Parity bits, majority voting, checksums and check digits for AQA A Level Computer Science (7517): twenty different sheets, each worked all the way through, and each with a complete answer key. Specification 4.5.5.3 names four methods. There are four questions, one for each. The AS version of the same bullet names only three - it leaves out checksums - so question 3 is A level content and A level only. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys, 3 pages each A contents table giving each sheet’s parity, number of copies and check-digit scheme, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 is parity: add the parity bit to the ASCII codes of four characters, check three received bytes, and then the byte with two bits changed that the parity check cannot see. The four characters include a digit, and the last part asks what a program gets if it reads all eight bits as a pure binary number - the character 5, its ASCII code 53, and the byte 181 are three different numbers. Question 2 is majority voting: an odd number of copies of an 8-bit message arrive, some corrupted. Recover the message bit by bit, name the positions that were corrected, and say how many copies may be wrong at once before the vote itself goes wrong. Question 3 is a checksum: the total of a block of five bytes, the checksum that goes with it, whether a received block is accepted, and the pair of errors - one byte up, another down by the same amount - that the checksum cannot see. Question 4 is a check digit: work one out from the weights, validate a different number, and then a transposition. Sometimes the scheme catches it. Sometimes it does not, and the answer key says why. Question 5 is written: what a parity bit can and cannot do, when majority voting is worth its cost, checksums against check digits, why the digits are weighted, or character codes and the eighth bit. Every sheet is 30 marks.

The whole of section 4.12 for AQA A Level Computer Science (7517): twenty different sheets on function types, partial application, composition, map, filter and reduce, and list processing - each worked all the way through, and each with a complete answer key. Six questions, one for each part of the specification. It is the only pack in this series with six; 4.12 has three sub-sections and seven bullets, and none of them is left out. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table giving each sheet’s question 1 function, its partially-applied function and its reduce, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 defines a function over a small set: write its type as f: A -> B, apply it twice, list what it actually produces, and give the smallest member of the co-domain that is never an output. That last part is there because the specification is explicit that not all of the co-domain need be produced, and because “give a member” would have had a hundred right answers. Question 2 is partial function application - the bullet that says candidates must be able to USE the notation. The type with brackets, the type without, the type of what is left after one argument, its value after two, and a three-argument type whose argument types are not all the same. Question 3 is composition. Which function is applied first, then g o f and f o g evaluated at the same point, so the answer to “are they the same function?” is worked rather than recited. Question 4 is map, filter and reduce, each on its own and then all three nested in one expression - where the filter applies to the mapped list, not to the original one. Question 5 is lists: the head and the tail, the head:tail form, prepend and append, the length of a tail, and what the tail of a one-item list is. Question 6 is written: first-class objects, the co-domain against the outputs, what makes a function higher-order, partial application, or the empty list. Every sheet is 30 marks.

Regular expressions for AQA A Level Computer Science (7517): twenty different expressions, each worked all the way through, and each with a complete answer key. Section 4.4.2.3 asks candidates to form and use simple regular expressions, and to write a regular expression that recognises the same language as a given finite state machine AND THE OTHER WAY ROUND. Most resources only go one way. Every sheet here goes both. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table giving each sheet’s expression and its written question, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 prints an expression and six strings. For each string, does the expression describe it? One of the six is chosen so that the answer turns on the rule the whole topic argues about. Question 2 gives a finite state machine and asks for a regular expression that describes exactly what it accepts, the shortest string it accepts, and how many strings of length four it accepts. Question 3 goes the other way. It prints an expression and three machines, only one of which accepts exactly what the expression describes. For each of the other two, the student writes the shortest string on which the machine and the expression disagree - and there is exactly one such string in each case. Question 4 is set notation: membership, union, intersection, difference, cardinality, and the compact form the specification itself writes. Question 5 is a written question on the equivalence of expressions and machines, on what makes a language regular, on the difference between * and +, on an expression as a set, or on whole-string matching. Every sheet is 30 marks.

