The development cycle, decomposition and structure diagrams

Four stages, in order

The program development life cycle is the set of stages a programmer goes through to make a program.

There are four stages: analysis, design, coding and testing.

It is called a cycle because problems found in testing send you back to an earlier stage.

Analysis: understand the problem

In analysis you find out exactly what the problem is and what the program must do. These needs are the requirements.

Abstraction means removing the details that do not matter, so you can focus on what does.

Decomposition means breaking a big problem into smaller parts that are easier to solve.

Design, coding and testing

In design you plan how the program will work. You decompose further and use structure diagrams, flowcharts and pseudocode.

In coding you write the program. You test each part as you write it. This is called iterative testing: write a bit, test it, fix it, repeat.

In testing you run the finished program with planned test data to check it works for every kind of input.

Sub-systems, and inputs, processes, outputs and storage

A system is a set of parts that work together to do a job. Each part is a sub-system. A sub-system can be split again until each part is small enough to code easily.

Example: a supermarket self-checkout is a system. Scanning, weighing, payment and printing the receipt are its sub-systems.

Every problem can also be decomposed into four parts.

Inputs: the data going in. Processes: what is done to the data. Outputs: the results that come out. Storage: data that must be kept for later.

Structure diagrams

A structure diagram shows how a system is broken down into sub-systems. The whole system is in the top box.

Each level below shows smaller parts. Read it top to bottom. Boxes on one level are usually read from left to right.

It shows what the parts are, not the order of every step or the decisions.

There is a diagram for this step in the IGCSE library.

Flowcharts: the six symbols

A flowchart shows the steps of an algorithm as shapes joined by arrows. Each shape has one meaning.

Follow the flow lines from START to STOP. A decision diamond has two exits, labelled Yes and No.

A loop is drawn as a flow line that goes back up to an earlier step.

There is a diagram for this step in the IGCSE library.

Pseudocode: Cambridge's planning language

Pseudocode is a way of writing an algorithm that looks like code but is not tied to one language.

Cambridge has its own rules. Keywords are in CAPITALS. Assignment uses `←`. Comments start with `//`. Not equal is `<>`.

Paper 2 answers must be in pseudocode. Program code only earns marks in the 15-mark scenario question.

DECLARE Mark : INTEGER
OUTPUT "Enter your mark: "
INPUT Mark
IF Mark >= 25
  THEN
    OUTPUT "Pass"
  ELSE
    OUTPUT "Fail"
ENDIF

mark = int(input("Enter your mark: "))
if mark >= 25:
    print("Pass")
else:
    print("Fail")