IGCSE2.1.2Paper 2 — Algorithms, Programming and Logic

Sub-systems and Decomposition

Breaking a big problem into smaller pieces that can each be solved on their own.

YOU SHOULD BE ABLE TO

Decompose a problem into inputs, processes, storage and outputs.
Use structure diagrams to show sub-systems.
Understand and use flowcharts and pseudocode.

TRACE TABLE

Total ← 0
FOR Count ← 1 TO N
    Total ← Total + Count
NEXT Count
OUTPUT Total

STARTN = 5

Count	Total	OUTPUT

One row per pass through the loop. Leave a cell blank if it does not change on that pass.

//Decomposing a problem

For any scenario, list four things: what goes IN, what must be DONE with it, what must be STORED, and what comes OUT. Doing this first turns a vague question into a plan you can code.

Problem: work out an average exam mark for a class

INPUT   : each student's mark
PROCESS : total the marks, count them, divide
STORAGE : an array of marks
OUTPUT  : the average, to 1 decimal place

//Flowchart symbols

Learn the shapes — the wrong shape loses marks. Oval = start/stop. Parallelogram = input/output. Rectangle = process. Diamond = decision, with labelled Yes/No branches. Arrows show the flow.

CHECK YOURSELF

1.Which flowchart shape is used for a decision?

2.Decomposing a problem means splitting it into which four parts?

Program Development Life Cycle Pseudocode and Trace Tables

IGCSE