Encryption for AQA A Level Computer Science (7517): twenty different sheets on the Caesar and Vernam ciphers, each worked all the way through, and each with a complete answer key. Section 4.5.6.10 asks candidates to BE ABLE TO APPLY each cipher to encrypt a plaintext AND to decrypt a ciphertext. That is four operations, and most resources show one. Every sheet here does all four. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table giving each sheet’s Caesar shift and the words it encrypts, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 is the Caesar cipher: fill in the shifted alphabet, encrypt one word, decrypt another that used the same shift, and say how many shifts an attacker would have to try. Question 2 encrypts five characters with a Vernam cipher. Each character’s 8-bit ASCII code is given, and so is the one-time pad’s; the student poses the exclusive OR bit by bit and writes the ciphertext byte. Question 3 goes the other way: an intercepted ciphertext and the pad that made it, and the plaintext to recover a letter at a time. Question 4 breaks a Caesar cipher of unknown shift. The first word of the plaintext is given, which is enough to find the shift and read the rest. Question 5 is a written question on perfect security, on computational security, on why the Caesar cipher falls, on the words this topic uses, or on what an attacker gains from a one-time pad that was used twice. Every sheet is 30 marks. THE CONVENTION THIS TOPIC ARGUES ABOUT, SETTLED ON THE PAGE The Vernam cipher on these sheets works on BITS. Each character becomes its 8-bit ASCII code, and plaintext and key are combined with exclusive OR. A good number of books instead add letter positions modulo 26 - which is a different method and a different answer to the same question. Neither is wrong; they are simply not the same, and a student who learned one gets marked wrong by a teacher who teaches the other. Every sheet states which it uses, at the top, before the first question. The smaller conventions are printed as well. Encrypting with a Caesar cipher moves each letter FORWARD through the alphabet, wrapping from z round to a. Messages are lower-case with no spaces, so that the lengths of the words do not show through the ciphertext. Every byte is written on all eight bits, leading zeros included. And a shift of 0 is not counted among the shifts an attacker must try, so the answer to that question is 25 rather than 26 - a number the textbooks disagree about.

Storage calculations for AQA A Level Computer Science (7517): twenty different sheets on bitmapped images and sound recordings, each worked all the way through, and each with a complete answer key. Two bullets of the specification say CALCULATE - storage requirements for bitmapped images, and sound sample sizes in bytes. Questions 1 to 4 do nothing else. WHAT YOU GET 20 worksheets, 2 pages each, numbered 01 to 20 20 matching answer keys A contents table giving each sheet’s image size, colour depth and sampling rate, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 gives an image in pixels with its colour depth: how many colours each pixel can take, the size in bits, the size in bytes, and then the size in KiB and in kB - the same file, twice. Question 2 starts from a printed original in inches scanned at a given number of dots per inch: pixels across, pixels down, pixels in total, bytes, and MiB. Question 3 is a recording: sampling rate, sample resolution, length and channels to a size in bytes and in MiB, then the lowest sampling rate the Nyquist theorem allows for a given highest frequency. Question 4 saves the image from question 1 with metadata, and asks how many complete files fit on a memory card. Question 5 is a written question on kilobytes against kibibytes, on the Nyquist theorem, on sample resolution and quality, on vector against bitmapped graphics, or on colour depth. Every sheet is 30 marks. THE CONVENTION THIS TOPIC ARGUES ABOUT, SETTLED ON THE PAGE A kilobyte is not a kibibyte. 1 kB is 1000 bytes and 1 KiB is 1024, so the same file is 937.5 KiB and 960.0 kB. A student who gives one where the other was wanted is marked wrong having done nothing wrong. AQA says so itself, in the additional information to 4.5.3.2: the terms kilobyte and megabyte have often been used where kibibyte and mebibyte were meant. Every question on these sheets names the unit it wants, and question 1 asks for both on one file so that the gap is seen rather than described. The smaller conventions are printed too. Metadata counts only when a question gives it. Resolution is in dots per inch and a dot is a pixel. A sound is mono unless the question says stereo. Sizes round to one decimal place with a half going upwards. NO AMBIGUOUS ROUNDING No sheet asks you to round a value that lands exactly on a half at the digit being dropped. “12.25 to one decimal place” is answered 12.2 or 12.3 depending on which rule you were taught, and both can be defended. Every sheet in this pack is built so that the question has one answer. FORMAT PDF, A4, print-ready. Twenty different images, scanner settings and recordings, so you already have a fresh sheet for every class, every resit and every homework. Licensed for the purchasing teacher and their own classes.

