Chapter03 Top Down Design with Function

Chapter 3
Top-Down
Design with
Functions
Instructor:
Kun-Mao Chao
( 台大資工趙坤
茂 )

Copyright ©2004 Pearson
Addison-Wesley. All rights
reserved. 3-2
Building Programs from Existing
Programs
• Programmers 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 maintained.
– Less code, less bugs.
– You have product (or homework) deadline.
– Many existing libraries have tune the performance, and
thus are more reliable and robust.

reserved. 3-3
The Previous miles-to-kilometers
Conversion Program
We have used some pre-written I/O functions provided in C.

reserved. 3-4
Predefined Functions and Code Reuse
• The 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>

reserved. 3-5
Some Mathematical C Library
Functions
Function Header File Description
abs(x) <stdlib.h> Return the absolute value of x
cos(x) <math.h> Return the cosine of angle x
exp(x) <math.h> Return the value of ex
pow(x, y) <math.h> Return the value of xy
sqrt(x) <math.h> Return the squre root of x

reserved. 3-6
An Example: The Usage of sqrt
Function (1/2)
Suppose we want to write a program which
reads a double value from the user.
Then it computes and outputs the square root
of the double value.

reserved. 3-7
An Example: The Usage of sqrt
Function (2/2)
Invoke the sqrt
functions

reserved. 3-8
Top-Down Design and 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, solve those
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.

reserved. 3-9
An Example: House and Stick Figures
House figure Stick figure
You are required to draw a house figure and a
female stick figure.

reserved. 3-10
An Example: House and Stick Figures
• The house consists of a triangle without its
base and a rectangle.
• The stick 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,
– and intersecting lines.

reserved. 3-11
Structure Chart for Drawing a Stick
Figure
Because the triangle is not a basic component, we have
to draw intersecting lines and a base to form a triangle.

reserved. 3-12
Declare Functions without Arguments
• One way that programmers implement top-down design
is by defining their own functions.
– Usually a subproblem is solved by a corresponding
subprogram.
• The programmer can create their own functions.
• Functions without arguments:
– Declaration: ftype fname(void);
– The identifier ftype specifies the data type of the function
result.
– Example: void fname(void);
– After fname() is called, the execution of the program
jumps to the subprogram defined by fname.

reserved. 3-13
An Example: Function Prototypes and
Main Function for Stick Figure (1/2)
The self-created functions must
be declared in advance.
Invoke declared
functions

reserved. 3-14
An Example: Function Prototypes and
Main Function for Stick Figure (2/2)
The subprogram (or called subfunction, subroutine)
corresponds to the draw_circle function.

reserved. 3-15
Flow of Control Between the Main
Function and a Subprogram
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 execute 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.

reserved. 3-16
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 subprograms can be executed more than once
in a program.
– Decrease the length of code and chance of error.

reserved. 3-17
Declare Functions 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.
– Output arguments are used to return results to
the calling function.

reserved. 3-18
Void Functions with Input Arguments
• We can create functions that receive input arguments
but does not return any result.
– void fname(type param1, type param2, …);

reserved. 3-19
Functions with Input Arguments and a
Single Result
• We can create functions which receive input arguments
and return a single result.
– type fname(type param1, type param2, …);

reserved. 3-20
An Example: Functions with a Single
Input Argument and A Result (1/2)
This function receives a double input
argument and returns a double value.
This function receives a double input
argument and returns a double value.

reserved. 3-21
An Example: Functions with a Single
Input Argument and A Result (2/2)
• After finishing execution of the subprogram, the
returned result may be stored into a variable.
– After invoking and executing find_circum(radius), the
variable circum is equal to 62.8318.

reserved. 3-22
An Example: Functions with Multiple
Input Arguments and A Result
• Function scale multiplies the first input argument by
10 raised to the power indicated by the second input
argument.
– e.g., scale(2.5, 2) returns 2.5 * 102
= 250.

reserved. 3-23
Data Areas After Call scale(num_1,
num_2)
• The local
variables x, n, and
scale_factor can
not be accessed in
the main function.
• The variables
num_1 and
num_2 in the
main function can
not be accessed in
the scale function. The value of scale_factor
is not ready.

reserved. 3-24
Homework #2
• Write a program to take a depth as input data.
• Compute and display the temperature in both
degree Celsius and degree Fahrenheit at this
depth inside the earth.
• Two useful formulas:
– Celsius = 10*(depth) + 20;
– Fahrenheit = 1.8*(Celsius) + 32;
• You are required to create two subfunctions to
compute the above two formulas respectively.
• Due date: to be determined by TA (demo in lab.
lecture)

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