20061004 chap3

Chapter 3

Top-Down Design with Functions

20061004 chap3 2

Objectives Building Programs from Existing

functions Some Mathematical Library Functions Create you own functions Top-Down Design & Structure Charts Declare Functions Order of Execution of Function

Subprograms and Main function The Function Data Area

20061004 chap3 3

Building Programs from Existing Information

Programmer seldom start off with an empty program

Reasons: Most basic or frequently used functions have been

written by other programmers If you write every function by yourself, the source

code may be messy and hard to maintain Less code, less bugs Deadline Many existing libraries have tuned the

performance, and are more reliable and robust.

20061004 chap3 4

Conversion Program We have used some pre-written I/O

functions provided in C

20061004 chap3 5

Predefined Functions and Code Reuse

A primary goal of software engineering is to write error-free code. Code reuse: reusing program fragments

that have been written and tested. C library functions provide most

commonly used functions. e.g., mathematical library <math.h>

To use existing C library functions, we have to include the header file of the corresponding library. e.g., #include <math.h>

20061004 chap3 6

Square Root computation

20061004 chap3 7

Square Root Program

20061004 chap3 8

Square Root Program (cont’d)

20061004 chap3 9

Some Mathematical Library Functions

20061004 chap3 10

Your own functions?

20061004 chap3 11

Top-Down Design & Structure Charts

Most likely a problem is complex, and we have to break up the problem into subproblems.

Top-Down Design is a method in which we break a problem into subproblems, and derive the solution for the original problem.

Structure Chart is a documentation tool that shows the relationships among the subproblems of a problem.

20061004 chap3 12

Example: Draw Simple Diagrams

House and Female Stick Figure

20061004 chap3 13

House and Female Stick Figure

The house consists of a triangle without its base and a rectangle.

The female stick figure consists of a circle, a triangle, and a triangle without its base.

Suppose we are given four basic drawing components (functions). A circle Parallel lines A base line Intersecting lines

20061004 chap3 14

Structure Chart

20061004 chap3 15

Use and define your own functions One way to implement top-

down design is by defining your own functions. Usually a subproblem is solved by

a corresponding subproblem. Functions without Arguments

Simple Functions that have no arguments and return no value.

20061004 chap3 16

Declare Functions without Arguments Declaration: ftype fname(void); The identifier ftype specifies the

data type of the function results. Example: void fname(void); After fname() is called, the

execution of the program jumps to the subprogram defined by fname.

20061004 chap3 17

Example: Function Prototypes & Main function for Stick Figure

20061004 chap3 18

Function Definitions

20061004 chap3 19

Order of Execution of Function Subprograms and Main function

1. When the computer executes a function call statement, it transfers the control to the function.

2. The computer allocates memory for variables used in the function and executes the statements in the function body.

3. After the execution of the function, the control is returned to the calling function and the allocated memory is released.

20061004 chap3 20

Advantages of Using Function Subprograms

Procedural Abstraction The main function consists of a sequence

of function calls. We defer implementation details until we

are ready to write the subprogram. We can focus on one function at a time.

Reuse of Function Subprograms The subprogram can be executed more

than once in a program Decrease the length of code and chance

of error.

20061004 chap3 21

Function with Arguments

The arguments of a function are used to transfer information between the calling and called functions. Input Arguments are used to pass

information into a subprogram from the calling function. (in this chapter)

Output Arguments are used to return results to the calling function. (in Chap 6)

20061004 chap3 22

Void Functions with Input Arguments We can create functions that receive input

arguments but do not return any results.

20061004 chap3 23

Functions with Input Arguments and A Single Result.

20061004 chap3 24

Example: Single Input & Single Output

20061004 chap3 25

Effect of Executing circum = find_circum (radius);

20061004 chap3 26

Example: Multiple Input Arguments & Single Output

Function scale multiples the first input argument by 10 raised to the power indicated by the second input argument.

20061004 chap3 27

The Function Data Area

20061004 chap3 28

The Function Data Area

20061004 chap3 29

Homework #3 Programming Project 4 (Textbook. Page 141)

due: 2006/10/11

讓使用者輸入一個物體發射的仰角、初速、目標所在的距離

將下列兩個計算寫成兩個 function 並印出你的計算結果 1. 此物體到達目標的時間

2. 此物體到達目標時離地面的高度

2

**)sin(*

2timegtimevelocityheight

)cos(*

tan

velocity

cedistime

20061004 chap3 30

Summary Building Programs from Existing

functions Some Mathematical Library Functions Create you own functions Top-Down Design & Structure Charts Declare Functions Order of Execution of Function

Subprograms and Main function The Function Data Area