Problem Solving and Programming

What Problem Solving and Programming covers

Problem Solving and Programming is examined on component 02 of OCR Computer Science, Algorithms and programming. 119 recall questions across 10 subtopics.

The 10 subtopics

Each one has its own page.

• Programming techniques: Programming constructsRecall questions on sequence, branching and iteration, count-controlled and condition-controlled loops, tracing and nested loops, infinite loops, elseif order and switch/case, plus reading and writing text files, string length and substrings, and the MOD, DIV and ^ operators.Open →
• Programming techniques: RecursionRecall questions on recursive subroutines, base and general cases, tracing factorial calls, the call stack and unwinding, stack overflow, recursion depth on large data, recursion compared with iteration, and slow recursive Fibonacci.Open →
• Programming techniques: Global and local variablesRecall questions on the scope and lifetime of variables, local and global variables, predicting output when a local variable shadows a global one, why globals are discouraged and when they are justified, and fixing out-of-scope values.Open →
• Programming techniques: modularity, functions and procedures, and parameter passingRecall questions on modularity and its benefits, functions and procedures, what a caller needs to know, reusable subroutines, side effects, passing parameters by value and by reference with traces, returning two results, and parameters versus arguments.Open →
• Programming techniques: Use of an IDE to develop/debug a programRecall questions on integrated development environments, breakpoints, stepping into and over, watch windows, the call stack view, syntax and error highlighting, auto-completion, refactoring tools, and debugging logic errors and stack overflows.Open →
• Programming techniques: Use of object oriented techniquesRecall questions on classes and objects, attributes and methods, constructors, separate object state, encapsulation with private attributes and get and set methods, inheritance and superclass constructors, polymorphism, and when object-oriented design fits.Open →
• Computational methods: solvable problems, problem recognition, decomposition, and divide and conquerRecall questions on features that make a problem solvable or unsolvable by computational methods, problem recognition, problem decomposition, divide and conquer, binary search comparisons, and algorithms that divide and combine.Open →
• Computational methods: Use of abstraction and backtrackingRecall questions on abstraction in computational solutions and the danger of abstracting too much, abstraction with decomposition, backtracking in mazes and the four queens problem, its limitations, and problems that suit backtracking.Open →
• Computational methods: data mining, heuristics and performance modellingRecall questions on data mining and its limitations, heuristics in route planning, satnavs and antivirus detection, what heuristics trade away, performance modelling with worked examples, and when modelling is the only sensible option.Open →
• Computational methods: pipelining, visualisation and choosing a methodRecall questions on pipelining outside the processor and what limits it, visualisation and its limitations, backtracking compared with heuristics, choosing the computational method that fits a problem, and combining methods in a satnav.Open →

What you need to know

Answer each one before you reveal it.

Take Problem Solving and Programming further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• Backtracking vs A heuristic

  Backtracking: returning to the last choice point when a path fails, and trying the next option. A heuristic: a rule of thumb that finds a good enough answer without guaranteeing the best one.

• Performance modelling vs Visualisation

  Performance modelling: predicting how a system will behave under load without building and running it. Visualisation: presenting data graphically so that patterns a table would hide become visible.

• A global variable vs A local variable

  A global variable: a variable reachable from anywhere in the program. A local variable: a variable reachable only inside the block or subroutine where it was declared.

Questions from this topic

Each one with the model answer underneath.

1. 1

   The three basic programming constructs are sequence, iteration and branching. What does each one do to the order in which statements run?

   Model answer

   Sequence: statements are carried out one after another in the order they are written, each exactly once. Branching (selection): a Boolean condition is tested and the program takes one of two or more alternative paths, so some statements are skipped altogether. Iteration: a block of statements is repeated, either a set number of times or until a condition changes. Every algorithm can be built from these three constructs alone.

2. 2

   Work through this and say what is printed, then explain why the order of the lines matters. price = 20 total = price * 3 price = 5 print(total)

   Model answer

   It prints 60. total was worked out while price still held 20, and assigning 5 to price afterwards does not re-run that calculation. In a sequence each line uses the values that exist at the moment it runs, so moving price = 5 above the calculation would make the program print 15 instead. This is why sequence is a construct in its own right: the same three statements in a different order give a different result.

3. 3

   Explain the difference between count-controlled and condition-controlled iteration, and give a situation that suits each.

   Model answer

   Count-controlled iteration repeats a fixed number of times that is known before the loop starts, using a counter that is stepped each time round (for example, for i = 1 to 10). It suits work where you already know how many repetitions are needed: processing every element of an array of 50 records, printing a times table, drawing a shape with a known number of sides. Condition-controlled iteration repeats while, or until, a condition holds, so the number of repetitions is not known in advance. It suits validating input until the user types something acceptable, reading lines until the end of a file is reached, or a game loop that runs until the player quits.

4. 4

   A loop is written as for i = 0 to 7. How many times does the body run, and why is this a classic place for an off-by-one error?

   Model answer

   Eight times, with i taking the values 0, 1, 2, 3, 4, 5, 6 and 7, because both end values are included. It is a classic error because someone reading it as 'up to 7' may expect seven repetitions. It is also where array indexing catches people out: arrays here are numbered from 0, so an array of eight items has its last item at index 7 and this loop is exactly right for it, whereas for i = 0 to 8 would run nine times and try to read an item that does not exist.

5. 5

   What is the one behavioural difference between a while loop and a do ... until loop, and when does it matter?

   Model answer

   A while loop tests its condition before the body, so if the condition is false at the very start the body never runs at all. A do ... until loop tests after the body, so the body always runs at least once. It matters whenever running the body once with no valid data would be wrong. 'While there are records left, process one' must be a while loop, because an empty file should cause no processing. 'Ask for a password and repeat until it is correct' suits do ... until, because you have to ask at least once before there is anything to test.

6. 6

   Trace this and give the two numbers printed. count = 0 total = 0 while total < 20 count = count + 1 total = total + count endwhile print(count) print(total)

   Model answer

   It prints 6 then 21. Working through: count 1 gives total 1; count 2 gives total 3; count 3 gives total 6; count 4 gives total 10; count 5 gives total 15 — still under 20, so the loop runs once more; count 6 gives total 21, the condition total < 20 is now false and the loop ends. Note that the loop finishes with total past 20 rather than exactly on it, because the test only happens between whole repetitions.

What is inside Levo

Six ways to work the same material.

Problem Solving and Programming revision questions

What does Problem Solving and Programming cover in OCR A-Level Computer Science?

10 subtopics: programming techniques: programming constructs, programming techniques: recursion, programming techniques: global and local variables, programming techniques: modularity, functions and procedures, and parameter passing, programming techniques: use of an ide to develop/debug a program, programming techniques: use of object oriented techniques, computational methods: solvable problems, problem recognition, decomposition, and divide and conquer, computational methods: use of abstraction and backtracking, computational methods: data mining, heuristics and performance modelling and computational methods: pipelining, visualisation and choosing a method. Each one has its own page with questions and model answers.

How is Problem Solving and Programming organised?

By the OCR H446 specification, subtopic by subtopic, so it matches the order you are taught it in.