vel13.htm

简单的说明如何使用VB,非常适合初学使用者,而且是用图表来解说的

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CLng()

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Converts the argument to an equivalent long integer data type</FONT>

<TR>

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CSng()

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Converts the argument to an equivalent single-precision data type</FONT>

<TR>

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CStr()

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Converts the argument to an equivalent string data type</FONT>

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CVar()

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Converts the argument to an 
equivalent variant data type</FONT>

</TABLE><BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE><B>Caution: </B>If the converted argument won't fit in the target data type, Visual Basic issues the Overflow error message shown in Figure 13.2. You've 
got to make sure, perhaps through an initial If statement, that the argument fits within the 
range of the converted data type.</NOTE>

<BR>

<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<P>Normally, the following assignment stores .1428571 in the label 
named lblValue:

<BR>

<BR>

<PRE>

<FONT COLOR="#000080">lblValue.Caption = (1 / 7) ' Assigns .1428571</FONT></PRE>

<P>The following, however, adds precision to the answer for a more accurate calculation:

<BR>

<BR>

<PRE>

<FONT 
COLOR="#000080">lblValue.Caption = CDbl(1 / 7) ' Assigns .142857142857143</FONT></PRE>

<P>

<a href="13vel02.gif"><B>Figure 13.2. Don't overflow the target data type.</B></a>

<BR>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE>Don't even <I>mention</I> other bases!: If you <I>really</I> want to get fancy with numeric conversions, 
Visual Basic supports four functions that convert their decimal base-10 arguments to octal (base-8) and hexadecimal (base-16) return values. 
You'll want to use these functions if you write advanced applications that deal with other number bases.

<BR>The 
two hexadecimal functions, Hex() and Hex$(), convert their numeric arguments to a variant and string hexadecimal value, respectively. The two octal functions, Oct() and Oct$(), convert their numeric arguments to a variant and string octal value, 

respectively.

<BR>If you think that you won't use these functions, forget them! These functions, especially the hexadecimal functions, come in handy for programmers who write system-level applications such as memory- and disk-maintenance programs.</NOTE>


<BR>

<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<P>The Asc() function converts its string argument to a corresponding ASCII table value. Generally, you'll pass a one-character string (or a variant that can equate to a string) value to Asc(). If the 
argument contains more than one character, Asc() 
ignores all characters following the first one.

<BR>

<P>The following statement stores the number 65 in the numeric variable named Initial because 65 is the uppercase letter <I>A</I> in the ASCII table:


<BR>

<BR>

<PRE>

<FONT COLOR="#000080">Initial = Asc(&quot;A&quot;)</FONT></PRE>

<P>Table 13.3 lists the remaining math functions that Visual Basic provides. Many of the functions are scientific and mathematical. You may not need any or all of the 
functions unless you write heavy, math-intensive applications.

<BR>

<BR>

<P ALIGN=CENTER>

<CENTER>

<FONT COLOR="#000080"><B>Table 13.3. The scientific and mathematical functions.</B></FONT></CENTER>

<BR>



<TABLE  BORDERCOLOR=#000040 BORDER=1 
CELLSPACING=2 WIDTH="100%" CELLPADDING=2 >

<TR>

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<I>Function</I>

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<I>Description</I></FONT>

<TR>

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><FONT COLOR=#000080>

Abs()

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Returns the absolute value of the argument</FONT>

<TR>

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Atn()

</FONT>

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><FONT COLOR=#000080>

Returns the computed arc tangent of the argument expressed in radians</FONT>

<TR>

<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>

Cos()

</FONT>

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Returns the computed 
cosine of the argument expressed in radians</FONT>

<TR>

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Exp()

</FONT>

<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>

Returns the base of the natural logarithm argument</FONT>

<TR>

<TD 
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Log()

</FONT>

<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>

Returns the natural logarithm of the argument.</FONT>

<TR>

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Sin()

</FONT>


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Returns the computed sine of the argument expressed in radians</FONT>

<TR>

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Sqr()

</FONT>

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Returns the square root of the argument</FONT>

<TR>

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Tan()

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Returns the computed tangent of the argument expressed in radians</FONT>


</TABLE><BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE><B>Tip: </B>If you compute trigonometric values on arguments expressed as degrees instead of radians, multiply the argument by <I>pi</I> and divide by 180. The following expression assigns 
the sine of 38 degrees to a variable named sVal:</NOTE>

<BR>

<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE>sVal = Sin(38 * 3.14159 / 180)</NOTE>

<BR>

<HR ALIGN=CENTER>


</BLOCKQUOTE></BLOCKQUOTE>

<P><FONT COLOR="#FF8000"><B><I>Review: </I></B></FONT>The numeric functions will help you write programs when you need to include mathematical calculations in the code. Many business, graphics, and research applications 
sometimes utilize complex routines, 
and Visual Basic's math functions will help shorten your programming time and improve the accuracy of your code.

<BR>

<BR>

<A NAME="E68E101"></A>

<H3 ALIGN=CENTER>

<CENTER>

<FONT SIZE=5 COLOR="#FF0000"><B>The 
String Functions</B></FONT></CENTER></H3>

<BR>

<P><FONT COLOR="#FF8000"><B><I>Concept: </I></B></FONT>There are several Visual Basic string functions that manipulate string data better than most other programming languages allow. The string functions 
allow you to strip away characters from string data 
and change strings in various ways.

<BR>

<P>The Visual Basic programming language supports strings using the variable-length and fixed-length strings that you've seen defined throughout the first half 
of this book. The string manipulation provided by Visual Basic is perhaps more powerful than any 
other non-BASIC programming language in widespread use today.

