IGCSE Computer Science 0478 — 7b Decomposition and Sub-systems Hangman

The 45 key terms

1digital system

A combination of hardware and software that inputs, processes, stores and outputs data for a purpose, such as a laptop, tablet, smart speaker or games console.

2sub-system

A smaller part of a computer system that does one job within the whole system, such as the CPU, memory or storage in a computer. It is not the whole system.

3hierarchy of sub-systems

Each sub-system can be split into smaller sub-systems, level by level, until every part does one simple job. A sub-system at one level is a system at the next.

4input, process, store, output

A digital system takes in data (input), works on it (process), keeps it (store) and shows a result (output).

5decomposition

Splitting a big problem into smaller sub-problems, such as input, process and output. Each part can be solved on its own, then the solutions are combined.

6input

Data that goes into a system from a user, a sensor or a file, such as a typed name, a temperature reading or a line read from a stored file.

7process

An action or calculation a solution carries out on its data, such as adding a total, comparing a value with a limit, sorting or searching, or making a decision.

8output

The result a solution gives out, such as a message on screen, a printout, a sound or a signal that switches a device on or off.

9storage

Data the solution must keep, in a variable, file or database, so it can be used again later. It is not the same as input or output.

10Input, Process, Output, Storage

Read the scenario and sort every part into input (data in), process (what is done), output (result shown) or storage (data kept). Then check nothing is missing or in the wrong group.

11IPO chart

A table with Input, Process and Output columns that lists what the user enters, the calculations and decisions the code makes, and the results or messages it displays.

12sub-problem

A small part of a bigger problem. It is easier to understand and solve than the whole, so splitting a problem up makes it manageable and makes mistakes easier to find.

13test each sub-problem separately

Each sub-problem is tested on its own before being joined to the rest, so an error is traced to the part that causes it and only that part needs fixing, not the whole program.

14team work and reuse

Different programmers write separate sub-problems at the same time, using their own skills, and finished parts such as a validation subroutine can be reused in other programs.

15analysis, design, coding, testing

The four ordered stages of building a program: analysis (what is needed), design (plan it), coding (write it) and testing (check it works).

16analysis

The first stage of the program development life cycle: identify the problem and list what the solution must do, using abstraction and decomposition, before any design starts.

17abstraction

Keeping only the detail that matters for the problem and hiding the rest, like a tube map showing stations and lines but not exact distances.

19abstract a scenario

Read the scenario, list the details the solution needs and cross out the rest, saying 'because' for each choice (e.g. a bus app keeps times, not bus colour).

20decomposition diagram

A tree with the whole problem at the top and boxes below for its smaller sub-problems. Each level adds detail, and you stop splitting when a part is simple.

21structure diagram

A diagram with the whole problem in a top box, joined by lines to lower boxes for each sub-problem, in levels. It shows the parts of the problem, not the order of the steps.

22top-down design

Name the whole problem in the top box, then split it into sub-problems level by level until each box is one simple task. Use clear box names.

23complete a structure diagram

Read the given boxes top down, then fill each gap with the sub-problem its parent box needs, so the sub-boxes together make up the parent. Ignore boxes that do not belong.

24algorithm

A set of clear steps, in order, that solves a problem or completes a task, like a recipe or morning routine. Each step must be precise so a person or computer can follow it.

26flowchart symbol

A fixed shape showing one step in a flowchart: oval start/end, arrow flow, rectangle process, parallelogram input/output, diamond decision, double-sided rectangle subroutine.

27flowchart sequence

A line of rectangles joined by arrows from the Start oval to the End oval, with one arrow in and one arrow out of each step. Each step is a short action phrase.

28parallelogram

The flowchart shape for an input or output step, like getting a name from the user or showing a result. Write the action and variable inside, e.g. Input age.

29decision diamond

A diamond shape in a flowchart that holds a yes/no question. One arrow labelled Yes and one labelled No lead to different steps, and the paths can join up again later.

30flowchart loop

An arrow that goes back to an earlier shape, usually from a decision diamond, so the steps repeat until the exit condition is met and the exit arrow is followed.

31Subroutine symbol

A rectangle with double side lines. It stands for a call to a subroutine whose steps are drawn in a separate flowchart, which keeps the main flowchart short.

32main flowchart

A flowchart that calls each sub-problem from the structure diagram with a subroutine symbol. Each sub-problem then gets its own separate flowchart.

33pseudocode

A way of writing algorithms using plain English and common symbols without the rules of any specific programming language, used to plan logic before coding begins

34KEYWORDS IN CAPITALS

Cambridge pseudocode writes keywords like INPUT, OUTPUT and IF in capital letters, puts one statement on each line, and indents the statements inside IF and loops.

35Total ← 0

The ← symbol is assignment: it stores the value on its right in the variable on its left. Total ← 0 sets Total to 0. Total ← Total + Score adds Score to Total.

36INPUT Name

INPUT reads a value from the user into the variable Name. OUTPUT displays a value or message on screen, such as OUTPUT "Hello" or OUTPUT Name.

37IF THEN ELSE ENDIF

Runs the indented lines after THEN if the condition is true, otherwise runs the ELSE lines. ENDIF closes the construct and sits at the same indent as IF. ELSE is optional.

38FOR Count ← 1 TO 10

A count-controlled loop that repeats the indented statements a fixed number of times, with Count going from 1 to 10. NEXT Count closes the loop and moves Count up by 1.

39WHILE ... DO ... ENDWHILE

A loop that tests its condition before each repeat, so it may run 0 times. REPEAT ... UNTIL runs the body first and tests after, so it always runs at least once.

40procedure vs function

A procedure carries out a task without returning a value, while a function returns a value to the code that called it, so the result can be stored or used.

41PROCEDURE Name() ... ENDPROCEDURE

Defines a named block of pseudocode for one sub-problem. It does not return a value. The block only runs when you write CALL Name() in the main program.

42FUNCTION Name(Param : TYPE) RETURNS TYPE

Starts a function that takes a typed parameter and RETURNS a value of the stated type. RETURN sends the value back, ENDFUNCTION closes it, and you can call it inside an expression.

43def main():

A function that calls other small functions, each doing one job, so the program is built in parts. Each box in a hierarchy chart can become one function.

44CALL

Runs a procedure. A main program is a short list of CALL lines, one per sub-problem from the structure diagram, in the right order, with no detail inside it.

45PROCEDURE

Each box in a structure diagram becomes a PROCEDURE or FUNCTION named after the box. A box with sub-boxes uses CALL statements to run them. Check that every box has code.

46abstract, decompose, draw, write

Abstract the scenario, split it into inputs, processes, outputs and storage, draw a structure diagram, then write a flowchart or pseudocode for each part and check it against the requirements.

47full-mark answer

Uses key terms (sub-system, decomposition, abstraction, input, process, output, storage) correctly and gives a reason for each point, for example by using the word 'because'.