements. First, clear: every step is exact and specific, never vague, so anyone following it gets the same result. Second, finite: the number of steps is limited, so it must finish and never loop forever. Third, ordered: the sequence is fixed, and swapping steps may break it — putting shoes on before socks does not work. A recipe helps here: one that never ends, or adds salt 'by mood', is not a sound algorithm. In answers, state all three requirements and attach a short example to each.
2. Everyday example: making lemon teaAlgorithms do not need computers — daily life is full of them. Take making lemon tea: after starting, boil the water; then decide 'is there a lemon?' If yes, put it in; if no, buy one first and carry on; finally add the tea, and it is done. This has step-by-step instructions plus one decision branch, so it is already an algorithm. An algorithm is simply 'a clear sequence of steps for solving a problem' — the design before coding, even the steps for teaching a classmate to fold a paper crane, all fit the definition.
3. Flowchart symbolsA flowchart expresses an algorithm with five standard symbols: an oval for start or end; a rectangle for process — an action or calculation; a diamond for decision, with yes and no paths; a parallelogram for input or output; and arrows for the direction of flow. Each symbol has its own job and must not be mixed up — a decision can never be drawn as a rectangle, nor input as one either. Memory trick: 'asking a question' takes a diamond, 'doing something' takes a rectangle, and 'data coming in or going out' takes a parallelogram.
4. Drawing rulesDrawing a flowchart follows several rules: exactly one start; every arrow must show its direction; and every diamond must have two exits — yes and no — never just one. For a pass/fail check, the diamond must split into a pass path and a fail path, and both must finally reach the End symbol before the chart is complete. Common errors: forgetting the start or end oval, arrows without direction, or one branch left hanging after a decision. After drawing, follow the arrows from start to end once, and these problems will show up.
5. Why draw firstSketching the flowchart before coding makes the logic clear and stops you getting lost halfway; a program is simply the flowchart translated into code. For example, with the grading chart finished first, no mark band is missed when coding, and debugging is faster too, because every branch can be checked against the chart. A Hong Kong example: the MTR journey planner proposes routes within seconds — a designed algorithm at work behind the scenes. Design first, code later: it is every programmer's habit and the core of computational thinking.
6. Drawing a loop on a flowchartRepeated work appears in a flowchart as a 'return arrow': the flow line bends from below back upward, forming a loop. To stack 10 blocks: a rectangle 'stack one block', then a diamond 'ten blocks yet?' — when the answer is no, the arrow bends back up to 'stack one block'; when yes, the arrow goes on to the end. A diamond exit bending back upward is the mark of a loop. When drawing, write the condition beside the diamond clearly, and make sure the loop finally has an exit — otherwise the flow circles forever.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• Algorithm = finite, clear, ordered steps
• Oval start/end, rectangle process, diamond decision, parallelogram input/output
• Diamonds have two exits; one start per chart
• Flowchart first, code second — logic stays clear
• Recipes and game guides are algorithms too — clear thinking needs no computer