C Programming for the Absolute Beginner phần 5 potx

a function that contains the logic and structures to handle this procedure and then reuse

that function when needed. Putting all the code into one function that can be called repeat￾edly will save you programming time immediately and in the future if changes to the function

need to be made.

Let me discuss another example using the printf() function (which you are already familiar

with) that demonstrates code reuse. In this example, a programmer has already implemented

the code and structures needed to print plain text to standard output. You simply use the

printf() function by calling its name and passing the desired characters to it. Because the

printf() function exists in a module or library, you can call it repeatedly without knowing

its implementation details, or, in other words, how it was built. Code reuse is truly a pro￾grammer’s best friend!

Information Hiding

Information hiding is a conceptual process by which programmers conceal implementation

details into functions. Functions can be seen as black boxes. A black box is simply a compo￾nent, logical or physical, that performs a task. You don't know how the black box performs

(implements) the task; you just simply know it works when needed. Figure 5.2 depicts the

black box concept.

FIGURE 5.2

Demonstrating

the black box

concept.

Consider the two black box drawings in Figure 5.2. Each black box describes one component;

in this case the components are printf() and scanf(). The reason that I consider the two

functions printf() and scanf() black boxes is because you do not need to know what’s inside

of them (how they are made), you only need to know what they take as input and what they

return as output. In other words, understanding how to use a function while not knowing

how it is built is a good example of information hiding.

Many of the functions you have used so far demonstrate the usefulness of information hiding.

Table 5.1 lists more common library functions that implement information hiding in struc￾tured programming.

Chapter 5 • Structured Programming 113

If you’re still put off by the notion of information hiding or black boxes, consider the following

question. Do most people know how a car’s engine works? Probably not, most people are only

concerned that they know how to operate a car. Fortunately, modern cars provide an interface

from which you can easily use the car, while hiding its implementation details. In other

words, one might consider the car's engine the black box. You only know what the black box

takes as input (gas) and what it gives as output (motion).

Going back to the printf() function, what do you really know about it? You know that the

printf() function prints characters you supply to the computer’s screen. But do you know

how the printf() function really works? Probably not, and you don’t need to. That’s a key

concept of information hiding.

In structured programming you build components that can be reused (code reusability) and

that include an interface that other programmers will know how to use without needing to

understand how they were built (information hiding).

FUNCTION PROTOTYPES

Function prototypes tell C how your function will be built and used. It is a common program￾ming practice to construct your function prototype before the actual function is built. That

statement was so important it is worth noting again. It is common programming practice

to construct your function prototype before the actual function is built.

Programmers must think about the desired purpose of the function, how it will receive

input, and how and what it will return. To demonstrate, take a look at the following function

prototype.

TABLE 5.1 C OMMON LIBRARY F UNCTIONS

Library Name Function Name Description

Standard input/output scanf() Reads data from the keyboard

Standard input/output printf() Prints data to the computer monitor

Character handling isdigit() Tests for decimal digit characters

Character handling islower() Tests for lowercase letters

Character handling isupper() Tests for uppercase letters

Character handling tolower() Converts character to lowercase

Character handling toupper() Converts character to uppercase

Mathematics exp() Computes the exponential

Mathematics pow() Computes a number raised to a power

Mathematics sqrt() Computes the square root

114 C Programming for the Absolute Beginner, Second Edition