1. Top-Down Design with Functions
Chapter 3
Top-Down Design with Functions
Dr. J.-Y. Pan
Dept. Comm. Eng.
Nat. Chung Cheng Univ.

2. Outline Building a program Top-Down Design and Structure Charts
Case Study Finding Area/Circumference of a circle
Top-Down Design and Structure Charts
Case Study: Drawing Simple Diagrams
Code reuse and functions

3. Building Programs from Existing Information (Design  Implementation)
Programs = Data structures + Algorithms
How to implement a program? You can…
editing data requirements conform to C syntax for constant macro and variable declarations (Data structures)
use the initial algorithm and refinements as program comments, and then write C statements for them (Algorithms)

4. Case Study Finding the Area and Circumference of a circle(1/4)
Step 1: Problem
Get the radius of a circle. Compute and display the circle’s area and circumference.

5. Case Study Finding the Area and Circumference of a circle(2/4)
Step 2: Analysis
Problem Constant PI
Problem Inputs
radius
Problem Outputs
area
circum
Relevant Formulas
Area of a circle = π x radius2
Circumference of a circle = 2 π x radius

6. Case Study Finding the Area and Circumference of a circle(3/4)
Step 3: Design
Initial Algorithm
1. Get the circle radius
2. Calculate the area
3. Calculate the circumference
4. Display the area and the circumference
Algorithm Refinement
2.1 Assign PI * radius * radius to area
3.1 Assign 2 * PI * radius to circum

7. Case Study Finding the Area and Circumference of a circle(4/4)
Step 4: Implementation (Figure 3.2、Figure 3.3)
Step 5: Testing

8. Outline of Program calculating Circle

9. Calculating Area and Circumference of a Circle

10. Top-Down Design and Structure Charts
When a problem is more complex than those we have seen so far…
Must break up the problem into subproblems to develop the program solution, and so on…
Top-down design
A small problem is easier to solve
A picture is worth a thousand words
Structure chart
a documentation tool that shows the relationship among the subproblems of a problem

11. Case Study: Drawing Simple Diagrams(1/2)
Step 1: Problem
You want to draw some simple diagrams on your printer or screen. For example, a female stick figure in Fig.3.9.
Step 2: Analysis
three basic components
a circle
a base line
Intersecting lines

12. Case Study: Drawing Simple Diagrams(2/2)
Step 3: Design (Figure 3.10)
Initial algorithm
1. Draw a circle.
2. Draw a triangle.
3. Draw intersecting lines.
Refinements
2.1 Draw intersecting lines.
2.2 Draw a base.

13. Figure 3.10 Structure Chart for Drawing a Stick Figure

14. Figure 3.11 Function Prototypes and Main Function for Stick Figure

15. Figure 3.12 Function draw_circle

16. Figure 3.13 Function draw_triangle

17. Figure 3.14 Whole Program to Draw a Stick Figure

18. Figure 3.14 Program to Draw a Stick Figure (cont’d)

19. Execution order of subprograms and main Figure 3
Execution order of subprograms and main Figure Flow of Control Between the main Function and a Function Subprogram
2. Allocate memory for
variables declared
1. When executes function call statement,
it transfer control to referenced function.
3. Execute statements in subprogram function
6. Execute next statement in the main function
4. Return control to the main function
5. Release memory

20. Code reuse and functions
Why reinvent the wheel?
C provides predefined functions
Standard library
Appendix B shows ANSI C standard libraries
P.800~801, stdio.h所列之functions
P.799, math.h所列之functions
P.802, string.h所列之functions (教到字串時請自學)
自行參考其功用，大略記一下
function call
y = sqrt(x);
Function sqrt as a “Black Box”
function name
argument

21. Advantages of Using Function Subprograms
Easier to apportion(分配) programming tasks in a team
Procedure abstraction
a programming technique in which a main function consists of a sequence of function calls and each function is implemented separately
Defer implementation details (How to do) from a subproblem (What to do).
Focusing on one function at a time is much easier
Reuse of function subprograms
Functions can be executed more than once
Functions can be used in other programs or functions

22. Functions with Input/output Arguments
Lego blocks “protrusions” and “cups”
Input arguments
Arguments used to pass information into a function subprogram
Output arguments
Arguments used to return results to the calling function

23. Testing Function scale
Actual argument
an expression used inside the parentheses of a function call
Formal parameter
an identifier that represents a corresponding actual argument in a function definition
Testing Function scale

24. Argument List Correspondence (The not rules)
The number of actual arguments used in a call to a function must be the same as the number of formal parameters listed in the function prototype.
The order of arguments in the lists determines correspondence.
Each actual argument must be of a data type that can be assigned to the corresponding formal parameter.

25. The Function Data Area When a function call is executed…
a memory area is allocated for storage of function data (ex. formal parameter, local var)
When function terminates…
function data area is always lost
Again?
Local variable
initially undefined (usually…)

26. Data Areas After Call scale(num_1, num_2);

27. Question?
A good question deserve a good grade…