IP addressing and subnet masking for AQA A Level Computer Science (7517): twenty different addresses and masks, each worked all the way through, and each with a complete answer key. Section 4.9.4.4 asks for candidates to know how a subnet mask is used to identify the network identifier part of an address. Every sheet does exactly that and then follows it through: the AND posed in binary, the network identifier, the size of the subnet, and whether two given addresses share it. WHAT YOU GET 20 worksheets, 2 pages each, numbered 01 to 20 20 matching answer keys A contents table giving each sheet’s mask, its host bits and how many hosts it holds, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 gives an address and a mask. Write both in binary, AND them octet by octet in the table provided, and give the network identifier in dotted decimal - then say how many bits are the network identifier and how many the host identifier. Question 2 sizes the subnet: how many addresses it holds, how many are usable, the broadcast address, and the first and last usable address. Question 3 gives three pairs of addresses under the same mask and asks which pairs are on the same subnet. The pairs are chosen so that the decimal addresses look close together whether or not they share a subnet - the only way through is to apply the mask. Question 4 goes the other way: a subnet has to hold a given number of machines, so give the smallest mask that will do, and say how many addresses would be spare. Question 5 is a written question on why IPv6 was introduced, on routable and non-routable addresses, on why a network is divided into subnets, on what the mask actually does, or on the two parts of an address. Every sheet is 25 marks. THE CONVENTION THIS TOPIC ARGUES ABOUT, SETTLED ON THE PAGE A subnet with h host bits holds 2^h addresses, but only 2^h - 2 of them can be given to a machine: the first is the network identifier and the last is the broadcast address. Textbooks quote both figures and very often do not say which they mean, so a student can answer correctly and still be marked wrong. Every sheet states on the page that usable means 2^h - 2, and every answer key uses it. The other conventions are printed too: octets are always written on all eight bits, leading zeros included, and network identifier is the specification’s own term. EVERY MASK CUTS INSIDE AN OCTET No /8, /16 or /24 anywhere in the pack. With an octet-aligned mask the network identifier can be read straight off without doing the AND at all, and the question stops testing anything. The masks here run from /20 to /30, so the binary has to be posed every time. FORMAT PDF, A4, print-ready. Twenty genuinely different addresses and masks, so you already have a fresh sheet for every class, every resit and every homework. Licensed for the purchasing teacher and their own classes.

Big-O and the complexity of algorithms for AQA A Level Computer Science (7517): twenty different fragments of pseudocode, each derived all the way through, and each with a complete answer key. Section 4.4.4.3 asks candidates to BE ABLE TO DERIVE the time complexity of an algorithm. Most resources on this topic are tables of orders to memorise. Every sheet here starts from code. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table giving each sheet’s fragment and its order, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 prints a numbered fragment of pseudocode and names one line. How many times does that line run for a given n? Then as an expression in n. Then the order in Big-O notation. The answer key explains where the count comes from, not just what it is. Question 2 does the same on two more fragments, each of a different order from the first. Question 3 covers the maths: the order of a polynomial, five orders to put in increasing order of growth, and two algorithms whose exact operation counts are given, compared at one particular size. Question 4 covers permutations and n factorial, which of two problems is tractable and why, and what is used when a problem is not. Question 5 is a written question on the Halting problem, on tractability, on why constant factors are dropped, on best case against worst case, or on the limits of computation. Every sheet is 30 marks. THE TWO CONVENTIONS THAT DECIDE THE ANSWERS Only the numbered line a question names is counted - not the FOR or WHILE lines, not the lines that set a variable up. Without that, “how many operations does this algorithm do” has no answer at all: textbooks count operations, comparisons or statements by turns, and three students who are all right write three different numbers. Naming a line removes the question rather than settling it arbitrarily. The second is the one marks are lost on: Big-O here always describes the WORST case, so a linear search of n items is O(n) even though it may find the item at the first comparison. Both are printed at the top of every sheet, and one of the five written questions asks the student to justify the second. EVERY COUNT IS EXACT No fragment relies on a rounding. The loop that halves n is only ever given a power of two, so the count is exactly log2 n rather than a floor or a ceiling depending on which textbook you read; the loop that steps by two is only ever given an even n. Where a question asks for a number, there is one number. FORMAT PDF, A4, print-ready. Twenty genuinely different fragments, so you already have a fresh sheet for every class, every resit and every homework. Licensed for the purchasing teacher and their own classes.

Backus-Naur Form for AQA A Level Computer Science (7517): twenty different worksheets, each built on a different grammar, and each with a complete answer key. Section 4.4.3 asks for two things. The first is to check whether a string is syntactically valid by referring to BNF or to a syntax diagram. The second is to explain why BNF can represent languages that a regular expression cannot. Every sheet does both, and the second one is done on a grammar the student has just spent four questions using. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table giving what each grammar describes and how long its derivation runs, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 gives a grammar and five strings to judge valid or not. The invalid ones are never obviously invalid: each is one character away from a string the grammar accepts, so the fault has to be found by following the rules rather than by glancing at the alphabet. Question 2 asks for one string to be derived step by step, writing the sentential form and the production rule used on every line. The sheet states that the leftmost non-terminal is replaced at every step, which is what makes one sequence correct rather than several. Question 3 shows one rule of the same grammar drawn as a syntax diagram, asks which non-terminal it defines, and then gives three more strings to judge by following the picture. The specification says BNF or syntax diagrams; this question is where the two meet. Question 4 has one symbol rubbed out of the grammar. Put it back, then answer this: if that whole alternative were deleted, the language would become finite - how many strings would be left? The answer is a number, and it is the shortest route to what recursion is actually for. Question 5 is a written question on why BNF goes beyond regular expressions, on terminals and non-terminals, on recursion, on why a language needs a formal definition at all, or on diagrams against notation. Sheets run from 22 to 26 marks.

