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The Program Life Cycle.

1. Phases of the life cycle.

2. Example of the Design Phase.3. Implementing the algorithm.

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Phases of the program life-cycle. A. The Design Phase.

Creation of an algorithm.

Analogy: drawing up a blueprint for a house.

B. The Implementation Phase.

Creation of a program.

Analogy: building a house.

C. The Maintenance Phase.

Use and modification of a program.

Analogy: adding an extension, repair of house.

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The Design Phase. 1. Receive the problem specification.

2. Understand / analyze the problem. 3. Create an algorithm.

4. Test and debug the algorithm

Why?

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Where may errors arise? 1. Failure to understand the problem.

May solve a different problem instead!

2. Incorrect algorithm.

Either the solution fails to be an algorithm (fails

one or more of four criteria) or it fails to give

the correct answer in all cases.

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How to avoid errors in design. We can reduce the number and severity

of errors by:

1. Taking time to fully understand the

problem.

2. Ensuring that we have a true algorithm

and that it works for both normal and

unusual datameaning?

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Importance of Design Phase. Studies show that time spent in the design

phase is always a good investment.

Analogy:

which is most cost-effective and causes

least problems ---

preventative care or treatment of illness?

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The Implementation Phase. After the design phase:

1. Code the program: translate the

algorithm into a programming language.

A program = an algorithm coded in a

programming language.

2. Compile the program.

3. Test and debug the program. Again?

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Where may errors arise? 1. Bad translation. The C# source code

does not say the same thing as the

algorithm. (Monty Python Danish-English

phrase book: Your hovercraft is full of

eels!)

2. Syntax errors. The code has errors ingrammar, spelling or punctuation.

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How to avoid errors in

implementation. 1. Master the programming language.In

this way, you will know what C# statement

corresponds to a statement ofEnglish.

2. Learn what the compilers syntax

error messages mean.

This will enable you to edit the code until it

is free of syntax errors.

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The Maintenance Phase. 1. Use the program.

2. Discover and fix errors missed in

testing.

Danger: fixing one problem may introduce

others.

3. Enhance the program by adding

additional capabilities.

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Example of Design Phase. 1. Receive problem.

Determine the change due to a customer.

Assume that the sales tax may vary.

2. Understand the problem.

Analyze the problem into the 3 main

activities of a program:

(1) INPUT (2) PROCESS (3) OUTPUT.

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Example of Design Phase (cont.). 3. Create algorithm.

Figure out the instructions required under

the 3 headings. These can be written as

pseudocode (English imperatives which

look like a program) e.g.

Input Price,

Calculate Change,

Display Change.

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Example of Design Phase (cont.). I. INPUT.

1. Input Amount Tendered.

2. Input Price.

3. Input Sales Tax.

How is this ambiguous?

How can we make it unambiguous?

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Example of Design Phase (cont.). II. PROCESS.

1. Calculate TaxAmount =

SalesTaxRate * Price.

2. Calculate TotalPrice =

Price + TaxAmount.

3. Calculate Change =

AmountTendered - TotalPrice

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Example of Design Phase (cont.).

III. OUTPUT.

1. Echo print the users input.

2. Display SalesTaxAmount

3. Display TotalPrice.

4. Display Change.

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Test and debug the algorithm. How?

Play computer. Try some data values.

Calculate what the change should be, then

follow the instructions of the algorithm

exactly as a computer would (i.e. blindly),

and see if you get the same result. Also, consider usual and unusual data.

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Test and debug the algorithm

(cont). You may think that our algorithm is

obviously correct.

But, can it handle unusual data?

No, if the AmountTendered is less than the

TotalPrice it will give negative change

instead of recording an error!

A validation check and loop would be

needed to fix this.

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Implementing an algorithm. The algorithm is just an idea in our head /

on paper. It has no power to make a

computer do anything.

So: how can we make the computer execute

the algorithm?

We must translate the algorithm into aprogramming language.

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Implementing an algorithm

(cont.) Why? Why is English unsuitable?

It is too vague. E.g several, a few,

many.

It is ambiguous. E.g. glasses, bat.

It can express actions which a computer

cannot perform e.g. vote your conscience.

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Implementing an algorithm

(cont.) Thus we need a programming language.

Definition: A programming language is a

formal (symbolic) language which is:

(1) precise;

(2) unambiguous;

(3) restricted to operations a CPU can

perform.

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Implementing an algorithm

(cont.) Thus programming requires 2 stages of

translation:

(1) the programmer translates the

algorithm into a programming language;

(2) the compiler translates the program

(source code) into binary (object code).

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Implementing an algorithm

(cont.) The compiler, like Gods law, is totally

unforgiving. It will only produce object

code if there are NO syntax errors. Sinceonly the object code can be run, a program

cannot run unless all syntax errors are

removed. Fortunately, the editor covers a multitude of

sins!