ch03.htm

good book for learning c++ standard language

long</FONT></H3>
</CENTER>
<P>One source of confusion for new C++ programmers is when to declare a variable
to be type <TT>long</TT> and when to declare it to be type <TT>short</TT>. The rule,
when understood, is fairly straightforward: If there is any chance that the value
you'll want to put into your variable will be too big for its type, use a larger
type.</P>
<P>As seen in Table 3.1, <TT>unsigned short</TT> integers, assuming that they are
two bytes, can hold a value only up to 65,535. <TT>Signed</TT> <TT>short</TT> integers
can hold only half that. Although <TT>unsigned long</TT> integers can hold an extremely
large number (4,294,967,295) that is still quite finite. If you need a larger number,
you'll have to go to <TT>float</TT> or <TT>double</TT>, and then you lose some precision.
Floats and doubles can hold extremely large numbers, but only the first 7 or 19 digits
are significant on most computers. That means that the number is rounded off after
that many digits.
<CENTER>
<H4><A NAME="Heading23"></A><FONT COLOR="#000077">Wrapping Around an unsigned Integer</FONT></H4>
</CENTER>
<P>The fact that <TT>unsigned long</TT> integers have a limit to the values they
can hold is only rarely a problem, but what happens if you do run out of room?</P>
<P>When an <TT>unsigned</TT> integer reaches its maximum value, it wraps around and
starts over, much as a car odometer might. Listing 3.4 shows what happens if you
try to put too large a value into a <TT>short</TT> integer.</P>
<P><A NAME="Heading24"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 3.4.A demonstration
of putting too large a value in an unsigned integer.</B></FONT></P>
<PRE><FONT COLOR="#0066FF">1: #include &lt;iostream.h&gt;
2:  int main()
3:  {
4:     unsigned short int smallNumber;
5:     smallNumber = 65535;
6:     cout &lt;&lt; &quot;small number:&quot; &lt;&lt; smallNumber &lt;&lt; endl;
7:     smallNumber++;
8:     cout &lt;&lt; &quot;small number:&quot; &lt;&lt; smallNumber &lt;&lt; endl;
9:     smallNumber++;
10:    cout &lt;&lt; &quot;small number:&quot; &lt;&lt; smallNumber &lt;&lt; endl;
11:        return 0;
<TT>12: }</TT>
Output: small number:65535
small number:0
small number:1
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis:</B></FONT><B> </B>On line 4, <TT>smallNumber</TT>
is declared to be an <TT>unsigned short int</TT>, which on my computer is a two-byte
variable, able to hold a value between 0 and 65,535. On line 5, the maximum value
is assigned to <TT>smallNumber</TT>, and it is printed on line 6. <BR>
<BR>
On line 7, <TT>smallNumber</TT> is incremented; that is, 1 is added to it. The symbol
for incrementing is <TT>++</TT> (as in the name C++--an incremental increase from
C). Thus, the value in <TT>smallNumber</TT> would be <TT>65,536</TT>. However, <TT>unsigned</TT>
<TT>short</TT> integers can't hold a number larger than 65,535, so the value is wrapped
around to <TT>0</TT>, which is printed on line 8.</P>
<P>On line 9 <TT>smallNumber</TT> is incremented again, and then its new value, <TT>1</TT>,
is printed.
<CENTER>
<H4><A NAME="Heading26"></A><FONT COLOR="#000077">Wrapping Around a signed Integer</FONT></H4>
</CENTER>
<P>A <TT>signed</TT> integer is different from an <TT>unsigned</TT> integer, in that
half of the values you can represent are negative. Instead of picturing a traditional
car odometer, you might picture one that rotates up for positive numbers and down
for negative numbers. One mile from 0 is either 1 or -1. When you run out of positive
numbers, you run right into the largest negative numbers and then count back down
to 0. Listing 3.5 shows what happens when you add 1 to the maximum positive number
in an <TT>unsigned short</TT> integer.</P>
<P><A NAME="Heading27"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 3.5. A demonstration
of adding too large a number to a signed integer.</B></FONT></P>
<PRE><FONT COLOR="#0066FF">1:  #include &lt;iostream.h&gt;
2:  int main()
3:  {
4:     short int smallNumber;
5:     smallNumber = 32767;
6:     cout &lt;&lt; &quot;small number:&quot; &lt;&lt; smallNumber &lt;&lt; endl;
7:     smallNumber++;
8:     cout &lt;&lt; &quot;small number:&quot; &lt;&lt; smallNumber &lt;&lt; endl;
9:     smallNumber++;
10:    cout &lt;&lt; &quot;small number:&quot; &lt;&lt; smallNumber &lt;&lt; endl;
11:        return 0;
<TT>12: }</TT>
Output: small number:32767
small number:-32768
small number:-32767
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis: </B></FONT>On line 4, <TT>smallNumber</TT>
is declared this time to be a <TT>signed</TT> <TT>short</TT> integer (if you don't
explicitly say that it is <TT>unsigned</TT>, it is assumed to be <TT>signed</TT>).
The program proceeds much as the preceding one, but the output is quite different.
To fully understand this output, you must be comfortable with how <TT>signed</TT>
numbers are represented as bits in a two-byte integer. For details, check Appendix
C, &quot;Binary and Hexadecimal.&quot; <BR>
<BR>
The bottom line, however, is that just like an <TT>unsigned</TT> integer, the <TT>signed</TT>
integer wraps around from its highest positive value to its highest negative value.
<CENTER>
<H3><A NAME="Heading29"></A><FONT COLOR="#000077">Characters</FONT></H3>
</CENTER>
<P>Character variables (<TT>type char</TT>) are typically 1 byte, enough to hold
256 values (see Appendix C). A <TT>char</TT> can be interpreted as a small number
(0-255) or as a member of the ASCII set. ASCII stands for the American Standard Code
for Information Interchange. The ASCII character set and its ISO (International Standards
Organization) equivalent are a way to encode all the letters, numerals, and punctuation
marks.


