Computer Programming Concepts and Visual Basic David I. Schneider

[COLOR]: Numbers written in base 16 are referred to as hexadecimal numbers. They are
written with the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A (=10), B (=11), C (=12), D (=13), E (=14),
and F (=15). A hexadecimal number such as rst corresponds to the decimal integer t + 16 *
s + 16 * r. Each color in Visual Basic is identified by a long integer (usually expressed as a
hexadecimal number of the form &H...&) and referred to as an RGB color number. This
number specifies the amounts of red, green, and blue combined to produce the color. The
amount of any color is a relative quantity, with 0 representing none of the color and 255 representing
the maximum available. Thus, black corresponds to 0 units each of red, green, and
blue, and white corresponds to 255 units each of red, green, and blue. The RGB color number
corresponding to r units of red, g units of green, and b units of blue is r + 256 * g + 65536
* b, which is the value returned by the function RGB(r, g, b). Hexadecimal notation provides
a fairly easy means of specifying RGB color numbers. If the amount of red desired is
expressed as a two-digit hexadecimal number, rr, the amount of green in hexadecimal as gg,
and the amount of blue in hexadecimal as bb, then the RGB color number for this color is
&H00bbggrr&. For example, the RGB color number for a bright green would come from 255
(FF in hexadecimal) units of green, so the RGB color number in hexadecimal is
&H0000FF00&.
[COORDINATE SYSTEMS]: The default coordinate system for a form, picture box, or the
printer defines the upper-left corner as the point (0, 0). In this coordinate system, the point
(x, y) lies x units to the right of and y units below the upper-left corner. The unit of measure
in the default coordinate system is a twip. A twip is defined as 1/1440 of an inch (though
varying screen sizes may result in 1440 twips not appearing as exactly an inch on the screen).
Custom coordinate systems can be created using the Scale method and ScaleMode property.
[DATE]: Functions dealing with dates and times use the type 7 variant data type. Dates and
times are stored as serial dates, double-precision numbers, with the whole part recording the
date and the decimal part recording the time. Valid whole parts range from –657434 to
2958465, which correspond to all days from January 1, 100 to December, 31, 9999. A whole
part of 0 corresponds to December 30, 1899. All decimal parts are valid, with .0 corresponding
to midnight, .25 corresponding to 6 a.m., .5 corresponding to noon, and so on. In
general, the decimal equivalent of sec/86400 corresponds to sec seconds past midnight. If a
given date corresponds to a negative whole part, then times on that day are obtained by
adding a negative decimal part to the negative whole part. For example, October, 24, 1898,
corresponds to a whole part of –432. A time of 6 p.m. corresponds to .75, so a time of 6 p.m.
on 10/24/1898 corresponds to –432 +–.75 = –432.75.
[DIRECTORIES]: Think of a disk as a master folder holding other folders, each of which
might hold yet other folders. Each folder, other than the master folder, has a name. Each
folder is identified by a path: a string beginning with a drive letter, a colon, and a backslash
character, ending with the name of the folder to be identified, and listing the names of the
intermediate folders (in order) separated by backslashes. For instance the path
“C:\DAVID\GAMES” identifies the folder GAMES, which is contained in the folder
DAVID, which in turn is contained in the master folder of drive C.
Each folder is called a directory and the master folder is called the root directory. When
a folder is opened, the revealed folders are referred to as its subdirectories. Think of a file as
a piece of paper inside one of the folders. Thus, each directory contains files and subdirectories.
[DYNAMIC VS. STATIC ARRAYS]: Visual Basic uses two methods of storing arrays:
dynamic and static. The memory locations for a static array are set aside the instant the program
is executed and this portion of memory may not be freed for any other purpose. The
memory locations for a dynamic array are assigned when a particular procedure requests that
an array be created (a Dim or ReDim statement is encountered) and can be freed for other
purposes. Although dynamic arrays are more flexible, static arrays can be accessed faster.
Arrays Dimensioned in the (Declarations) section of (General) use static allocation, except
Appendix C 455