OCR GCSE Computer Science 2.3 Producing robust programs and 2.4 Boolean logic: defensive design, testing, errors, logic operators and gates

What topics 2.3 and 2.4 actually demand

These two short topics finish Component 02. Topic 2.3 is about making programs robust (coping with misuse, well tested, with known error types), and 2.4 is Boolean logic (the operators, truth tables and gates). Both are examined in Paper J277/02, and both reward precise definitions and accurate working (a correct test plan, a correct truth table).

This guide ties together the five dot-point pages across the two topics.

Defensive design

Defensive design writes programs that anticipate misuse. Input validation checks data is acceptable before use (range, type, length, presence and format checks), usually re-prompting in a loop. Authentication confirms who a user is (username and password). Maintainability makes code easy to change later through comments, consistent indentation, sensible names and subprograms.

Testing

Iterative testing happens repeatedly during development; terminal (final) testing happens at the end on the finished program. The three types of test data are normal (typical, accepted), boundary (at the edges of the valid range, where off-by-one errors hide) and erroneous (invalid, rejected). A test plan tabulates the input, its type and the expected result.

Error types

A syntax error breaks the language rules so the program will not run, and the translator reports it (missing bracket, misspelled keyword, missing endif). A logic error has correct syntax so the program runs, but gives the wrong result (wrong operator or condition). Logic errors are harder to find because there is no error message, so you trace or debug.

Boolean logic

The three operators are AND (true only when both inputs are true), OR (true when at least one is true) and NOT (reverses a single input). A truth table lists every input combination and the output: two inputs give four rows, three give eight. For combined expressions, use intermediate columns.

Logic gates and circuits

The three gates are AND (D-shaped), OR (curved back, pointed) and NOT (triangle with a circle, an inverter). In a circuit, the output of one gate can feed another. To analyse a circuit, write its Boolean expression by following the signal, then build the truth table with a column per gate. Gates are how the processor physically carries out logic.

Check your knowledge

A mix of recall and applied questions covering topics 2.3 and 2.4. Attempt them, then check against the solutions.

1. State what input validation is and name two validation checks. (2 marks)
2. State two techniques that make a program easier to maintain. (2 marks)
3. State the difference between iterative and terminal testing. (2 marks)
4. Give one example each of normal, boundary and erroneous data for a 1 to 10 range. (3 marks)
5. State the difference between a syntax error and a logic error. (2 marks)
6. State when the expression A AND B is true. (1 mark)
7. How many rows does a truth table with three inputs have? (1 mark)
8. Name the logic gate that has a single input and reverses it. (1 mark)