Programming: free AQA GCSE Computer Science practice questions

The mistakes that cost the most marks

These are the errors that turn up in answer after answer.

• Confusing validation with verification. Validation checks that data is sensible. Verification checks that it was entered or copied correctly, for example by asking for a password twice.
• Assuming arrays start at 1. In the languages used at GCSE they normally start at 0, so the last index of an array of n items is n minus 1.
• Mixing up DIV and MOD. 17 DIV 5 is 3, which is how many whole times. 17 MOD 5 is 2, which is what is left over.
• Calling a subroutine that returns a value a procedure. If it returns something it is a function.
• Saying a logic error stops the program working. It usually runs perfectly and gives the wrong answer, which is what makes it dangerous.
• Giving only normal test data. Marks are usually for normal, boundary and erroneous data together.

Data types and the two kinds of named store

The core data types are integer for a whole number, real or float for a number with a decimal part, Boolean for True or False, character for one symbol, and string for a sequence of characters. Whether a user is logged in is a Boolean. A height in metres is a real, because 1.72 has a decimal part and an integer would lose it. A variable is a named store whose value can change while the program runs. A constant is a named value that cannot change once set, which makes a program easier to read and to update.

Every program is built from three constructs

Sequence is one step after another. Selection chooses between paths with IF, ELSE IF and ELSE. Iteration repeats. Iteration comes in two kinds. Count-controlled loops, such as FOR, repeat a fixed number of times. Condition-controlled loops, such as WHILE and REPEAT UNTIL, repeat while or until a condition holds. Arithmetic operators include the usual four plus integer division, written DIV, and remainder, written MOD. MOD is the standard way to test whether a number is even or divisible by something. Relational operators compare two values and give a Boolean. Boolean operators combine conditions with AND, OR and NOT.

Arrays, records and subroutines

An array is a set of items of the same type stored under one name and reached by an index. Arrays are usually indexed from 0, so an array of 5 items has indexes 0 to 4. A two-dimensional array is an array of arrays, indexed with two values, typically row then column. A record stores several related values that may be of different types under one name, for example a name, an age and a member flag. A subroutine is a named block of code you can call. A procedure carries out a task. A function returns a value. Parameters pass data in and the return value passes a result back. Subroutines make programs shorter, easier to test and easier to reuse. A local variable exists only inside the subroutine that declares it. A global variable is visible throughout the program. Local variables are preferred because they cannot be changed by accident elsewhere.

Strings, files, validation and authentication

String handling covers finding the length, extracting a substring or a single character by position, joining strings together, which is concatenation, and converting the case. Programs read from and write to files so that data survives after the program ends. The usual steps are open, read or write, then close. Validation checks that input is sensible before it is used: range checks, presence checks, length checks, type checks and format checks. Authentication checks who the user is, usually with a username and password. The two are different jobs and a question will usually want you to name the right one.

Errors and testing

A syntax error breaks the rules of the language and stops the program running at all. A logic error runs fine and produces the wrong result, which is why testing matters. Test data should cover normal data, which is typical valid values, boundary data, which sits right at the edge of what is allowed, and erroneous data, which should be rejected. Iterative testing happens while you build, module by module. Final testing checks the whole finished program against what it was supposed to do.