Download - True BASIC

Libraries and Modules
107
SELECT CASE how$
CASE “L”
INPUT PROMPT how$: angle
CALL Left(angle)
CASE “R”
INPUT PROMPT how$: angle
CALL Right(angle)
CASE “F”
INPUT PROMPT how$: distance
CALL Forward(distance)
CASE “B”
INPUT PROMPT how$: distance
CALL Back(distance)
CASE “C”
CALL ClearScreen
CASE “S”
STOP
CASE ELSE
END SELECT
END IF
LOOP
END
Sharing Channel Numbers
Channel numbers may be shared and passed as parameters, but they cannot be public. Because channel numbers
may be shared, modules are often used to isolate the interface to files, logical windows, or a printer. You will find
much more information on channel numbers and their use in Chapter 12 “Files for Data Input and Output.”
The following example uses logical windows to illustrate shared channels. Logical windows are described in Chapter
13 “Graphics.” Briefly, however, you need to know that True BASIC defines the default logical window with coordinates
0 to 1 from left to right and 0 to 1 from bottom to top. Thus, the lower left corner of the default window is 0, 0 and
the upper right is 1, 1. You can define smaller logical windows within a physical window by specifying the left, right,
bottom, and top coordinate limits in an OPEN statement, as in the following module. The WINDOW statement tells
True BASIC where to send subsequent output (see Chapter 13 “Graphics”).
MODULE Timer
SHARE start, #1, #2
OPEN #1: SCREEN 0, .8, 0, 1
OPEN #2: SCREEN .85, 1, 0, 1
LET start = Time
CALL Clock
SUB Clock
LET t = Time
WINDOW #2
CLEAR
PRINT Round(t - start, 1)
WINDOW #1
END SUB
! Starting time, 2 windows
! Working window
! Time window
! Use built-in function
! Print to time window
! Switch back to working window
END MODULE
In this example, True BASIC would open the logical windows and print the initial time in window #2 as part of
module initialization before it executes the main program. Output from the main program will be printed in window
#1. The main program can update the running time whenever it wishes with the statement:
CALL Clock