Stacks, circular queues and hash tables for AQA A Level Computer Science (7517): twenty different worksheets, each tracing all three structures, and each with a complete answer key. This is the topic textbooks disagree about most, and every disagreement is a place where a student can be right and still be marked wrong. Every one of them is settled on the sheet itself. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table giving the size of each queue, the number of slots in each hash table and the final written question, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 traces a stack through a sequence of push, pop and peek operations, recording the contents, the top pointer and the value returned. The stack empties at least once on every sheet, so the row where the pointer goes back to -1 is always there. Question 2 traces a circular queue, and shows every slot of the array rather than just the items in it - which is the only way the wrap is visible. Front, rear and the count are recorded on every row. Every queue on every sheet really does wrap round to slot 0. Question 3 adds keys to an empty hash table, rehashing to the next free slot on a collision, and asks how many keys collided. Every table has two or three collisions: enough to have to rehash, few enough to stay readable. Question 4 asks for the test that shows the stack is empty and the test that shows the queue is full. Question 5 is a written question on static and dynamic structures, circular versus linear queues, collisions, peek, or priority queues. Sheets run from 31 to 36 marks. THE CONVENTIONS ARE PRINTED, NOT ASSUMED Top is the index of the top item and is -1 when the stack is empty, not the next free slot. Rear is the index where the next item will be stored, not the index of the last one added. The queue keeps a count, so it is full when the count equals the size of the array - rather than the other common convention, which leaves one slot permanently empty and gives a capacity one smaller. A slot that has been removed keeps its value, which is what actually happens in memory. All four are printed at the top of every sheet, and the answer key for question 4 says which alternative answer not to accept, and why. WRITTEN FOR AQA The operations are the ones the specification lists: push, pop, peek or top, test for empty and full for stacks; add, remove, test for empty and full for queues; simple hashing algorithms, collisions and rehashing for hash tables. The wording of the answers follows the specification’s own - a collision is when two key values compute the same hash. FORMAT PDF, A4, print-ready. Twenty genuinely different sets of operations, so you already have a fresh sheet for every class, every resit and every homework. Licensed for the purchasing teacher and their own classes.

Finite state machines for AQA A Level Computer Science (7517): twenty different worksheets, each with a machine to read, a table to complete, two strings to trace and a Mealy machine to run, and each with a complete answer key. The specification asks for four things in one sentence - state transition diagrams AND state transition tables, for machines with no output AND with output, Mealy machines only. Most resources stop at the first half. Every sheet here does all four. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table giving each sheet’s alphabet, the length of the string it traces and its final written question, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 gives a state transition diagram and asks for the matching state transition table. The first row is filled in, so the table’s layout is never the thing being tested. Question 2 traces a string through that machine, one row per symbol read, and asks whether it is accepted. Question 3 gives three more strings to judge. At least one is accepted and at least one is not, so the question cannot be answered by guessing one way. Question 4 is a Mealy machine: a diagram with an output on every transition, a trace table with an Output column, and the whole output string to write out. The diagram and the table it feeds are always on the same page. Question 5 is a written question - machines with and without output, regular languages and regular expressions, why every state has exactly one arrow per symbol, tables against diagrams, or the shortest string the machine accepts. Sheets run from 24 to 27 marks. EVERY ANSWER HAS EXACTLY ONE READING Every machine is deterministic and complete: one arrow per state per symbol, so a trace can never get stuck and never has a choice to make. No start state is an accepting state, so the empty string is never quietly at stake. And where a sheet asks for the shortest string a machine accepts, it is only asked when that string is the only one of its length - so there is a single answer to mark rather than an argument to have. The rules the answers depend on are printed on the sheet: where the machine starts, what a double circle means, that a string is accepted on the state reached AFTER the last symbol, and what a label such as 1/0 on a transition stands for. FORMAT PDF, A4, print-ready. Twenty genuinely different pairs of machines, so you already have a fresh sheet for every class, every resit and every homework. Licensed for the purchasing teacher and their own classes.

