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

<P>Earlier, you learned how to use Str$() to convert a number to a string. Because Str$() always converts positive numbers to strings with a leading blank (where the imaginary plus sign appears), you can combine LTrim$() with Str$() to eliminate the 

leading blank. The first of the following two statements stores the leading blank in st1. The second uses LTrim$() to get rid of the blank before storing the string into st2.

<BR>

<PRE>

<FONT COLOR="#000080">st1 = Str$(234) ' Stores &quot; 234&quot;


st2 = LTrim$(Str$(234)) ' Stores &quot;234&quot;</FONT></PRE>

<P><FONT COLOR="#FF8000"><B><I>Review: </I></B></FONT>The string functions manipulate strings and return values based on string arguments. As you write more powerful programs, you'll need to 
extract substrings and convert the case of strings in various 
ways to display string values on the form and in controls.

<BR>

<BR>

<A NAME="E68E102"></A>

<H3 ALIGN=CENTER>

<CENTER>

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

<BR>

<P><FONT COLOR="#FF8000"><B><I>Concept: </I></B></FONT>There are a handful of general functions that don't fall into a strict numeric or string category. These general-purpose functions add power to your 
programming skills. This section explains how to 
detect data types using the Is...() and <I>VarType</I>() functions. You may not see a use for these functions quite yet, but familiarize yourself with them. In Lesson 8, you'll learn how to pass data back 
and forth between Visual Basic routines, and there 
are times when you'll need to know which data type was passed to you using one of the functions taught here.

<BR>

<P>Table 13.5 contains <I>data inspection</I> functions, often called the <I>Is...()</I> 
functions. When you store a data value in a variant variable (which can accept any data type), Table 13.5's functions returns a true or false result that indicates 
whether the argument can be converted to a specific data type.

<BR>

<BR>

<P 
ALIGN=CENTER>

<CENTER>

<FONT COLOR="#000080"><B>Table 13.5. The </B><B>Is...()</B><B> data inspection functions.</B></FONT></CENTER>

<BR>



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

<TR>

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

<I>Function Name</I>

</FONT>

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

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

<TR>

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

IsDate()

</FONT>

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

Determines whether its argument can be converted to a valid date</FONT>

<TR>

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

IsEmpty()

</FONT>

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


Determines whether its argument has been initialized with any value since the argument's original definition</FONT>

<TR>

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

IsNull()

</FONT>

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


Determines whether its argument holds a Null value (such as an empty string)</FONT>

<TR>

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

IsNumeric()

</FONT>

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

Determines whether its argument 
holds a value that can be converted to a valid number</FONT>

</TABLE><P>The following code ensures sure that a valid string appears in the variable named ans before applying a UCase$() function:

<BR>

<PRE>

<FONT COLOR="#000080">If IsString(ans) Then

 
NewAns = UCase$(ans)

Else

 MsgBox &quot;You didn't type a letter!&quot;

End If</FONT></PRE>

<P>Empty variables differ from Null values and zero values. Empty variables indicate that nothing has been entered into variables. The IsNull() function checks 
to see whether its control argument contains a Null value.

<BR>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE><B>Tip: </B>Use IsNull() to see whether a control or field on a form contains data. Use IsEmpty() just for variables.</NOTE>

<BR>


<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<P>The VarType() function determines the data type of its argument. Table 13.6 lists the return values from the VarType() function. VarType() returns no values other than the nine listed in the table.

<BR>


<BR>

<P ALIGN=CENTER>

<CENTER>

<FONT COLOR="#000080"><B>Table 13.6. The </B><B>VarType()</B><B> function's return values.</B></FONT></CENTER>

<BR>



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

<TR>

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

<I>Return</I>

</FONT>

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

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

<TR>

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

0

</FONT>

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

Indicates that the argument is Empty</FONT>

<TR>

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

1

</FONT>

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

Indicates that the argument is Null</FONT>

<TR>

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

2

</FONT>

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

Indicates that the argument is Integer</FONT>

<TR>

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

3

</FONT>

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

Indicates that the argument is Long</FONT>

<TR>

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

4

</FONT>

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

Indicates that the argument is Single</FONT>


<TR>

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

5

</FONT>

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

Indicates that the argument is Double</FONT>

<TR>

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

6

</FONT>

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

Indicates that the argument is Currency</FONT>

<TR>

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

7

</FONT>

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

Indicates that the argument 
is Date</FONT>

<TR>

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

8

</FONT>

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

Indicates that the argument is String</FONT>

</TABLE><P><FONT COLOR="#FF8000"><B><I>Stop and Type: 
</I></B></FONT>Listing 13.1 makes sure that the user entered a value into the txtAge text box control.

<BR>

<P><FONT COLOR="#FF8000"><B><I>Review: </I></B></FONT>The general-purpose functions return values that indicate data contents and data types. You 
can use these functions to see whether variables and controls have been initialized before calculating with 
those values.

