03-Program-Design

2009 S2 COMP1921 3. Program Design
Oliver Diessel odiessel@.au
Last updated: 13:00 8 Aug 2009
3. Program design
Outline:
Review program design techniques Practice these on the substr example The importance of testing A couple of examples to practice on Next week’s practice exam problem
Note: “position” here refers to the character position in the string where the substring starts, so if the substring is found to start at the leftmost character of the string, it is said to be found at position 1
COMP1921_09s2
Week 3
3. Program Design 3
Program development procedure
1.
Understand the specification
Determine what outputs, inputs and processing are required
End of substring, Chars don’t match, or End of search string
4.
5. 6. 7.
8.
If at end of substring, return position of first char of substring in string If at end of search string, return 0 Reset subp to start of substring Set strp to position after the candidate position where we previously matched Continue from step 1.
Week 3 3. Program Design 9
COMP1 development
Notes:
1. 2.
3.
We have a big loop over steps 1-8 There are multiple exit points from the loop at steps 2, 4 & 5 We can’t have multiple return points, so we will need to use a flag to indicate we are “finished” and set a “return value” i.e. we will use a structured programming approach
Write a rough solution in pseudocode (a natural language style that approximates what you would write in a programming language, but doesn’t worry about syntax or low-level details) Start at a high level, numbering each step, and then refine your solution by adding sub-steps that include more and more detail You should eventually aim for a one-to-one correspondence between the level of detail in your pseudocode and the actual statements you would write in C – note that this algorithmic elaboration is iterative in itself
Facilitated through the development of a test harness to implement your program from the start Use regression testing to check that all previously correct functions continue to operate correctly when new functionality is added
5.
Repeat the above steps until the program works as desired
Week 3 3. Program Design 5
COMP1921_09s2
Example: substring position
Let’s return to the problem we began to consider at the end of last week and apply the above techniques to its solution:
COMP1921_09s2 Week 3 3. Program Design 6
Substring position
T h i s
strp
i s
a
t e s t \0 …
i s \0 …
subp
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Week 3
3. Program Design 7
Substring position
// return the position of the first // occurrence of the substring // needle in search string haystack; // return 0 if needle is not found int substr(char *haystack, char *needle);
3.
Elaborate the functions identified in step 2. one by one
An approach that can work well is to start by implementing a skeleton of the code in the form of a main function together with its calls to function stubs (a test harness) Identify data structures, e.g., number and type of variables that would be helpful
candp
T h i s
i s
strp
a
t e s t \0 …
i s
b \0 …
subp
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Week 3
3. Program Design 8
substr algorithm development
1. 2. 2a) 3.
a) b) c)
Advance strp until match or until end of string reached If at end of string, return 0 Remember the candidate position Advance both strp and subp until
2.
Decompose the processing into a sequence of high level sub-problems
Identify repeating sub-problems and natural boundaries in terms of the data that needs to be processed and the type of operations that need to be performed This is known as functional decomposition
COMP1921_09s2
Week 3
3. Program Design 4
Program development procedure
3.
Elaborate the functions identified (cont’d)
Design algorithms to solve each sub-problem
Who hasn’t tried to help a friend hunt down a bug and found their code too difficult to understand?
Use a unit testing approach to test each “part” of a program in relative isolation
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Week 3
3. Program Design 10
Testing
Avoid putting off testing until your whole program is implemented!
Tracking down bugs, even in 10s of lines (let alone 1000s) can be very tedious and unpredictable
4.
Test each function as it is developed