Reverse Polish notation practice for AQA A Level Computer Science (7517): twenty different worksheets on converting to and from RPN and evaluating it with a stack, each with a complete answer key. The specification asks students to convert expressions from infix to Reverse Polish “and vice versa”. Most resources only show the first direction. Every sheet here does both: question 1 converts an infix expression to RPN, and question 2 converts a different RPN expression back to infix. WHAT YOU GET 20 worksheets, 2 pages each, numbered 01 to 20 20 matching answer keys A contents table listing the expression each sheet asks to convert, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 converts infix to Reverse Polish. Question 2 converts Reverse Polish back to infix, with brackets showing the order of evaluation. Question 3 evaluates an RPN expression with a stack in a trace table, one row per token read, with the first row completed. Question 4 asks why RPN needs no brackets and why interpreters store expressions in it. Sheets run from 17 to 19 marks. THREE PLACES A QUESTION COULD HAVE HAD TWO ANSWERS Precedence and associativity are stated in the conventions at the top of every sheet: times and divide before plus and minus, equal precedence left to right. The order of the operands of a subtraction - the slip where a b - is read as b - a - is stated too, with a worked example. And the expressions themselves are bracketed so that they say what they mean under that left-to-right rule. An expression like A * B * C looks harmless, but if the intended grouping is A * (B * C) then a student following the printed rule would produce a different RPN and be marked wrong for doing exactly what the sheet told them. Every expression in this pack is printed with the brackets that make its order unambiguous, and no more. Every expression to be evaluated was generated so that no intermediate result is negative or fractional: a student who reaches one knows they have slipped. WRITTEN FOR AQA The specification wording is “be able to convert simple expressions in infix form to Reverse Polish notation (RPN) form and vice versa”, with “eliminates need for brackets in sub-expressions” and “expressions in a form suitable for evaluation using a stack” in the notes beside it - AQA 7517, section 4.3.3, printed in the header of every page. Question 4’s answers use those words. This is an AQA topic: OCR H446 does not name Reverse Polish notation, so these sheets are shaped for AQA and are not offered as an OCR resource. FORMAT PDF, A4, print-ready. Twenty genuinely different versions, so you already have a fresh sheet for every class, every resit and every homework. Licensed for the purchasing teacher and their own classes.

Floating point binary practice for AQA A Level Computer Science (7517): twenty different worksheets on reading, writing and normalising floating point numbers, and on the error in a value the format cannot store, each with a complete answer key. AQA states in the specification that exam questions use a format in which both the mantissa and the exponent are two’s complement, but the number of bits changes from paper to paper: June 2018 used a 7-bit mantissa with a 5-bit exponent, June 2023 used 8 and 4. These sheets do the same. They use 8+4, 7+5 and 6+4, and every sheet prints its own format at the top of the page. WHAT YOU GET 20 worksheets, 2 pages each, numbered 01 to 20 20 matching answer keys A contents table listing the format and the final written question of each sheet, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 reads a floating point number and asks for the mantissa, the exponent and the value, in decimal. Question 2 goes the other way: a decimal value, written first in fixed point binary, then normalised into the boxes. Question 3 normalises two numbers - one with a positive mantissa and one with a negative mantissa. Question 4 gives a value the format cannot store exactly, together with the pattern the computer stores instead, and asks for the absolute and the relative error. Question 5 is a written question on precision, range, underflow, overflow, or why numbers are normalised at all. That covers specification 4.5.4.4 to 4.5.4.9. Sheets run from 21 to 22 marks. BOTH SIGNS, WHICH IS WHAT THE SPECIFICATION ASKS FOR The specification says students should “be able to normalise un-normalised floating point numbers with positive or negative mantissas”. Most resources work through the positive case and stop. Question 3 does both on every single sheet, and the answer key shows how many places the mantissa shifted and what the exponent became. NO QUESTION HAS TWO CORRECT ANSWERS Every value a student is asked to convert fits its format exactly. That is deliberate: when a value does not fit, the answer depends on whether your department rounds or truncates, and the AQA mark schemes themselves accept both. None of these sheets asks a student to make that choice. The one value that cannot be stored exactly is in question 4 - and there the stored pattern is given, so what is asked for is the error, not the rounding. Where the binary point sits, what counts as normalised, and how many bits go to each field are printed in the conventions at the top of every page, not assumed from a textbook. FORMAT PDF, A4, print-ready. No macros, nothing to enable, nothing that stops working when your IT policy changes. Twenty genuinely different versions, so you already have a fresh sheet for every class, every resit and every homework. Licensed for the purchasing teacher and their own classes.

