Qualities of software and its development

Transcription

1 1 / 17 Qualities of software and its development Miaoqing Huang University of Arkansas Spring 2010

2 Representative qualities 2 / 17 Correctness Robustness Performance Usability Maintainability Portability Process qualities

3 Correctness 3 / 17 A software has to function correctly, i.e., meeting the requirement specifications Methods to assess the correctness Experimental testing, i.e., using test bench Example: Encryption algorithm AES, Plaintext: 0x aabbccddeeff Key: 0x a0b0c0d0e0f Ciphertext:0x69c4e0d86a7b0430d8cdb78070b4c55a Formal analysis Example: knapsack problem Approaches to improving correctness Use high-level languages (e.g., Java, C) instead of low-level or middle level languages (e.g., assembly language) Use standard (and optimized) libraries and algorithms Don t reinvent the wheel!!!

4 Knapsack Problem? 4 / 17 Given a set of items, each with a weight and a profit, determine the number of each item to include in a collection so that the total weight is less than a given limit and the total profit is as large as possible

5 Knapsack Problem? 5 / 17 Given a set of items, each with a weight and a profit, determine the number of each item to include in a collection so that the total weight is less than a given limit and the total profit is as large as possible Use dynamic programming approach

6 Robustness 6 / 17 A software is able to behaves correctly or reasonably in circumstances, which is not anticipated in requirement specification Examples: hardware failure, incorrect or unanticipated input data Algorithm 1: Quick Sort Input: A sequence of unsorted numbers Output: A sequence of numbers in increasing order Start sorting right away;

7 Robustness 7 / 17 A software is able to behaves correctly or reasonably in circumstances, which is not anticipated in requirement specification Examples: hardware failure, incorrect or unanticipated input data Algorithm 2: Quick Sort Input: A sequence of unsorted numbers Output: A sequence of numbers in increasing order Check the inputs to make sure (1) all inputs are numbers, (2) the length of inputs are within the acceptance region...; Start sorting right away;

8 Robustness 8 / 17 A software is able to behaves correctly or reasonably in circumstances, which is not anticipated in requirement specification Examples: hardware failure, incorrect or unanticipated input data...

9 Performance 9 / 17 The performance of a software consists of two components Speed Resource requirement, e.g., memory, disk space, and power Mutual influence among performance, productivity & scalability Java programs generally are easier to program (i.e., more productivity), but slower in speed (i.e., less performance) compared with C programs A software that requires much resources generally is difficult to scale Evaluate the performance of a software system Two factors contributing to the software system Algorithm, mostly affecting speed Data structures, affecting both speed and resource requirement Three approaches Mathematical analysis of time and space complexity of the algorithms and the data structures Simulate the product Measure the performance of the product

10 Performance Example 10 / 17 Sorting The most straightforward way Algorithm 3: Straightforward Sort Input: A sequence of n unsorted numbers stored in link ln A Output: A sequence of numbers in increasing order stored in link ln B for i=0:n 1 do Walk down link ln A to find the smallest element x; /*n i operations*/ Add x at the tail of link ln B ; /*1 operation*/ Delete x from ln A ; /*1 operation*/ O(T ) = O(n) + O(n 1) + O(n 2) + + O(2) + O(1) = O(n 2 )

11 Performance Example 11 / 17 Sorting Quick Sort (divide and conquer approach) Algorithm 4: Quick Sort Input: A array of unsorted number, A[0..n 1] Output: A array of sorted number in increasing order, A[0..n 1] Take any arbitrary element of the input array A[0..n 1], say A[0]; Partition A[0..n 1] around A[0] into two parts: the left part consisting of elements A[0], and the right part consisting of elements > A[0]; Sort the left part and the right part recursively; O(T ) = O(n log n)

12 Usability 12 / 17 Ease of Use / User friendliness Have different targets Novice user: a window interface Experienced user: standardized hotkeys, commands (e.g., vim editor) and advanced features pipe, shell scripts under Unix/Linux Development team member: modules that are easy to be configured through module interface (e.g., parameters or arguments) Achieve ease of use through standardization Example: follow the standard way to design interface on Windows platform

13 Maintainability 13 / 17 Modify the existing software code to Fix bugs Enhance features Three categories of software maintenance Corrective (20%): removing residual errors Adaptive (20%): adjusting to environment changes New OS, new database system, or new file system Perfective (>50%): quality improvements New software release (e.g., Adobe Acrobat) Two sub-qualities Reparability: ability to correct defects in reasonable time Modularize the design properly Use proper languages and tools Evolvability: ability to adapt software to environment changes and to improve it in reasonable time

14 Portability 14 / 17 Portability Software can run on different hardware platforms or software environments Example In 1980s, application needed to take care of the endianness of the processor, i.e., big-endian on Motorola processor, little-endian on Intel processor On a 32-bit processor, long int is 32-bit; on a 64-bit processor, it may be 64-bit Improve the portability through Modularization, i.e., only a few modules need to be modified to port software to a different platform Use high-level languages that support portability Java, C

15 Understandability 15 / 17 Understandability Good documentation and good programming style Give detailed description for each subroutine, including function, algorithm and data structure (if they are complex) Use indention, {} or pair of begin/end to define the start and the end of code block, use () to clarify logic combination In python language, indention is used to define a code block...

16 Understandability 16 / 17 Understandability Good documentation and good programming style Give detailed description for each subroutine, including function, algorithm and data structure (if they are complex) Use indention, {} or pair of begin/end to define the start and the end of code block, use () to clarify logic combination In python language, indention is used to define a code block...

17 Typical process qualities 17 / 17 Productivity Team management and individual effort Tradeoff between productivity and performance Timeliness Visibility All of its steps and current status are documented clearly and transparent Use proper tools, such as Subversion (SVN)