<BR>

<P>

<FONT COLOR="#000080"><B>Listing 13.1. Checks to make sure that the user entered a value.</B></FONT>

<BR>

<PRE>

<FONT 
COLOR="#000080">1: If IsNull(txtAge.Text) Then

2: MsgBox &quot;You didn't type anything!&quot;

3: Else

4: MsgBox &quot;Thanks for entering a value.&quot;

5: End If</FONT></PRE>

<P><FONT COLOR="#FF8000"><B><I>Analysis: </I></B></FONT>Line 1 ensures 
that the user has typed <I>something</I> in the control named txtAge. Subsequent code could then use the VarType() function to make sure that the user entered a value of the correct 
data type required by the user.

<BR>

<BR>

<A NAME="E68E103"></A>

<H3 
ALIGN=CENTER>

<CENTER>

<FONT SIZE=5 COLOR="#FF0000"><B>Homework</B></FONT></CENTER></H3>

<BR>

<BR>

<A NAME="E69E90"></A>

<H4 ALIGN=CENTER>

<CENTER>

<FONT SIZE=4 COLOR="#FF0000"><B>General Knowledge</B></FONT></CENTER></H4>

<BR>

<OL>

<LI>What is 
a function <I>argument</I>?

<BR>

<BR>

<LI>True or false: A function's return value must match the argument list's data type.

<BR>

<BR>

<LI>True or false: Some functions require no arguments, some one, and some several.

<BR>

<BR>

<LI>True or false: 
The function's arguments must all match in data type.

<BR>

<BR>

<LI>Why are there three integer functions?

<BR>

<BR>

<LI>What value does Visual Basic store in Ans given the following assignment statements?

<BR>Ans = Int(61.32)

<BR>Ans = Int(-61.32)


<BR>Ans = Fix(-61.32)

<BR>Ans = Cint(421.51)

<BR>

<BR>

<LI>True or false: Int(), CInt(), and Fix() all return the same value for positive arguments.

<BR>

<BR>

<LI>What function performs the opposite of Chr$()?

<BR>

<BR>

<LI>What value appears in 
the string variable named AList after these assignments?

<BR>AList = UCase$(&quot;AbCdEfG&quot;)

<BR>AList = LCase$(&quot;AbCdEfG&quot;)

<BR>

<BR>

<LI>What would the following assignment statement store in the variant variable named Anything?


<BR>Anything = Str$(Val(&quot;78.1&quot;))

<BR>

<BR>

<LI>Write the values stored in each of the following assignment statements:

<BR>AStr = Left$(&quot;Sams&quot;, 1)

<BR>AStr = Left$(&quot;Sams&quot;, 3)

<BR>AStr = Right$(&quot;Sams&quot;, 1)


<BR>AStr = Right$(&quot;Sams&quot;, 3)

<BR>AStr = Mid$(&quot;Sams&quot;, 2, 3)

<BR>

<BR>

<LI>True or false: Depending on the arguments, Mid$() could return the same values as Left$() and Right$().

<BR>

<BR>

<LI>True or false: Both of the following 
produce the same value when used inside an expression:

<BR>Chr$(67)

<BR>Asc(&quot;C&quot;)

<BR>

<BR>

<LI>What is the name of the function that converts a number to a string?

<BR>

<BR>

</OL>

<BR>

<A NAME="E69E91"></A>

<H4 ALIGN=CENTER>

<CENTER>


<FONT SIZE=4 COLOR="#FF0000"><B>Write Code That...</B></FONT></CENTER></H4>

<BR>

<OL>

<LI>Suppose that you needed to display a numeric variable's value inside the prompt string of an input box. Describe how you would you concatenate such a variable to 
the prompt string?

<BR>

<BR>

<LI>How could you find out the amount of memory consumed by 250 double-precision variables?

<BR>

<BR>

<LI>Write the code that uses an input box to get a number from the user and then uses a message box to display the 
square root of that number.

<BR>

<BR>

<LI>Write the code that uses two separate input boxes to ask the user for a first and last name, and then display in a message box the total number of letters in both names.

<BR>

<BR>

<LI>Write an assignment 
statement that stores the ASCII value of &quot;P&quot; in a variable named ValAsc.

<BR>

<BR>

</OL>

<BR>

<A NAME="E69E92"></A>

<H4 ALIGN=CENTER>

<CENTER>

<FONT SIZE=4 COLOR="#FF0000"><B>Find the Bug</B></FONT></CENTER></H4>

<BR>

<OL>

<LI>Rudy 
gets an overflow message when attempting the following assignment into the integer variable named Weight:

<BR>Weight = CInt(32768 * Num)

<BR>Describe the problem for Rudy.

<BR>

<BR>

</OL>

<BR>

<A NAME="E69E93"></A>

<H4 ALIGN=CENTER>

<CENTER>


<FONT SIZE=4 COLOR="#FF0000"><B>Extra Credit</B></FONT></CENTER></H4>

<BR>

<OL>

<LI>Add to Listing 13.1 to make sure that the user entered an integer in the txtAge.Text control.

<BR>

<BR>

</OL>

<P>Write a program that stores the 256 ASCII characters 
(from ASCII 0 to ASCII 255) in a string array that's defined to hold 256 elements.

<BR>

<P ALIGN=LEFT>

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