Assembly language practice for AQA A Level Computer Science (7517): twenty different programs to trace, each written in the instruction set AQA gives candidates in the exam, and each with a complete answer key. Most low-level programming resources teach the Little Man Computer. AQA does not use it. AQA publishes its own instruction set - LDR, STR, MOV, CMP, ADD, SUB, B and the conditional branches, the bitwise operators and the shifts - and hands it to candidates with the paper. A student who has only ever traced LMC meets that language for the first time in the exam. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table giving what each program does and how long its trace is, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET The instructions that program uses are printed at the top of the sheet, in AQA’s own wording, so nothing has to be memorised to answer. Question 1 is the trace table: one row per instruction executed, recording the line number and every register, with the first row completed. Question 2 asks what the program leaves in memory. Question 3 runs it again with one value changed and asks what it leaves then - without tracing it a second time. Question 4 names the addressing mode of two operands and asks for the difference. Question 5 separates the opcode from the operands and asks why the program will not run on a different processor. Sheets run from 16 to 32 marks, depending on how long the trace is. TWENTY PROGRAMS THAT ACTUALLY DO SOMETHING Not random instructions. The programs add a list of values, multiply by repeated addition, divide by repeated subtraction, find the larger of two values, count values above a threshold, multiply and divide using shifts, and mask and combine bits. Eight shapes, different registers, addresses and constants each time, and traces from eight instructions to twenty-three - a short one is a starter, a loop of twenty fills a lesson. NOTHING IS LEFT TO THE TEXTBOOK Every value in every trace stays between 0 and 255, so no answer depends on how wide a register is - which the instruction set does not settle. The conventions the trace table depends on are printed on the sheet: one row per instruction executed including HALT, the line number in the Line column, and every register written in every row even when it has not changed. Every program halts, and every conditional branch is shown going both ways across the two runs. FORMAT PDF, A4, print-ready. Twenty genuinely different programs, so you already have a fresh sheet for every class, every resit and every homework. Written for AQA 7517. The instruction set is AQA’s own, so these sheets are not offered as an OCR or GCSE resource. Licensed for the purchasing teacher and their own classes.

Turing machines for AQA A Level Computer Science (7517): twenty different machines, each with a state transition diagram to read, a transition function to write, a tape to hand-trace, and a complete answer key. Section 4.4.5 asks for the transition rules to be written BOTH as a function and as a state transition diagram, and for the two to be understood as the same thing. Every sheet here gives the diagram and asks for the function, so that conversion is the work rather than an aside. It also says “hand-trace” - so question 2 is a table the student fills in row by row, not a box for a final answer. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table giving what each machine computes and how long its trace runs, so you can pick one without opening all twenty WHAT IS ON EVERY SHEET Question 1 gives the state transition diagram and asks for the transition function, one rule for every arrow. The first is written out as a model. Question 2 is the hand-trace: the machine is started on a numbered tape with the head on a marked square, and the student writes the state, the head position and the whole tape after every step. The last row is marked “stop” - the state the machine halts in and where the head is left. One mark per row. Question 3 asks for the final tape, the halting state and the number of steps. Question 4 runs the same machine on a second tape and asks for the same three things, this time without a trace table to lean on. The second tape always gives a different result from the first, so it cannot be answered by copying. Question 5 is a written question on the Universal Turing machine, on how halting states are recognised, on why the tape is defined as infinite, on the function against the diagram, or on what Turing machines are used to define. Sheets run from 21 to 31 marks.

Binary tree traversal practice for AQA A Level Computer Science (7517): twenty different worksheets on pre-order, in-order and post-order traversal, each with a complete answer key. Every worksheet is built on a different binary tree, and no two trees in the pack have the same shape. You can set one sheet in class, a second for homework and a third for a resit without ever setting the same questions twice, and without drawing new trees yourself at eleven o’clock at night. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table listing the number of nodes, the mark total and the traversal shown in question 3 for every sheet, so you can pick a seven-node tree for a ten-minute starter or a twelve-node one for a full period WHAT IS ON EVERY SHEET Question 1 asks for all three traversals of the same tree, which is the point: pre-order, in-order and post-order only make sense against each other. Question 2 is answered from a table of nodes rather than from the diagram - Index, Left, Data, Right, with -1 for a missing child - and asks for a named child, the leaves and the depth. Question 3 shows a recursive subroutine and asks which traversal it performs, what it is used for, what stops the recursion, and what the same subroutine would output if it were called on a subtree. Mark totals are printed at the top of each sheet and beside every question. Sheets run from 17 to 20 marks, on trees of seven to twelve nodes.

