ch21.htm

Why C++ is the emerging standard in software development. The steps to develop a C++ program. How

14:       cin.getline(stringOne,80);
15:
16:       cout << "Enter a second string: ";
17:       cin.getline(stringTwo,80);
18:
19:       cout << "String One: " << stringOne << endl;
20:       cout << "String Two: " << stringTwo << endl;
21:
22:       strcat(stringOne," ");
23:       strncat(stringOne,stringTwo,10);
24:
25:       cout << "String One: " << stringOne << endl;
26:       cout << "String Two: " << stringTwo << endl;
27:
28:     return 0;
<TT>29: }</TT></FONT>
<FONT COLOR="#0066FF">
Output: Enter a string: Oh beautiful
Enter a second string: for spacious skies for amber waves of grain
String One: Oh beautiful
String Two: for spacious skies for amber waves of grain
String One: Oh beautiful for spacio
String Two: for spacious skies for amber waves of grain
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis:</B></FONT><B> </B>On lines 7 and 8, two character
arrays are created, and the user is prompted for two strings, which are put into
the two arrays.<BR>
<BR>
A space is appended to <TT>stringOne</TT> on line 22, and on line 23, the first ten
characters of <TT>stringTwo</TT> are appended to <TT>stringOne</TT>. The result is
printed on lines 25 and 26.
<H4 ALIGN="CENTER"><A NAME="Heading11"></A><FONT COLOR="#000077">Other String Functions</FONT></H4>
<P>The string library provides a number of other string functions, including those
used to find occurrences of various characters or &quot;tokens&quot; within a string.
If you need to find a comma or a particular word as it occurs in a string, look to
the string library to see whether the function you need already exists.
<H3 ALIGN="CENTER"><A NAME="Heading12"></A><FONT COLOR="#000077">Time and Date</FONT></H3>
<P>The time library provides a number of functions for obtaining a close approximation
of the current time and date, and for comparing times and dates to one another.</P>
<P>The center of this library is a structure, <TT>tm</TT>, which consists of nine
integer values for the second, minute, hour, day of the month, number of the month
(where January=0), the number of years since 1900, the day (where Sunday=0), the
day of the year (0-365), and a Boolean value establishing whether daylight saving
time is in effect. (This last may not be supported on some systems.)</P>
<P>Most time functions expect a variable of type <TT>time_t</TT> or a pointer to
a variable of this type. There are conversion routines to turn such a variable into
a <TT>tm</TT> data structure.</P>
<P>The standard library supplies the function <TT>time()</TT>, which takes a pointer
to a <TT>time_t</TT> variable and fills it with the current time. It also provides
<TT>ctime()</TT>, which takes the <TT>time_t</TT> variable filled by <TT>time()</TT>
and returns an ASCII string that can be used for printing. If you need more control
over the output, however, you can pass the <TT>time_t</TT> variable to <TT>local_time()</TT>,
which will return a pointer to a <TT>tm</TT> structure. Listing 21.5 illustrates
these various time functions.</P>

<P><A NAME="Heading13"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 21.5. Using
ctime().</B></FONT>
<PRE><FONT COLOR="#0066FF">1:     #include &lt;time.h&gt;
2:     #include &lt;iostream.h&gt;
3:
4:     int main()
5:     {
6:        time_t currentTime;
7:
8:        // get and print the current time
9:        time (&amp;currentTime); // fill now with the current time
10:       cout &lt;&lt; &quot;It is now &quot; &lt;&lt; ctime(&amp;currentTime) &lt;&lt; endl;
11:
12:       struct tm * ptm= localtime(&amp;currentTime);
13:
14:       cout &lt;&lt; &quot;Today is &quot; &lt;&lt; ((ptm-&gt;tm_mon)+1) &lt;&lt; &quot;/&quot;;
15:       cout &lt;&lt; ptm-&gt;tm_mday &lt;&lt; &quot;/&quot;;
16:       cout &lt;&lt; ptm-&gt;tm_year &lt;&lt; endl;
17:
18:       cout &lt;&lt; &quot;\nDone.&quot;;
19:     return 0;
<TT>20: }</TT>
Output: It is now Mon Mar 31 13:50:10 1997

