soft_engg_lecture18 - Software Engineering Asim Banerjee...

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Software Engineering Asim Banerjee 23 rd March 2010 [email protected]
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IT314 - Software Engineering 2 Lecture Seating Plan (Sample) #1 #2 #3 #4 #5 #6 #7 #8 #9 Stage #10 #11 #12 #13 #20 #21 #24 #23 #22
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IT314 - Software Engineering 3 Error, Fault & Failure (1/2) The IEEE definitions of the three terms are Error is used in two different ways Discrepancy between a computed, observed or measured value and the true, specified or theoretically calculated value i.e. it is a measure of the difference between the actual and the ideal. Human action that results in software containing a defect or fault. NOTE: This definition is quite general and encompasses all phases. Fault is a condition that causes a system to fail in performing its required function
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IT314 - Software Engineering 4 Error, Fault & Failure (2/2) The IEEE definitions of the three terms are (Continued) NOTE: Fault is the basic reason for software to malfunction and is synonymous with the commonly used term bug. Failure is the inability of a system or component to perform a required function according to its specifications. NOTE: Failure is produced only when there is a fault in the system, but presence of a fault does not guarantee a failure.
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IT314 - Software Engineering 5 Debugging (1/3) What is Debugging? Once the existence of an error is known, it is necessary to locate it (and fix it) Brute force method: Involves incorporating (instrumenting) “print statements” to print intermediate values with the hope that this will help in manual debugging Note : This is the most common but least efficient method of debugging Backtracking: Beginning from the location where the symptom is observed, source code is tracked backwards till the error is discovered Note : With increase in the size of code, the number of paths to be back tracked also increases.
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IT314 - Software Engineering 6 Debugging (2/3) Cause elimination approach: List of cause (which could possibly affect the outcome) is made Tests are conducted to eliminate these causes one by one (“fault tree analysis” is a related technique) Program slicing: This technique is similar to back tracking It reduces the search space by defining slices Note : “Slice of a program for a particular variable for a particular statement” is a set of source lines preceding this statement that can influence the value of that variable.
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IT314 - Software Engineering 7 Debugging (3/3) General guidelines for debugging: Thorough understanding of design is essential Do not treat the symptom, but treat the cause Note : Sometimes, it may be necessary to completely redesign the software Beware that, while attempting to fix an error, you may introduce some new errors
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IT314 - Software Engineering 8 Test Summary Report This report is generated towards the end of testing It provides summary of tests applied to the entire software i.e. all modules How many test cases applied and to which module How many test cases were successful How many test cases were unsuccessful (including the degree of unsuccessfulness)
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IT314 - Software Engineering 9 Any Questions?
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