Searching algorithms practice for AQA A Level Computer Science (7517): twenty different worksheets on linear search, binary search and binary tree search, each with a complete answer key. Every sheet runs all three algorithms on the same values, looking for the same target: the unsorted list, the same list sorted, and the binary search tree built from it. A student finishes the sheet having counted, say, twelve comparisons against four and five - and question 4 then asks them to explain the gap they have just measured, rather than to recite O(n) and O(log n) from memory. WHAT YOU GET 20 worksheets, 4 pages each, numbered 01 to 20 20 matching answer keys A contents table giving the number of values, the mark total and the comparison counts of every sheet, so you can set one where the contrast is stark or one where it is narrower WHAT IS ON EVERY SHEET Question 1 traces a linear search of an unsorted list and asks for the number of comparisons. Question 2 traces a binary search of the same values sorted, against the pseudocode printed on the sheet, in a table with Bottom, Top, Midpoint and the value there. Question 3 traces a binary tree search of the same values. Question 4 asks for the time complexity of all three, for an explanation of the gap between the first two counts, and for the largest number of comparisons a binary search could need on a list of several thousand items. The first row of every trace table is completed, so there is no doubt about where a trace begins. Sheets run from 19 to 20 marks. THE MIDPOINT RULE IS PRINTED ON THE SHEET On an even number of items there is no middle item, and a trace question that does not say which way to round has two correct answers. Every worksheet states the rule in its conventions and again in the pseudocode: Midpoint is (Bottom + Top) DIV 2, integer division, rounded down. Whatever your department teaches, the trace the sheet asks for is the one determined by what the student is looking at. WRITTEN FOR AQA The specification asks students to “know and be able to trace and analyse the complexity of the linear search algorithm”, and the same for binary search and binary tree search - AQA 7517, section 4.3.4, printed in the header of every page. Both verbs are on the sheet: three traces, then the complexity of each with a reason, in the specification’s own terms. The pseudocode follows AQA’s own written conventions: DIV for integer division, LEN for the length of an array, arrays indexed from 0, and the not- equal and less-than-or-equal symbols as AQA writes them. FORMAT PDF, A4, print-ready. Twenty genuinely different versions, so you already have a fresh sheet for every class, every resit and every homework. Licensed for the purchasing teacher and their own classes.

Sorting algorithms practice for AQA A Level Computer Science (7517): twenty different worksheets on bubble sort and merge sort, each with a complete answer key. Every sheet sorts the same eight values twice. The bubble sort is traced pass by pass, and the merge sort is a diagram the student completes: the list comes already split down to single values, and the three rows of merging are the question. A student finishes the sheet having counted, say, seven passes and twenty-two swaps one way against three levels of merging the other - and question 3 then asks them to explain the gap. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table giving the number of passes, the number of swaps and the mark total of every sheet, so you can pick a four-pass list for a starter or a seven-pass one for a full lesson WHAT IS ON EVERY SHEET Question 1 gives the bubble sort in pseudocode and a table to complete, one row per pass, showing the list after that pass and the number of swaps in it. The first pass is done for the student. Question 2 is the merge sort diagram, with boxes sized to write in. Question 3 asks for the time complexity of both algorithms, for what that complexity means for a list twice as long, and for the number of levels of merging a much larger list would need. Sheets run from 17 to 20 marks, depending on how many passes the list happens to need. TWO CONVENTIONS, HANDLED TWO DIFFERENT WAYS Textbooks disagree on whether a bubble sort stops as soon as a pass makes no swaps or always runs the same number of passes. The two produce exactly the same list after every pass, so the trace table suits both; they differ only in how many passes get counted, and the stopping rule is printed in the pseudocode on the worksheet, so that question is answerable from the sheet. These worksheets never ask for the number of comparisons a bubble sort makes, because that one genuinely does depend on the version taught. They ask for the number of swaps, which does not: it is the number of pairs that started out in the wrong order, whatever route the algorithm takes. For merge sort, each list is eight values long, so every split is even and the question of which half gets the odd item never arises. WRITTEN FOR AQA The specification asks students to “know and be able to trace and analyse the time complexity of the bubble sort algorithm” and the same for merge sort - AQA 7517, section 4.3.5, printed in the header of every page. The pseudocode follows AQA’s own written conventions: the arrow for assignment, FOR … TO, LEN for the length of an array, and arrays indexed from 0. FORMAT PDF, A4, print-ready. Twenty genuinely different versions, so you already have a fresh sheet for every class, every resit and every homework. Licensed for the purchasing teacher and their own classes.

