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S D D  Program  Development  Tools

S D D Program Development Tools






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    S D D  Program  Development  Tools S D D Program Development Tools Presentation Transcript

    • 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:
      5 4 True Frog 0 4 0 Frog True Search (Count) Count Upper Lower FindItem Linear
    • Activity 2
      • Now perform the desk check for Linear(Animals(), “Cat”) call is reproduced below:
      Search (Count) Count Upper Lower FindItem Linear
    • Activity 2
      • Now perform the desk check for Linear(Animals(), “Cat”) call is reproduced below:
      5 4 True Cat 2 Dog 1 Frog 0 4 0 Cat True Search (Count) Count Upper Lower FindItem Linear
    • Activity 3
      • There are actually 2 errors in this program, what are they?
      • The function always returns TRUE regardless of the existence of the search item. Should be set to false.
      • 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.