<BR>

<P>You've already seen the Chr$() function that converts an ASCII number to its character 
equivalent. (There is a related Chr() function that returns a character in the variant data type also.) The Str$() (and the related variant-returning Str() function) 
converts a number to a string.

<BR>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>


<NOTE><B>Tip: </B>Convert numbers to strings when you want to display numeric quantities inside a message box.</NOTE>

<BR>

<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<P>Suppose that the user's age were stored in a numeric variable named Age. If you 
wanted to display the user's age in a message box, you couldn't do so without first converting the age to a string value as follows:

<BR>

<BR>

<PRE>

<FONT COLOR="#000080">MsgBox &quot;Your age is now&quot; &amp; Str$(Age)</FONT></PRE>

<P>The string 
case-conversion functions convert their string arguments to uppercase or lowercase character strings. Table 13.4 lists these functions.

<BR>

<BR>

<P ALIGN=CENTER>

<CENTER>

<FONT COLOR="#000080"><B>Table 13.4. The case-conversion 
functions.</B></FONT></CENTER>

<BR>



<TABLE  BORDERCOLOR=#000040 BORDER=1 CELLSPACING=2 WIDTH="100%" CELLPADDING=2 >

<TR>

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<I>Function</I>

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<I>Description</I></FONT>

<TR>

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LCase()

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Returns the string argument as a variant data type that's all lowercase 
letters</FONT>

<TR>

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LCase$()

</FONT>

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Returns the string argument as a string data type that's all lowercase letters</FONT>

<TR>

<TD 
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UCase()

</FONT>

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Returns the string argument as a variant data type that's all uppercase letters</FONT>

<TR>

<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT 
COLOR=#000080>

UCase$()

</FONT>

<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>

Returns the string argument as a string data type that's all uppercase letters</FONT>

</TABLE><P>If the argument already contains one or more characters in the 
target case, the functions leave those characters alone. The remarks following these statements indicate the function return values:

<BR>

<PRE>

<FONT COLOR="#000080">lowerName = LCase(&quot;Larry&quot;) ' Returns larry

upperName = 
UCase(&quot;Smythe&quot;) ' Returns SMYTHE</FONT></PRE>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE><I>Definition: </I>A <I>substring</I> is a portion of a string.</NOTE>

<BR>

<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<P>Visual Basic 
contains three string functions that return left, center, or right portions of a string. These are the Left$(), Mid$(), and Right$() functions. All three functions are said to return a substring value. Here are the formats of these 
functions:

<BR>

<PRE>


<FONT COLOR="#000080">Left$(StringVal, length)

Right$(StringVal, length)

Mid$(StringVal, startVal, length)</FONT></PRE>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE><B>Note: </B>The Mid$() function is called the <I>midstring</I> 
function.</NOTE>

<BR>

<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<P>The Left$() function returns <I>length</I> characters from the <I>StringVal</I>. The <I>StringVal</I> might be a string constant or variable. Likewise, Right$() returns <I>length</I> 
characters from the <I>StringVal</I>. Mid$() returns <I>length</I> 
characters from the <I>StringVal</I> beginning at the <I>startVal</I> character.

<BR>

<P>The remarks following these statements indicate the function return values:

<BR>

<PRE>

<FONT 
COLOR="#000080">Title = &quot;Something Good!&quot;

lTitle = Left$(Title, 4) ' Some

rTitle = Right$(Title, 5) ' Good!

mTitle = Mid$(Title, 5, 8) ' thing Go</FONT></PRE>

<P>Figure 13.3 shows how Visual Basic strips these substrings from the longer 
string values.

<BR>

<P><B> <A HREF="13vel03.gif">Figure 13.3. Substring functions pull substrings from strings.</A></B>

<BR>

<P>The Len() function returns the number of characters stored in strings. Often, you'll have to adjust control sizes to hold 
long strings. You can use the Len() function as a guide to calculate how wide a control must be. The following statement stores 14 
in the variable named Length:

<BR>

<P>length = Len(&quot;Blue and Green&quot;)

<BR>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR 
ALIGN=CENTER>

<BR>

<NOTE><B>Note: </B>The Len() function works for numeric values as well. If you pass a numeric variable, constant, or expression to Len(), Len() returns the amount of memory needed to hold that numeric value.</NOTE>

<BR>

<HR 
ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<P>The LTrim$() and RTrim$() functions trim extra spaces from the beginning or end of a string. If extra spaces don't exist, the LTrim$() and RTrim$() functions do nothing. LTrim$() returns the argument's string 
without any leading spaces. RTrim$() returns 
the argument's string without any trailing spaces. The Trim$() function handles both jobs by trimming both leading and trailing spaces from a string.

<BR>

<P>Here are the formats of the string-trimming 
functions:

<BR>

<PRE>

<FONT COLOR="#000080">LTrim[$](StringExpression)

RTrim[$](StringExpression)

Trim[$](StringExpression)</FONT></PRE>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE>There are also equivalent variant functions called 
LTrim(), RTrim(), and Trim().</NOTE>

<BR>

<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<P>The following statements trim spaces from the beginning, end, or both sides of strings:

<BR>

<PRE>

<FONT COLOR="#000080">LStr1 = LTrim$(&quot; Jonah&quot;) ' 
Stores Jonah

RStr2 = RTrim$(&quot;Jonah &quot;) ' Stores Jonah

AnySt3 = Trim$(&quot; Jonah &quot;) ' Stores Jonah</FONT></PRE>

<P>Without the trimming functions, the spaces are copied into the target variables along with the name Jonah.

<BR>