Programming Control Structures

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1Summary 重點摘要

All program logic is built from three control structures: sequence executes step by step, selection branches on a condition, and iteration repeats; combined with variables, assignment and Boolean conditions, pseudocode expresses language-neutral algorithms.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: Variables and assignmentA variable can be pictured as a labelled storage cell whose contents the program reads or changes at any time; assignment puts data into it, written with the variable on the left and a value or expression on the right. Assignment overwrites the old value, and the right-hand expression may reference the variable itself: setting total to total plus mark updates the running total with a new mark. For example, beginners ask how 'total equals total plus mark' can be true — the right side is evaluated with the old value first and stored back on the left, which is exactly how accumulation works.
2. Step 2: Sequence and the importance of orderSequence executes instructions one by one in written order; the next instruction starts only after the previous one finishes. A wrong order ruins the result: enter the size and the marks before computing the average; set the counter to zero before accumulating. The classic case is swapping two variables: setting A to B directly destroys the original value of A; the correct way is to store A in a temporary variable, then set A to B, then set B to the temporary. For example, stepping through fifty and twenty shows the temporary preserving the fifty, completing the swap.
3. Step 3: Selection — single-way and two-waySelection chooses paths by condition. A single-way branch runs a block only when the condition holds and skips it otherwise — adding one to the pass count only when a mark is at least fifty. A two-way branch arranges both sides: one block when the condition holds, another when it fails — outputting pass or fail accordingly; exactly one of the two paths runs, never both, never neither. For example, judging pass or fail suits a two-way branch because both outcomes need handling, while crediting only passing marks suits a single-way branch — the requirement decides which form to use.
4. Step 4: Multi-way and nested branchesWith more than two cases, use nested branches judging layer by layer, or a multi-way branch sorting at once. For grading: at least eighty is grade A, at least sixty is B, the rest is C — nested, first test whether the mark reaches eighty; if yes, grade A; if not, test whether it reaches sixty; if yes, grade B; otherwise C. Nesting must keep the layers clear: the inner test runs only when the outer condition fails. For example, the order of tests cannot be shuffled — testing sixty first would grade an eighty as B; judging from strictest to loosest is the basic principle of multi-way branching.
5. Step 5: Conditions and Boolean logicConditions are built from comparison operators — greater than, less than, equal to, greater than or equal to, less than or equal to, not equal to — and evaluate to Boolean values, true or false only. Complex conditions combine them with logical operators: AND requires both sides to hold; OR requires either side; NOT flips true to false. For example, 'mark at least fifty AND mark below sixty' pins the range fifty to fifty-nine, while 'aged twelve or above OR at least 1.4 metres tall' needs only one side; before writing a condition, decide whether both parts or either part must hold.
6. Step 6: Count-controlled and condition-controlled loopsIteration comes in two forms. A count-controlled loop suits a known number of repeats: the counter runs from one to the class size, entering and accumulating one mark per pass — thirty students means exactly thirty passes. A condition-controlled loop, headed by WHILE, suits repeats decided by a condition: the condition is checked before every pass, and the loop continues only while it holds — asking for the password until it is correct. The two are interchangeable, but a known count reads clearer as a count-controlled loop. For example, averaging a class uses counting since the size is known, while input that stops only when the user clicks stop suits a condition-controlled loop.
7. Step 7: Causes of infinite loops and combining the structuresIf a condition-controlled loop never updates the variable its condition depends on, the condition stays true forever and the program never ends — an infinite loop; forgetting to add one to the counter while the condition compares it with the class size is the usual cause. The remedy is updating the variable inside every pass so the condition eventually fails. The three structures nest freely: a selection inside a loop judges passes, a loop inside a branch retries input. For example, a class statistics program is sequence for initialising, a loop for reading, a selection inside the loop counting passes, and sequence again for output — any complex logic is layered from these same pieces.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• A variable is a named data location; assignment stores the right-hand result into the left-hand variable.
• Three structures: sequence runs step by step, selection branches on conditions, iteration repeats — and they nest freely.
• Known repeats take a count-controlled loop, unknown ones a condition-controlled loop; the loop variable must update every pass.
• Conditions yield Boolean values, built from comparison operators combined with AND, OR and NOT.
• Multi-way tests run from strict to loose; accumulating variables must be initialised to zero before use.