1. Unit 1: Principles of Computer Science
Decomposition in Computer Programming
Mr Chandler
This presentation discusses decomposition in computer programming, what it is, how we do it and why we would want to do it

2. Decomposition – Yuck!
There is nothing rotten about computer programming
We mean the other definition – breaking into parts
OK so when I think about decomposition I think about, well, dead bodies. So what do dead bodies and computer programming have in common - not much thank goodness.
there is absolutly nothing rotten about computer programming. When we talk about decomposition in computer programming we mean the other definition of decomposition – breaking into something into its component parts

3. How to Create a Computer Program
Computer program = list of instructions
To Write a Computer Program
Identify the problem
Break it into separate parts (Decomposition)
Sometimes the problem must be simplified first (Abstraction)
Create the code for each part
Combine the pieces of code to create the program
So remember that a computer program is just a list of steps or instructions that you want the computer to perform to solve a problem.
So when you write a computer program you need to know what those steps are
No problem, it’s a piece of cake. Here what you need to do
Identify the problem you want the computer to solve
Break it into components or steps that can be solved (that is DECOMPOSITION which we will be discussing in this video).
Sometimes the problem has to be simplified first before you can break it down into steps (that is called abstraction which we will talk about in another video
Then create code for each component or part
Then you combine the pieces of code to create the computer program!

4. Decomposition Steps Divide and Conquer Step 1: Identify the parts
Roughly the same level of detail
Can be written up separately
Combine to solve the original problem
Step 2: Write the steps for each part
Step 3: Combine the steps for each part to solve the problem
So you can think of Decomposition as dividing and conquering the problem.
When you decompose a problem, the first thing you do is identify the parts of the problem.
Each of these parts should have about the same level of detail and you should be able to write them up or do them separately.
When you combine the pieces, they should solve the original problem.
After you break the problem down into parts, the next step in decomposition is writing out the steps in code for each part.
Then you put all the steps together in the correct order and you get the code that solves the problem
So what we need to do is look at an example… but we don’t know enough yet to look at a programming example so lets decompose a different type of problem

5. Example: I Want Cake! Step 1: Identify the Components or Parts
Gather the Equipment
Gather the Ingredients
Measure the Ingredients
Mix the Ingredients
Bake the Cake
Lets look at an example. I want cake. I absolutely, desperately want cake but I need to make a cake if I want it. Lets decompose the problem. The first step is to break the problem into its components or parts. So the the components of my problem of wanting cake are.. …
Roughly the same level of detail
Can be written up separately
Combine to solve the original problem
It is important to know that someone else might decompose the problem differently and that is OK too.

6. Example: I Want Cake! (continued)
Step 2: Write the Steps
A. Gather the Equipment
Get the mixing bowls
Get small bowls to measure ingredients into
Get cake pans
Get mixer
Get measuring spoons
Get measuring cups
So now we proceed to step two of the decomposition process – writing steps for each part. Lets look at the fist part – gathering the equipment. Here are the steps ….

7. Example: I Want Cake! (continued)
Step 2: Write the Steps (continued)
B. Gather the Ingredients
Get the flour
Get the sugar
Get the oil
Get the vanilla
Get the eggs
Get the baking powder
Get the milk
Get the pam
Lets look at the next part of the decomposition of my cake problem – gathering the ingredients

8. Example: I Want Cake! (continued)
Step 2: Write the Steps (continued)
C. Measure the Ingredients
Measure 1 ½ cups flour
Put flour in small bowl
Measure1 cup sugar
Put in another bowl ….
You get the idea (continue for parts D and E)
And now part three – measuring the ingredients…. Well you get the idea. You would do the same for parts D and E

9. Example: I Want Cake! (continued)
Step 3: Put the pieces back together
Gather the Equipment
Gather the Ingredients
Measure the Ingredients
Mix the Ingredients
Bake the Cake
Then carry out all the small steps……….
………… And when you are done you have Cake!
The next step in the decomposition of my wanting cake problem is putting the pieces and their steps back together. Then we would carry out the steps – which would be equivalent to running the program and voila when we are done we would Have Cake

10. Advantages of Decomposition
Easier to think about smaller pieces of the problem than the whole problem at once.
Different people can work on each part (speeds up programming). Someone might gather the ingredients while someone else measure them.
Parallel Processing might be possible (speeds up execution). You could preheat the oven while you are preparing the batter.
So that is what decomposition looks like… some advantages of decomposition are

11. Disadvantages of Decomposition
Computers are Stupid
They do exactly what you tell them to do even if it is WRONG.
They do the steps in exactly the order you tell them to do even if that’s WRONG too!
Sometimes problems cannot be decomposed without simplifying them first
Too difficult
Not well understood
Sometimes when you put the pieces back together it does not work!

12. Summary Decomposition This is good because But Remember-
Break the problem into its components
Write the steps (code) for each component
Put the pieces together to make the program and solve the problem!
This is good because
It is easier to think about
Programming and execution can be faster
But Remember-
Computers are stupid - they do:
Exactly what you tell them to do
Exactly how you tell them to do it
Sometimes putting the pieces together doesn’t work