Loops-The For-Next Command - Commodore 128D User Manual

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Loops—The
FOR-NEXT
Command
Now change line 10 to read N = 20, and RUN the program again.
Notice you can tell the computer to execute more than one state
ment when N is less than 5. You can put any statement(s) you want
after the THEN command. Remember that the GOTO 15 will not be
reached until N<5 is true. Any command that should be followed
whether or not the specified condition is met should appear on a
separate line.
In the first RUN of the program used in the previous example, we
made the computer print the variable N five times by telling it to
increase or "increment" the variable N by units of one, until the
value of N equalled five; then we ended the program. There is a sim
pler way to do this in BASIC. We can use a FOR-NEXT loop, like this:
10FORN = 1 TO 5
20 ?N; "IS LESS THAN OR EQUAL TO 5"
30 NEXT N
40 END
Type and RUN this program and compare the result with the result of
the IF-THEN program—they are similar. In fact, the steps taken by
the computer are almost identical for the two programs. The FOR-
NEXT loop is a very powerful programming tool. You can specify the
number of times the computer should repeat an action. Let's trace
the computer's steps for the program above.
First, the computer assigns a value of 1 to the variable N. The 5 in
the FOR statement in line 10 tells the computer to execute all state
ments between the FOR statement and the NEXT statement, until N
is equal to 5. In this case there is just one statement—the PRINT
statement.
This is how the computer interprets the inner workings of a FOR...
NEXT loop—it operates in much the same way as the IF... THEN
example on the previous page. First, the C128 assigns a value of 1 to
the variable N. It then executes all instructions between the FOR and
NEXT keywords. When the NEXT statement is encountered, it tells
the computer to increment the counter variable N (in this case by 1),
compare N to 5 and continue with another cycle through the FOR
... NEXT loop if N > 5 is false. The increment defaults to 1 if no other
increment is specified in the FOR statement. After five passes
through the loop, and once N > 5 is true, the computer processes
the statement which immediately follows the NEXT statement and
resumes with the rest of the program. Since the computer does not
compare the value of N to the start value of the loop variable until the
NEXT statement is encountered, every loop is executed at least
once.
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USING C128 MODE—Advanced BASIC Programming