Dijkstra’s algorithm practice for AQA A Level Computer Science (7517): twenty different worksheets on the shortest path algorithm, each with a complete answer key. Every worksheet is built on a different weighted network. That is the point of the pack: you can set one sheet in class, a second for homework and a third for a resit without any two students ever holding the same questions, and without drawing new networks yourself at eleven o’clock at night. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys A contents table listing the number of vertices and the mark total of every sheet, so you can pick a six-vertex network for a ten-minute starter or a nine-vertex one for a full period WHAT IS ON EVERY SHEET Question 1 asks students to trace the algorithm: one row per vertex settled, recording every tentative distance and the vertex it was reached from, written as a subscript. Question 2 asks for the length of the shortest path and then for the path itself, read back off the completed table - the step most students never practise and most lose marks on. Question 3 asks why the distance to a settled vertex can never be improved by a later step, for a typical application of shortest path algorithms, and one further question that varies from sheet to sheet.

Graph traversal practice for AQA A Level Computer Science (7517): twenty different worksheets on breadth-first and depth-first search, each with a complete answer key. Every worksheet is built on a different graph. That is the point of the pack: you can set one sheet in class, a second for homework and a third for a resit without any two students ever holding the same questions, and without having to invent new graphs yourself at eleven o’clock at night. WHAT YOU GET 20 worksheets, 3 pages each, numbered 01 to 20 20 matching answer keys, 2 pages each A contents table listing the size and mark total of every sheet, so you can pick a short one for a cover lesson or a longer one for a full period WHAT IS ON EVERY SHEET Question 1 asks students to complete a trace table for a breadth-first traversal: the node removed from the queue, the state of the queue after each step, and the running visit order. Question 2 does the same for depth-first search, including the backtracking steps that most students forget. Question 3 shows a piece of pseudocode and asks which traversal it implements, and for a typical application of it. The first row of each trace table is completed for students, which removes any doubt about where the trace begins. Mark totals are printed at the top of each sheet and beside every question. WRITTEN FOR AQA, NOT ADAPTED FROM SOMEWHERE ELSE The pseudocode uses the conventions that appear in real 7517 question papers: the arrow assignment, SUBROUTINE and ENDSUBROUTINE, OUTPUT, FOR EACH … IN, and adjacency written as ConnectedNodes[N]. Helper subroutines are declared in prose before the code, exactly as AQA does. Question 3 asks for an application of the traversal rather than its time complexity, because that is what specification 4.3.1 actually requires. THE ANSWER KEYS DO SOMETHING UNUSUAL Textbooks disagree about how depth-first search is traced. These sheets use the alphabetically-first convention that mark schemes normally show, and state it plainly on the worksheet. But each answer key also lists the visit orders produced by the other two conventions in common use, with a note telling you to credit them in full. Whichever version your department teaches, you can mark with these keys without arguing with them. FORMAT PDF, A4, print-ready. You do not need to edit these: there are twenty genuinely different versions, so you already have a fresh sheet for every class, every resit and every homework. Licensed for the purchasing teacher and their own classes.

Two free sample worksheets on error checking and correction for AQA A Level Computer Science (7517), with full answer keys, so you can see exactly what the full pack contains before buying anything. Each sheet has one question for each of the four methods the specification names: parity bits, majority voting, checksums and check digits. The two sheets deliberately differ - one uses odd parity, the other even; one votes on five copies, the other on three; one works out an ISBN-10 check digit, the other an ISBN-13. Put them side by side and the point of the pack is visible in a minute: the same figures give different check digits, so the sheet has to say which rule it means. That is what these sheets do. The checksum rule is framed above question 3 and the check-digit scheme above question 4, weights and modulus and all, and the parity and bit-numbering conventions are at the top of the page. These two sheets are not in the paid pack - they are extra, so nothing is wasted if you buy it afterwards. The full pack contains 20 worksheets and 20 answer keys. Search Tes for “AQA A Level Computer Science error checking and correction” to find it. If these are useful, a review is genuinely appreciated - it is the only way other teachers find resources like this.

Two free sample worksheets on functional programming for AQA A Level Computer Science (7517), with full answer keys, so you can see exactly what the full pack contains before buying anything. Each sheet has six questions, one for each part of section 4.12: the type of a function and what it actually produces, partial application in the notation the specification names, composition in both orders, map and filter and reduce on the same list, and the head and tail of a list. The two sheets are deliberately different. One folds its reduce with add, where the direction does not matter; the other folds with sub, where it does - and the answer key shows why the rule has to be printed rather than assumed. These two sheets are not in the paid pack - they are extra, so nothing is wasted if you buy it afterwards. The full pack contains 20 worksheets and 20 answer keys. Search Tes for “AQA A Level Computer Science functional programming” to find it. If these are useful, a review is genuinely appreciated - it is the only way other teachers find resources like this.