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Empty Loops—Inserting Delays In a Program
Before you proceed any further, it will be helpful to understand about
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loops and some ways they are used to get the computer to do what
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you want. You can use a loop to slow down the computer (by now
you have witnessed the speed with which the computer executes
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commands). See if you can predict what this program will do before
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you run it.
10 A$ = "COMMODORE C128"
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20 FOR J = 1 TO 20
30 PRINT
40 FOR K = 1 TO 1500
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50 NEXT K
60 PRINT A$
70 NEXT J
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80 END
Did you get what you expected? The loop contained in lines 40 and
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50 tells the computer to count to 1500 before executing the remain-
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der of the program. This is known as a delay loop and is often useful.
Because it is inside the main loop of the program, it is called a
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nested loop. Nested loops can be very useful when you want the
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computer to perform a number of tasks in a given order, and repeat
the entire sequence of commands a certain number of times.
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Section 5 describes an advanced way to insert delays through use
of the new BASIC 7.0 command, SLEEP.
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The STEP Command
You can tell the computer to increment your counter by units (e.g. 10,
0.5 or any other number). You do this by using a STEP command with
the FOR statement. For example, if you want the computer to count
bytens(to100,type:
10 FOR X = 0 TO 100 STEP 10
20 ?X
30 NEXT
Notice that you do not need the X in the NEXT statement if you are
only executing one loop at a time—NEXT refers to the most recent
FOR statement. Also, note that you do not have to increase (or
"increment") your counter—you can decrease (or "decrement") it
as well. For example, change line 10 in the program above to read:
10 FOR X = 100 TO 0 STEP-10
54
USING C128 MODE-Advanced BASIC Programming
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