<BLOCKQUOTE>
	<P>
<HR>
Computers do not know about letters, punctuation, or sentences. All they understand
	are numbers. In fact, all they really know about is whether or not a sufficient amount
	of electricity is at a particular junction of wires. If so, it is represented internally
	as a <TT>1</TT>; if not, it is represented as a <TT>0</TT>. By grouping ones and
	zeros, the computer is able to generate patterns that can be interpreted as numbers,
	and these in turn can be assigned to letters and punctuation. 
<HR>


</BLOCKQUOTE>

<P>In the ASCII code, the lowercase letter &quot;a&quot; is assigned the value <TT>97</TT>.
All the lower- and uppercase letters, all the numerals, and all the punctuation marks
are assigned values between 1 and 128. Another 128 marks and symbols are reserved
for use by the computer maker, although the IBM extended character set has become
something of a standard.
<CENTER>
<H4><A NAME="Heading30"></A><FONT COLOR="#000077">Characters and Numbers</FONT></H4>
</CENTER>
<P>When you put a character, for example, <TT>`a'</TT>, into a <TT>char</TT> variable,
what is really there is just a number between 0 and 255. The compiler knows, however,
how to translate back and forth between characters (represented by a single quotation
mark and then a letter, numeral, or punctuation mark, followed by a closing single
quotation mark) and one of the ASCII values.</P>
<P>The value/letter relationship is arbitrary; there is no particular reason that
the lowercase &quot;a&quot; is assigned the value <TT>97</TT>. As long as everyone
(your keyboard, compiler, and screen) agrees, there is no problem. It is important
to realize, however, that there is a big difference between the value <TT>5</TT>
and the character <TT>`5'</TT>. The latter is actually valued at <TT>53</TT>, much
as the letter <TT>`a'</TT> is valued at <TT>97</TT>.</P>
<P><A NAME="Heading31"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 3.6. Printing
characters based on numbers</B></FONT></P>
<PRE><FONT COLOR="#0066FF">1:   #include &lt;iostream.h&gt;
2:   int main()
3:   {
4:   for (int i = 32; i&lt;128; i++)
5:         cout &lt;&lt; (char) i;
6:         return 0;
<TT>7: }</TT>
 Output: !&quot;#$%G'()*+,./0123456789:;&lt;&gt;?@ABCDEFGHIJKLMNOP
_QRSTUVWXYZ[\]^'abcdefghijklmnopqrstuvwxyz&lt;|&gt;~s
</FONT></PRE>
<P>This simple program prints the character values for the integers 32 through 127.
<CENTER>
<H4><A NAME="Heading33"></A><FONT COLOR="#000077">Special Printing Characters</FONT></H4>
</CENTER>
<P>The C++ compiler recognizes some special characters for formatting. Table 3.2
shows the most common ones. You put these into your code by typing the backslash
(called the escape character), followed by the character. Thus, to put a tab character
into your code, you would enter a single quotation mark, the slash, the letter t,
and then a closing single quotation mark:</P>
<PRE><FONT COLOR="#0066FF">char tabCharacter = `\t';
</FONT></PRE>
<P>This example declares a <TT>char</TT> variable (<TT>tabCharacter</TT>) and initializes
it with the character value <TT>\t</TT>, which is recognized as a tab. The special
printing characters are used when printing either to the screen or to a file or other
output device.</P>