Today is 3/31/97

Done.
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis:</B></FONT><B> </B>On line 6, <TT>CurrentTime</TT>
is declared to be a variable of type <TT>time_t</TT>. The address of this variable
is passed to the standard time library function <TT>time()</TT>, and the variable
<TT>currentTime</TT> is set to the current date and time. The address of this variable
is then passed to <TT>ctime()</TT>, which returns an ASCII string that is in turn
passed to the <TT>cout</TT> statement on line 12.The address of <TT>currentTime</TT>
is then passed to the standard time library function <TT>localtime()</TT>, and a
pointer to a <TT>tm</TT> structure is returned, which is used to initialize the local
variable <TT>ptm</TT>. The member data of this structure is then accessed to print
the current month, day of the month, and year. 
<H3 ALIGN="CENTER"><A NAME="Heading14"></A><FONT COLOR="#000077">stdlib</FONT></H3>
<P><TT>stdlib</TT> is something of a miscellaneous collection of functions that did
not fit into the other libraries. It includes simple integer math functions, sorting
functions (including <TT>qsort()</TT>, one of the fastest sorts available), and text
conversions for moving from ASCII text to integers, <TT>long</TT>, <TT>float</TT>,
and so forth.</P>
<P>The functions in <TT>stdlib</TT> you are likely to use most often include <TT>atoi()</TT>,
<TT>itoa()</TT>, and the family of related functions. <TT>atoi()</TT> provides ASCII
to integer conversion. <TT>atoi()</TT> takes a single argument: a pointer to a constant
character string. It returns an integer (as you might expect). Listing 21.6 illustrates
its use.</P>

<P><A NAME="Heading15"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 21.6. Using
atoi() and related functions.</B></FONT>
<PRE><FONT COLOR="#0066FF">1:     #include &lt;stdlib.h&gt;
2:     #include &lt;iostream.h&gt;
3:
4:     int main()
5:     {
6:        char buffer[80];
7:        cout &lt;&lt; &quot;Enter a number: &quot;;
8:        cin &gt;&gt; buffer;
9:
10:       int number;
11:       // number = buffer; compile error
12:       number = atoi(buffer);
13:       cout &lt;&lt; &quot;Here's the number: &quot; &lt;&lt; number &lt;&lt; endl;
14:
15:       // int sum = buffer + 5;
16:       int sum = atoi(buffer) + 5;
17:       cout &lt;&lt; &quot;Here's sum: &quot; &lt;&lt; sum &lt;&lt; endl;
18:     return 0;
<TT>19: }</TT></FONT>
<FONT COLOR="#0066FF">
Output: Enter a number: 9
Here's the number: 9
Here's sum: 14
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis:</B></FONT><B> </B>On line 6 of this simple
program, an 80-character buffer is allocated, and on line 7 the user is prompted
for a number. The input is taken as text and written into the buffer.<BR>
On line 10, an <TT>int</TT> variable, <TT>number</TT>, is declared, and on line 11
the program attempts to assign the contents of the buffer to the <TT>int</TT> variable.
This generates a compile-time error and is commented out.</P>
<P>On line 12, the problem is solved by invoking the standard library function <TT>atoi()</TT>,
passing in the buffer as the parameter. The return value, the integer value of the
text string, is assigned to the integer variable number and printed on line 13.</P>
<P>On line 15, a new integer variable, <TT>sum</TT>, is declared, and an attempt
is made to assign to it the result of adding the integer constant <TT>5</TT> to the
buffer. This, too, fails and is solved by calling the standard function <TT>atoi()</TT>.


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>NOTE:</B></FONT><B> </B>Some compilers implement standard
	conversion procedures (such as <TT>atoi()</TT>) using macros. You can usually use
	these functions without worrying about how they are implemented. Check your compiler's
	documentation for details. 
<HR>


