Computer Programming Concepts and Visual Basic David I. Schneider

264 Computer Programming Concepts and Visual Basic
text appropriately in the picture box is essential to good-looking graphs. A picture box has
two properties, CurrentX and CurrentY, and two methods, TextHeight and TextWidth, that
allow us to precisely position text alongside graphics.
The properties CurrentX and CurrentY record the precise horizontal and vertical location
at which the next character of text will be printed. By assigning appropriate values to
these properties before executing a Print method, text can be positioned very precisely in the
picture box. In the following event procedure, the coordinates of the right end of the tick
mark are (x, y) = (.3, 3). As a first attempt at labeling a tick mark on the y axis, the CurrentX
and CurrentY properties are set to these coordinates. The results are shown in Figure 9-7(a).
Private Sub cmdDraw_Click()
picOutput.Cls picOutput.Scale (-4, 4)-(4, -4)
picOutput.Line (-4, 0)-(4, 0) ‘Draw x-axis
picOutput.Line (0, -4)-(0, 4) ‘Draw y-axis
picOutput.Line (-.3, 3)-(.3, 3) ‘Draw tick mark
picOutput.CurrentX = .3 ‘Right end of tick mark
picOutput.CurrentY = 3 ‘Same vertical position as tick mark
picOutput.Print “y=3”
End Sub
‘Label for tick mark
FIGURE 9-7 Placing Labels: (a) First Attempt and (b) Second Attempt
Note that the top of the text is even with the tick mark. This reflects the fact that the
value of the CurrentY property used by Visual Basic is the location for the top of the character
cursor. Ideally, the text should be moved up so that the tick mark aligns with the middle
of the text. To do this, the value of the CurrentY property needs to be increased by
one-half the height of the text. The following statement assigns a corrected value to the CurrentY
property by using the TextHeight method to obtain the height of the text being used as
the tick mark label. (Since c < d in the scale method, TextHeight returns (–1) � [height of
text].)
picOutput.CurrentY = 3 - picOutput.TextHeight(“y=3”) / 2
The result of using this corrected value for CurrentY is shown in Figure 9-7(b).
When the TextHeight method is used, all characters have the same height. Thus the
height of a string can be obtained by asking for the height of any single character. The following
procedure uses the TextHeight method with a space character to center the text cursor
at the requested graphic point.
Private Sub PositionText(x As Single, y As Single)
‘Center text cursor at the point (x, y)
picOutput.CurrentX = x
picOutput.CurrentY = y – picOutput.TextHeight(“ ”) / 2
End Sub
Another useful picture box method is TextWidth. Whereas the Len function returns the
number of characters in a string, the TextWidth method considers the varying widths of characters
and returns the physical width of the entire string in the units of the current scale for