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<title>Butterfly Basic</title>
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<h2>Your first BASIC program </h2>
<p>While it抯 entertaining to display messages on the LCD using immediate mode,
it抯 much more useful to write programs that can control the LCD. Type in the
following: </p>
<blockquote> <tt> >NEW <br />
>10 FOR I = 1 TO 1000 <br />
>20 PRINT #3, "I="; I <br />
>30 NEXT <br />
>40 END <br />
>_</tt></blockquote>
<p>The <b>NEW</b> command clears out any program already in memory and prepares
for you to enter a new one. Individual program instructions are entered using
a line number. The interpreter uses line numbers to sort the program instructions.
When you run the program, the instructions execute in line number order. Now
let's run the program:</p>
<blockquote>
<p><tt> >RUN</tt></p>
</blockquote>
<p>There抯 a blur of activity on the display and it finally shows:</p>
<blockquote>
<p><tt> I=1000</tt></p>
</blockquote>
<p>The problem is the program has executed so quickly that all the intermediate
display commands, which show values counting from 1 to 1000, are almost instantaneously
overwritten with new contents. So, we抣l alter the program to slow it
down so we can see it running:</p>
<blockquote>
<p><tt>>25 FOR J = 1 TO 100: NEXT<br />
>LIST <br />
10 FOR I = 1 TO 1000 <br />
20 PRINT #3, "I="; I <br />
25 FOR J = 1 TO 100: NEXT<br />
30 NEXT <br />
40 END <br />
></tt></p>
</blockquote>
<p>The <b>LIST</b> command instructs the interpreter to list the program. Notice
the interpreter has sorted the program instructions and that line 25 that is
shown between lines 20 and 30, not after line 40. Because you抣l need to edit
your program during development and debugging, it抯 a good idea to enter line
numbers in steps of ten which leaves space for new lines to be inserted. </p>
<p>The new line we entered causes the program to pause for a short time before
continuing—it's a do-nothing loop that just delays for a little while.
When you run the program a second time, again using <b>RUN</b>, you抣l see
the display change much more slowly, allowing you to read the intermediate
values. </p>
<p>Now there抯 a new problem: the display is easily read, but it抯 taking too
long to run through all 1,000 values. You can stop the program at any time
by typing Ctrl+C (that is, holding the Ctrl button while pressing C), and if
you try it now the interpreter responds with: </p>
<blockquote> <tt>
<p>>RUN<br />
BREAK IN LINE 25<br />
></p>
</tt> </blockquote>
<p>The line number in the message will be 10, 20, 25, or 30, depending on which
line was executing when you interrupted, or 揵roke into,⌨️ 快捷键说明
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