<DL>
	<DD>
<HR>
<FONT COLOR="#000077"><B>New Term:</B></FONT><B> </B>An <I>escape character</I> changes
	the meaning of the character that follows it. For example, normally the character
	<TT>n</TT> means the letter n, but when it is preceded by the escape character (<TT>\</TT>)
	it means new line. 
<HR>

</DL>

<P><FONT SIZE="4"><B>Table 3.2. The Escape Characters</B></FONT>. 
<TABLE BORDER="0">
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT"><B><I>Character</I></B></TD>
		<TD ALIGN="LEFT"><B><I>What it means</I></B></TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT"><TT>\n</TT></TD>
		<TD ALIGN="LEFT">new line</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT"><TT>\t</TT></TD>
		<TD ALIGN="LEFT">tab</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT"><TT>\b</TT></TD>
		<TD ALIGN="LEFT">backspace</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT"><TT>\&quot;</TT></TD>
		<TD ALIGN="LEFT">double quote</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT"><TT>\'</TT></TD>
		<TD ALIGN="LEFT">single quote</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT"><TT>\?</TT></TD>
		<TD ALIGN="LEFT">question mark</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT"><TT>\\</TT></TD>
		<TD ALIGN="LEFT">backslash</TD>
	</TR>
</TABLE>

<CENTER>
<H3><A NAME="Heading34"></A><FONT COLOR="#000077">Constants</FONT></H3>
</CENTER>
<P>Like variables, constants are data storage locations. Unlike variables, and as
the name implies, constants don't change. You must initialize a constant when you
create it, and you cannot assign a new value later.
<CENTER>
<H4><A NAME="Heading35"></A><FONT COLOR="#000077">Literal Constants</FONT></H4>
</CENTER>
<P>C++ has two types of constants: literal and symbolic.</P>
<P>A literal constant is a value typed directly into your program wherever it is
needed. For example</P>
<PRE><FONT COLOR="#0066FF">int myAge = 39;
</FONT></PRE>
<P><TT>myAge</TT> is a variable of type <TT>int</TT>; <TT>39</TT> is a literal constant.
You can't assign a value to <TT>39</TT>, and its value can't be changed.
<CENTER>
<H4><A NAME="Heading36"></A><FONT COLOR="#000077">Symbolic Constants</FONT></H4>
</CENTER>
<P>A symbolic constant is a constant that is represented by a name, just as a variable
is represented. Unlike a variable, however, after a constant is initialized, its
value can't be changed.</P>
<P>If your program has one integer variable named <TT>students</TT> and another named
<TT>classes</TT>, you could compute how many students you have, given a known number
of classes, if you knew there were 15 students per class:</P>
<PRE><FONT COLOR="#0066FF">students = classes * 15;
</FONT></PRE>


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>NOTE:</B></FONT><TT><B> </B>*</TT> indicates multiplication.
	
<HR>


</BLOCKQUOTE>

<P>In this example, <TT>15</TT> is a literal constant. Your code would be easier
to read, and easier to maintain, if you substituted a symbolic constant for this
value:</P>
<PRE><FONT COLOR="#0066FF">students = classes * studentsPerClass
</FONT></PRE>
<P>If you later decided to change the number of students in each class, you could
do so where you define the constant <TT>studentsPerClass</TT> without having to make
a change every place you used that value.</P>
<P>There are two ways to declare a symbolic constant in C++. The old, traditional,
and now obsolete way is with a preprocessor directive, <TT>#define</TT>. Defining
Constants with #define To define a constant the traditional way, you would enter
this:</P>
<PRE><FONT COLOR="#0066FF">#define studentsPerClass 15
</FONT></PRE>
<P>Note that <TT>studentsPerClass</TT> is of no particular type (<TT>int</TT>, <TT>char</TT>,
and so on). <TT>#define</TT> does a simple text substitution. Every time the preprocessor
sees the word <TT>studentsPerClass</TT>, it puts in the text <TT>15</TT>.</P>
<P>Because the preprocessor runs before the compiler, your compiler never sees your
constant; it sees the number <TT>15</TT>. Defining Constants with const Although
<TT>#define</TT> works, there is a new, much better way to define constants in C++:</P>
<PRE><FONT COLOR="#0066FF">const unsigned short int studentsPerClass = 15;
</FONT></PRE>
<P>This example also declares a symbolic constant named <TT>studentsPerClass</TT>,
but this time <TT>studentsPerClass</TT> is typed as an <TT>unsigned short int</TT>.
This method has several advantages in making your code easier to maintain and in
preventing bugs. The biggest difference is that this constant has a type, and the
compiler can enforce that it is used according to its type.


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>NOTE:</B></FONT><B> </B>Constants cannot be changed while
	the program is running. If you need to change <TT>studentsPerClass</TT>, for example,
	you need to change the code and recompile. 
<HR>
</P>