Decomposition and Abstraction

1 State what is meant by the term decomposition. 2

One mark for each of the following, to a maximum of 2.

• Breaking a large or complex problem into smaller parts or sub problems
• So that each part can be solved, tested or understood separately

Model answer. Decomposition means breaking a large and complex problem down into a number of smaller sub problems. Each sub problem is small enough to be understood, solved and tested on its own, and the solutions are then combined to solve the original problem.

2 A supermarket is developing an app that lets customers scan items and pay without visiting a till. Explain how decomposition would be used when developing this app. 4

One mark for each of the following, to a maximum of 4.

• The overall problem is broken into smaller sub problems
• Names sensible sub problems for this scenario, such as scanning a barcode, keeping a running total, taking payment or checking the customer out
• Each sub problem can be written and tested independently
• Different developers can work on different parts at the same time, and a fault can be traced to a single part

Model answer. Decomposition would be used to break the whole app down into smaller sub problems that can each be tackled on their own. Sensible sub problems here would be reading a barcode from the camera, looking that barcode up to find the product and its price, keeping a running total of the basket, applying any offers, taking a card payment and producing a digital receipt. Each of these can be written and tested independently, so the payment code can be proved correct before it is ever joined to the scanning code. Splitting the work this way also allows several developers to build different parts at the same time, and if the finished app produces a wrong total the fault can be traced to a single sub program rather than searched for across the whole system.

3 Explain what is meant by abstraction and give one reason why it is used. 3

One mark for each of the following, to a maximum of 3.

• Removing or hiding detail that is not needed to solve the problem
• Only the information relevant to the problem is kept
• So the problem is simpler to solve, or the solution needs less storage or processing

Model answer. Abstraction means removing or hiding the detail that is not needed to solve a particular problem, so that only the information that is genuinely relevant remains. It is used because a problem with less detail in it is easier to understand and quicker to design a solution for, and because the resulting program stores and processes less data, so it runs faster and uses less memory.

4 A weather app models rainfall across a country. Describe two details the developers would keep and two they would remove, and justify your choices. 4

One mark for each of the following, to a maximum of 4.

• Names a relevant detail that is kept, such as location, rainfall amount, time or temperature
• Justifies keeping it because it affects the forecast or the output the user needs
• Names an irrelevant detail that is removed, such as building colours, street names or the shape of individual clouds
• Justifies removing it because it does not change the result and would waste storage or processing

Model answer. The developers would keep the location of each measurement and the amount of rain recorded there, because the whole purpose of the app is to tell a user how much rain is expected at a place, so neither can be removed without breaking the app. They would also keep the time of each reading, since a forecast is meaningless without knowing when it applies. They would remove details such as the colour of the buildings in each town and the exact shape of individual clouds, because neither changes the rainfall figure a user is shown. Storing and processing that extra detail would use more memory and slow the forecast calculation down for no benefit at all.

5 Compare decomposition and abstraction, making clear how they differ. 4

One mark for each of the following, to a maximum of 4.

• Decomposition splits a problem into smaller sub problems
• Abstraction removes or hides detail that is not needed
• Both reduce the complexity the programmer has to deal with at once
• They are usually used together, with a problem first broken up and detail then removed from each part

Model answer. Decomposition and abstraction are both ways of making a large problem manageable, but they act on it differently. Decomposition splits the problem into smaller sub problems, so a library system becomes separate tasks for issuing books, returning books and calculating fines. Abstraction does not split anything: it removes detail from the problem, so the book record keeps the title, author and ISBN but not the colour of the cover. What they share is that each reduces how much a programmer has to hold in mind at one time, which makes the solution quicker to design and easier to test. In practice they are used together, because a problem is normally decomposed into parts first and then unnecessary detail is stripped out of each part.