</BLOCKQUOTE>

<H4 ALIGN="CENTER"><A NAME="Heading16"></A><FONT COLOR="#000077">qsort()</FONT></H4>
<P>At times you may want to sort a table or an array; <TT>qsort()</TT> provides a
quick and easy way to do so. The hard part of using <TT>qsort()</TT> is setting up
the structures to pass in.</P>
<P><TT>qsort()</TT> takes four arguments. The first is a pointer to the start of
the table to be sorted (an array name works just fine), the second is the number
of elements in the table, the third is the size of each element, and the fourth is
a pointer to a comparison function.</P>
<P>The comparison function must return an <TT>int</TT>, and must take as its parameters
two constant <TT>void</TT> pointers. <TT>void</TT> pointers aren't used very often
in C++, as they diminish the type checking, but they have the advantage that they
can be used to point to items of any type. If you were writing your own <TT>qsort()</TT>
function, you might consider using templates instead. Listing 21.7 illustrates how
to use the standard <TT>qsort()</TT> function.</P>

<P><A NAME="Heading17"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 21.7. Using
qsort().</B></FONT>
<PRE><FONT COLOR="#0066FF">1:     /* qsort example */
2:
3:     #include &lt;iostream.h&gt;
4:     #include &lt;stdlib.h&gt;
5:
6:     // form of sort_function required by qsort
7:     int sortFunction( const void *intOne, const void *intTwo);
8:
9:     const int TableSize = 10;  // array size
10:
11:    int main(void)
12:    {
13:       int i,table[TableSize];
14:
15:       // fill the table with values
16:       for (i = 0; i&lt;TableSize; i++)
17:       {
18:          cout &lt;&lt; &quot;Enter a number: &quot;;
19:          cin &gt;&gt; table[i];
20:       }
21:       cout &lt;&lt; &quot;\n&quot;;
22:
23:       // sort the values
24:       qsort((void *)table, TableSize, sizeof(table[0]), sortFunction);
25:
26:       // print the results
27:       for (i = 0; i &lt; TableSize; i++)
28:          cout &lt;&lt; &quot;Table [&quot; &lt;&lt; i &lt;&lt; &quot;]: &quot; &lt;&lt; table[i] &lt;&lt; endl;
29:
30:       cout &lt;&lt; &quot;Done.&quot; &lt;&lt; endl;
31:     return 0;
32:    }
33:
34:    int sortFunction( const void *a, const void *b)
35:    {
36:       int intOne = *((int*)a);
37:       int intTwo = *((int*)b);
38:       if (intOne &lt; intTwo)
39:          return -1;
40:       if (intOne == intTwo)
41:          return 0;
42:       return 1;
<TT>43: }</TT>
Output: Enter a number: 2
Enter a number: 9
Enter a number: 12
Enter a number: 873
Enter a number: 0
Enter a number: 45
Enter a number: 93
Enter a number: 2
Enter a number: 66
Enter a number: 1

Table[0]: 0
Table[1]: 1
Table[2]: 2
Table[3]: 2
Table[4]: 9
Table[5]: 12
Table[6]: 45
Table[7]: 66
Table[8]: 93
Table[9]: 873
Done.
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis:</B></FONT><B> </B>On line 4, the standard library
header is included, which is required by the <TT>qsort()</TT> function. On line 7,
the function <TT>sortFunction()</TT> is declared, which takes the required four parameters.<BR>
<BR>
An array is declared on line 13 and filled by user input on lines 16-20. <TT>qsort()</TT>
is called on line 24, casting the address of the array name <TT>table</TT> to be
a <TT>void*</TT>.</P>
<P>Note that the parameters for <TT>sortFunction</TT> are not passed to the call
to <TT>qsort()</TT>. The name of the <TT>sortFunction</TT>, which is itself a pointer
to that function, is the parameter to <TT>qsort()</TT>.</P>
<P>Once <TT>qsort()</TT> is running, it will fill the constant <TT>void</TT> pointers
<TT>a</TT> and <TT>b</TT> with each value of the array. If the first value is smaller
than the second, the comparison function must return <TT>-1</TT>. If it is equal,
the comparison function must return <TT>0</TT>. Finally, if the first value is greater
than the second value, the comparison function must return <TT>1</TT>. This is reflected
in the <TT>sortFunction()</TT>, as shown on lines 34 to 43.
<H4 ALIGN="CENTER"><A NAME="Heading18"></A><FONT COLOR="#000077">Other Libraries</FONT></H4>
<P>Your C++ compiler supplies a number of other libraries, among them the standard
input and output libraries and the stream libraries that you've been using throughout
this book. It is well worth your time and effort to explore the documentation that
came with your compiler to find out what these libraries have to offer.
<H3 ALIGN="CENTER"><A NAME="Heading19"></A><FONT COLOR="#000077">Bit Twiddling</FONT></H3>
<P>Often you will want to set flags in your objects to keep track of the state of
your object. (Is it in <TT>AlarmState</TT>? Has this been initialized yet? Are you
coming or going?)</P>
<P>You can do this with user-defined Booleans, but when you have many flags, and
when storage size is an issue, it is convenient to be able to use the individual
bits as flags.</P>
<P>Each byte has eight bits, so in a four-byte <TT>long</TT> you can hold 32 separate
flags. A bit is said to be &quot;set&quot; if its value is <TT>1</TT>, and clear
if its value is <TT>0</TT>. When you set a bit, you make its value <TT>1</TT>, and
when you clear it, you make its value <TT>0</TT>. (Set and clear are both adjectives
and verbs). You can set and clear bits by changing the value of the <TT>long</TT>,
but that can be tedious and confusing.


