Flashcards · 9618 · A Level Topic 13

Data Representation (A Level) flashcards

14 cards on A Level Topic 13, subtopics 13.1 to 13.3. Pick a subtopic or study them all.

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What is a user-defined data type and why is it used?: A data type created by the programmer from the built-in types, to model the data of a particular problem more closely.
What is an enumerated data type? Give a 9618 pseudocode example.: A non-composite type whose values are a fixed ordered list of names. TYPE Season = (Spring, Summer, Autumn, Winter)
What is a pointer data type? Give a 9618 pseudocode example.: A type whose values are memory addresses. TYPE TIntPointer = ^INTEGER MyPtr^ refers to the value at the address MyPtr holds (dereferencing).
What is the difference between composite and non-composite data types?: Non-composite: defined without referring to another type (e.g. enumerated, pointer). Composite: built from other data types (e.g. record, set, class).
What is a set data type?: A composite type holding an unordered collection of values with no duplicates, supporting set operations such as union, intersection and membership.

What are serial, sequential and random file organisation?: Serial: records stored in the order they were added. Sequential: records stored in order of a key field. Random: each record is stored at an address calculated from its key by a hashing algorithm.
What is the difference between sequential and direct access?: Sequential access reads records one after another from the start. Direct (random) access goes straight to a record’s location, using an index or a hashing algorithm.
What is a hashing collision and how can it be handled?: Two keys give the same address. The record can be stored in the next free position (searching forwards) or in a separate overflow area.
How do you write a record to a random file in 9618 pseudocode?: OPENFILE "Seats.dat" FOR RANDOM SEEK "Seats.dat", Position PUTRECORD "Seats.dat", NewRecord CLOSEFILE "Seats.dat" (GETRECORD reads one.)
Which file organisation suits a payroll run and which suits a booking system?: Payroll processes every record in order, so sequential suits it. A booking system needs one record found quickly, so random (direct access) suits it.

What are the mantissa and exponent in floating-point representation?: The number is mantissa × 2^exponent. The mantissa holds the significant digits (precision); the exponent sets where the binary point goes (range). Both are in two’s complement.
What is the trade-off between mantissa and exponent size?: More bits for the mantissa give greater precision. More bits for the exponent give a greater range of values.
What does it mean for a floating-point number to be normalised?: The mantissa starts 01 (positive) or 10 (negative), so the first two bits differ. This gives the maximum precision for the bits available and a unique representation of each value.
Why can some numbers not be stored exactly in floating point?: Values such as 0.1 have no exact binary fraction, so they are rounded, giving rounding errors. Results too large give overflow; too small to represent give underflow.