S D D Program Development Tools

Implementation

Program Development Techniques

Program Development

AIM Highlight a number of techniques and tools

available to the software developer to assist in code generation.

These resources will help to reduce the number of errors in the final product.

First we will look what makes a well developed solution and what are the errors that may occur.

The Structured Approach

As mentioned before the function of the translator is to report any syntax errors.

However, just because a program is syntactically correct it doesn’t mean that it is free of errors.

Following a structured approach to developing programs, particularly complex one’s is a way of improving the level of ‘correctness’ in a program.

Structured Approach

The structured approach promotes the following concepts: Use of a clear modular structure. One logical task per subroutine. Appropriate Documentation.

Error free code is more likely to be produced under these circumstances.

Modular Structure

The use of modularisation is the breaking down of the program into smaller more achievable sub-programs. (Top-down)

Each module is tested independently and is only included into the project once it is error free.

What are two examples of modelling diagrams that show the top-down approach?

One Logical Task

What do we mean? Making sure that each sub-program only has

one task. e.g. Print, Save, Update. If sub-programs are complex then they

become harder to debug. Restricting sub-programs to one task also

enhances the reusability of the code.

One Logical Task

Example In a program that requires THREE processes

to be carried out. Read an array from file Sort the array Print the array

Each process is a logical task and should therefore be created as a sub-program.

Appropriate Documentation

Clear documentation within the code, such as comments, appropriate identifier names and indenting of control structures will help in the understanding of the logic of the code.

Identifier naming is when you give a variable or constant an intrinsic or meaningful name.

Errors

There are three types of errors. Syntax Logic Run-time

Syntax errors are detected by the translator. Run-time and logic errors occur when the

code is executed.

Syntax

Syntax errors are any errors that prevent the translator from converting the source code into machine code.

Examples of syntax errors include: typing mistakes, incorrect punctuation and spelling mistakes.

Most syntax errors are picked up in the process of syntactical analysis within translation.

Logic Errors

Are errors in the design of code that result in incorrect output.

Examples include continuos loops and incorrect calculations.

i.e. Sum = Num1+Num2/2 Logic errors are the most difficult to detect

and can only be identified through desk checking or peer checking.

Run-Time Errors

Caused through the inability of the computer to perform a task.

The error or bug is usually linked to a software or hardware problem.

Possible sources of run-time errors include: Incorrect BIOS settings Incorrect use or settings of hardware drivers Application software.

Run-Time Errors

Errors in the software are due to: Division By Zero errors – This process is not

defined by mathematics so it cannot be completed by a computer. i.e. Finding the average of a list if the list is empty.

Arithmetic Overflow – Occurs when a value is outside ranges (-32768 to 32767) or there are to many calculations for the processor.

Accessing inappropriate memory locations – occur when assign a value to identifier that doesn’t exist.

Techniques for Detecting Errors

1. Stubs A stub is a small routine which is yet to be

coded into the solution. The use of stubs allows high level modules

to be tested. Stubs do not perform any real processing

they only simulate processing will occur. Stubs are used to determine if parameters

are passed and modules are called.

Techniques for Detecting Errors

2. Flags Are used to indicate that a certain condition

has been met. (Boolean) Flags are usually used in the termination of

a loop. However in debugging they are used to

signify that an event has occurred correctly.

Techniques for Detecting Errors

Debugging Output Statements Is a statement inserted as a line of code to

display the value of variables at a particular point in the execution of a program.

This allows the programmer to monitor the contents of variables to determine the precise point at which an error has occurred.

Activity 1

You are writing a function that sorts an array of data. Unfortunately you have a number of problems.

Discuss how the following techniques could be used to isolate the source of errors in your sort function. Stubs Flags Debugging Output Statements.

Some Important Points

If the function includes calls to other modules these modules could be replaced with stubs where values returned can be known.

Temporary flags could be used to identify whether or not particular statements have been executed.

Debugging output statements could be used to show the contents of variables at crucial points during operation.

Peer Checking

Errors often occur that are impossible to correct for the original programmer.

Colleagues are able to see the problem from a fresh point of view.

Many software development companies require each individual module to be peer checked and formally signed off.

Desk Checking

Is the process of working through a module by hand.

Highlights the contents of each variable and the changes that occur when data is inputted.

Often called a structured walkthrough. The process of working through the code

statement by statement helps to make the logic clear.

Activity 2

Below is a function that will perform a linear search of an array in VB.Public Function Linear(Search(), FindItem) As Boolean

Dim Lower, Upper, Count

Linear = True

Lower = LBound(Search)

Upper = UBound(Search)

Count = Lower

WHILE Count < Upper

IF Search(Count) = SearchItem THEN

Linear = True

Count = Upper

END IF

Count = Count + 1

END Function

Activity 2

Below is the array that is to be searched. Animals(0) = “Frog” Animals(1) = “Dog” Animals(2) = “Cat” Animals(3) = “Cow” Animals(4) = “Ant”

To perform a desk check a call routine is entered i.e. Linear(Animals(), “Frog”)

Activity 2 The desk check for Linear(Animals(), “Frog”) call

is reproduced below:

Linear FindItem Lower Upper Count Search

(Count)

True Frog 0 4 0 Frog

True 4

5

Activity 2 Now perform the desk check for Linear(Animals(),

“Cat”) call is reproduced below:

Linear FindItem Lower Upper Count Search

(Count)

Activity 2 Now perform the desk check for Linear(Animals(),

“Cat”) call is reproduced below:

Linear FindItem Lower Upper Count Search

(Count)

True Cat 0 4 0 Frog

1 Dog

2 Cat

True 4

5

Activity 3

There are actually 2 errors in this program, what are they?

1. The function always returns TRUE regardless of the existence of the search item. Should be set to false.

2. The identifier used as the second parameter is called Finditem yet within the code it is SearchItem.

Activity 4

“If every path and every boundary condition have been tested and the output matches exactly the expected output then surely the code must be free of errors.”

This statement is not true why?

Activity 4

Unexpected inputs with different data types or ranges can cause errors.

Mathematical errors such as division by zero. Arithmetic errors can occur when the code is

logically correct. Communication errors in regards to validating

data sent from outside devices.

Software Tools for Detecting Errors in Code

Most software development environments include tools to assist the programmer in the task of isolating errors in source code.

The following three tools are commonly available to assist in the detection of logic and run-time errors. Breakpoints Program Traces Single Line Stepping

Breakpoints

Are used to temporarily halt the execution of the code.

Once the execution has stopped it is possible to view the current value of variables.

By adding breakpoints at strategic points in the code it is possible to locate the source of the error.

Program Traces

Tracing in terms of error detection, refers to tracking the execution of statements and recording the changing contents of variables.

Many software environments produce a trace file. The file maintains a log of all the transactions that have taken place.

Single-Line Stepping

Is the process of halting the execution after each statement is executed.

After each step, the values of variables can be displayed and minor changes made.

This process can become monotonous. However, variations are provided which allow users to step over functions which may be error-free.

Activity 5

“A subroutine is under development that reads a sequential file of records. As each record is read various calculations are made. A logic error exists as some calculations are incorrect.”

Explain how the use of single line stepping could be used to isolate the problem.

Activity 5

First set up a series of breakpoints to allow the viewing of relevant variables contents.

Start the single line stepping process with the watch window open. Progressively step through the code observing the changes occurring in the window watch.

The records affected by the logic error should now be identifiable.