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>NOTE:</B></FONT><B> </B>Appendix C, &quot;Binary and Hexadecimal,&quot;
	provides valuable additional information about binary and hexadecimal manipulation.
	
<HR>


</BLOCKQUOTE>

<P>C++ provides bitwise operators that act upon the individual bits.These look like,
but are different from, the logical operators, so many novice programmers confuse
them. The bitwise operators are presented in Table 21.1. <BR>
<BR>
<FONT SIZE="4"><B>Table 21.1. The Bitwise Operators. </B></FONT>
<TABLE BORDER="0" WIDTH="145">
	<TR ALIGN="LEFT" rowspan="1">
		<TD WIDTH="64" ALIGN="LEFT"><B><I>symbol</I></B></TD>
		<TD ALIGN="LEFT"><B><I>operator</I></B></TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD WIDTH="64" ALIGN="LEFT"><TT>&amp;</TT></TD>
		<TD ALIGN="LEFT"><TT>AND</TT></TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD WIDTH="64" ALIGN="LEFT"><TT>|</TT></TD>
		<TD ALIGN="LEFT"><TT>OR</TT></TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD WIDTH="64" ALIGN="LEFT"><TT>^</TT></TD>
		<TD ALIGN="LEFT">exclusive <TT>OR</TT></TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD WIDTH="64" ALIGN="LEFT"><TT>~</TT></TD>
		<TD ALIGN="LEFT">complement</TD>
	</TR>
</TABLE>

<CENTER>
<H4><A NAME="Heading20"></A><FONT COLOR="#000077">Operator AND</FONT></H4>
</CENTER>
<P>The <TT>AND</TT> operator (<TT>&amp;</TT>) is a single ampersand, as opposed to
the logical <TT>AND</TT>, which is two ampersands. When you <TT>AND</TT> two bits,
the result is <TT>1</TT> if both bits are <TT>1</TT>, but <TT>0</TT> if either or
both bits are <TT>0</TT>. The way to think of this is: The result is <TT>1</TT> if
bit 1 is set and if bit 2 is set.
<CENTER>
<H4><A NAME="Heading21"></A><FONT COLOR="#000077">Operator OR</FONT></H4>
</CENTER>
<P>The second bitwise operator is <TT>OR</TT> (<TT>|</TT>). Again, this is a single
vertical bar, as opposed to the logical <TT>OR</TT>, which is two vertical bars.
When you <TT>OR</TT> two bits, the result is <TT>1</TT> if either bit is set or if
both are.
<CENTER>
<H4><A NAME="Heading22"></A><FONT COLOR="#000077">Operator Exclusive OR</FONT></H4>
</CENTER>
<P>The third bitwise operator is exclusive <TT>OR</TT> (<TT>^</TT>). When you exclusive
<TT>OR</TT> two bits, the result is <TT>1</TT> if the two bits are different.
<CENTER>
<H4><A NAME="Heading23"></A><FONT COLOR="#000077">The Complement Operator</FONT></H4>
</CENTER>
<P>The complement operator (<TT>~</TT>) clears every bit in a number that is set
and sets every bit that is clear. If the current value of the number is <TT>1010