Programming Fundamentals

Every value stored in a program has a data type that determines how it is represented in memory and what operations are valid on it. AQA specifies the following:

Data type	Description	Example
Integer	Whole number (positive, negative, zero)	-5, 0, 42
Real (float)	Number with a fractional part	3.14, -0.5
Boolean	Two values only	True, False
Character	Single symbol from a character set	'A', '7', '?'
String	Sequence of characters	"Hello", "AQA"
Date/time	Calendar date or clock time	2024-03-15
Pointer/reference	Stores a memory address	(address of variable)
Record	Collection of fields (mixed types)	Student(name, age, grade)

Variables store values that can change during execution. Constants store fixed values — defined once, not reassigned. Using constants for fixed values (e.g. MAX_SIZE = 100) makes code easier to read and safer to maintain.

Arithmetic operators produce a numeric result:

Operator	Operation	Example	Result
+	Addition	7 + 3	10
-	Subtraction	7 - 3	4
*	Multiplication	7 * 3	21
/	Real division	7 / 2	3.5
DIV	Integer division	7 DIV 2	3
MOD	Remainder	7 MOD 2	1
^	Exponentiation	2 ^ 8	256

Comparison operators produce a Boolean result: <, <=, >, >=, =, ≠.

Operator precedence (highest first): exponentiation → *, /, DIV, MOD → +, - → comparisons. Use brackets to override: (2 + 3) * 4 = 20, not 2 + 3 * 4 = 14.

Boolean operators combine or negate Boolean values. They are essential for conditions in selection and iteration.

Operator	Meaning	Example	Result
NOT	Negation	NOT True	False
AND	Both true	True AND False	False
OR	At least one true	True OR False	True
XOR	Exactly one true	True XOR True	False

(Extra context — not required by AQA 7517) Short-circuit evaluation: in A AND B, if A is False, B is not evaluated. In A OR B, if A is True, B is not evaluated. This can affect program behaviour when B has side effects.

Worked example — express "x is between 1 and 10 inclusive":

IF x >= 1 AND x <= 10 THEN
    OUTPUT "In range"
ENDIF

Strings support a standard set of operations. These appear frequently in AQA exam questions:

Operation	Pseudocode	Effect
Length	LEN(s)	Number of characters in s
Position	POSITION(s, sub)	Position of first occurrence of sub in s (1-indexed)
Substring	SUBSTRING(s, start, length)	Extract part of string
Concatenation	s1 + s2	Join two strings
Upper/lowercase	UPPER(s), LOWER(s)	Convert case
Character code	ASC(c)	ASCII code of character c
Code to character	CHR(n)	Character from code n
String to integer	INT(s)	Convert string "42" to integer 42
Integer to string	STR(n)	Convert integer 42 to string "42"

Worked example — extract the first three characters of a string:

name ← "Alexander"
initials ← SUBSTRING(name, 1, 3)  // "Ale" (1-indexed, 3 chars)
OUTPUT initials

Indexing convention: AQA pseudocode uses 1-based indexing — the first character is at position 1, not 0.

Random number generation is useful for simulations, games, and sampling. In AQA pseudocode:

n ← RANDOM_INT(a, b)    // Random integer in range a to b inclusive

Worked example — simulate a six-sided die:

roll ← RANDOM_INT(1, 6)
OUTPUT "You rolled: " + STR(roll)

(Extra context — not required by AQA 7517) In most languages, the random number generator produces pseudo-random numbers — sequences that appear random but are generated by a deterministic algorithm seeded with an initial value. True randomness requires hardware (e.g. thermal noise sampling).

An exception is an error condition that occurs at runtime — for example, dividing by zero, reading beyond the end of a file, or receiving unexpected input. Without handling, exceptions crash the program.

Try-except (try-catch) structure:

TRY
    x ← INT(userinput)
    result ← 100 / x
    OUTPUT result
EXCEPT
    OUTPUT "Invalid input or division by zero"
ENDTRY

The TRY block contains code that might fail. If an exception occurs, control jumps to the EXCEPT block. Execution continues after ENDTRY.

Exception handling should catch specific, anticipated errors — not suppress all errors indiscriminately. A broad catch-all can hide bugs.

Common runtime exceptions: division by zero, invalid type conversion, index out of bounds, file not found, null/None reference.

Type casting (type conversion) converts a value from one data type to another:

// String to integer
age_str ← "17"
age_int ← INT(age_str)       // 17 (as integer)

// Integer to string (for concatenation)
score ← 95
message ← "Your score: " + STR(score)   // "Your score: 95"

// String to real
price ← REAL("19.99")        // 19.99 (as real number)

Implicit vs explicit casting: some languages cast automatically (implicit), others require an explicit call. AQA pseudocode uses explicit conversion functions.

Pitfall: converting a non-numeric string to a number raises an exception — always validate input before casting, or use exception handling.

1. Confusing integer division and real division

7 / 2 gives 3.5; 7 DIV 2 gives 3. Forgetting DIV causes type errors when an integer is required.

2. Misusing the MOD operator

MOD gives the remainder, not a fraction. 7 MOD 3 = 1 (not 2.33). Common use: test if a number is even with n MOD 2 = 0.

3. Using = for string comparison vs assignment

In AQA pseudocode, ← is assignment and = is comparison. Writing name = "Ali" in a condition is correct; writing name = "Ali" as a statement is wrong — it should be name ← "Ali".

4. 1-based vs 0-based string indexing

AQA pseudocode uses 1-based indexing. SUBSTRING("Hello", 1, 3) returns "Hel", not "ell". Python uses 0-based indexing — be aware when translating between pseudocode and code.

5. Forgetting that RANDOM_INT includes both endpoints

RANDOM_INT(1, 6) can return 1 and 6. Students sometimes assume the upper bound is excluded (as in Python's range()).