yabasic.htm

一个小巧的BASIC解释器的源代码很小可它的确做到了

        gives you a specific ascii-character: <font
        face="Courier New"><strong>print asc(&quot;e&quot;)</strong></font>
        gives 101 as a result, because the character <font
        face="Courier New"><strong>&quot;e&quot;</strong></font>
        has position <font face="Courier New"><strong>101</strong></font>
        within the ascii-charset. Likewise the function <font
        face="Courier New"><strong>chr$()</strong></font> returns
        the ascii-char for a given position within the charset,
        e.g. <font face="Courier New"><strong>chr$(98)</strong></font>
        returns <font face="Courier New"><strong>&quot;b&quot;</strong></font>.</dd>
    <dt><a name="Escape-sequences"><strong>Escape-sequences</strong></a></dt>
    <dd>Nevertheless you won't use <font face="Courier New"><strong>chr$()</strong></font>
        as often as you might think, because the most important
        nonprintable characters can be constructed using
        escape-sequences with the <font face="Courier New"><strong>\</strong></font>-character:
        You might use <font face="Courier New"><strong>\n</strong></font>
        instead of <font face="Courier New"><strong>chr$(10)</strong></font>
        wherever you want to use the newline-character.</dd>
    <dd>The following table lists all escape sequences of yabasic
        (of course, these are just the sequences known within the
        C-language):<br>
        </dd>
    <dd><table border="1">
            <tr>
                <td align="center"><strong>Escape-sequence</strong></td>
                <td><strong>Resulting Char</strong></td>
            </tr>
            <tr>
                <td align="center"><font face="Courier New"><strong>\n</strong></font></td>
                <td>newline</td>
            </tr>
            <tr>
                <td align="center"><font face="Courier New"><strong>\t</strong></font></td>
                <td>tabulator</td>
            </tr>
            <tr>
                <td align="center"><font face="Courier New"><strong>\v</strong></font></td>
                <td>vertical tabulator</td>
            </tr>
            <tr>
                <td align="center"><font face="Courier New"><strong>\b</strong></font></td>
                <td>backspace</td>
            </tr>
            <tr>
                <td align="center"><font face="Courier New"><strong>\r</strong></font></td>
                <td>carriage return</td>
            </tr>
            <tr>
                <td align="center"><font face="Courier New"><strong>\f</strong></font></td>
                <td>formfeed</td>
            </tr>
            <tr>
                <td align="center"><font face="Courier New"><strong>\a</strong></font></td>
                <td>alert</td>
            </tr>
            <tr>
                <td align="center"><font face="Courier New"><strong>\\</strong></font></td>
                <td>backslash</td>
            </tr>
            <tr>
                <td align="center"><font face="Courier New"><strong>\`</strong></font></td>
                <td>single quote</td>
            </tr>
            <tr>
                <td align="center"><font face="Courier New"><strong>\&quot;</strong></font></td>
                <td>double quote</td>
            </tr>
        </table>
    </dd>
    <dd><br>
        These escape sequences are replaced within every pair of
        doublequotes (<font face="Courier New"><strong>&quot;&quot;</strong></font>),
        i.e. within literal strings; user input read with the<font
        face="Courier New"><strong> input</strong></font>-statement
        is not affected in any way.<br>
        Finally note, that escape sequences have a profound
        impact, when specifying <a
        href="#Specifying Window-pathnames">Window-pathnames</a>.</dd>
    <dd>&nbsp;</dd>
</dl>

<p>Here is another example which introduces the rest of yabasic's
string-functions:</p>

<blockquote>
    <dl>
        <dd><pre><strong>label loop
  print &quot;Please enter a string containing the word \&quot;yabasic\&quot;&quot;
  input a$
if (instr(lower$(a$),&quot;yabasic&quot;)&lt;&gt;0) then
  gosub thanx
else
  print &quot;No, please try again !&quot;
endif
goto loop

label thanx
  print &quot;Thanks a lot !&quot;
return</strong></pre>
        </dd>
    </dl>
</blockquote>

<p>If you run this program you will receive the following output:</p>

<blockquote>
    <dl>
        <dd><pre><strong>Please enter a string containing the word &quot;yabasic&quot;
?</strong><em><strong><u>thequickbrownfox</u></strong></em><strong>
No, please try again !
Please enter a string containing the word &quot;yabasic&quot;
?</strong><em><strong><u>jumpedyabasicoverthelazydog</u></strong></em><strong>
Thanx.</strong></pre>
        </dd>
    </dl>
</blockquote>

<dl>
    <dt><a name="Marking locations in a program"><strong>Marking
        locations in a program</strong></a></dt>
    <dd>The first line in the example-program (<font
        face="Courier New"><strong>label loop</strong></font>) is
        a label: As yabasic has no line-numbers, you need labels
        to mark a specific location within your program. You can
        compose labels out of letters and digits; the keyword <font
        face="Courier New"><strong>label</strong></font> is
        required and the label itself should be unique within
        your program. Note that yabasic allows for <a
        href="#linenumber">line numbers</a> too.</dd>
    <dt><a name="Jumping around in your program"><strong>Jumping
        around in your program</strong></a></dt>
    <dd>A label by itself causes no special action. Only in
        conjunction with the <font face="Courier New"><strong>goto</strong></font>-statement
        (or <font face="Courier New"><strong>gosub</strong></font>
        or <font face="Courier New"><strong>restore</strong></font>)
        does a label have any function. If yabasic encounters a <font
        face="Courier New"><strong>goto</strong></font>-statement
        (here: <font face="Courier New"><strong>goto loop</strong></font>)
        then it searches for the matching label (here: <font
        face="Courier New"><strong>label loop</strong></font>)
        and proceeds to execute at the position of the label.<br>
        Note that you can even leave (and enter !) a for-next
        loop with goto.<br>
        <br>
        Closely related to the <font face="Courier New"><strong>goto</strong></font>-command
        is the <font face="Courier New"><strong>gosub</strong></font>-command;
        if yabasic encounters a <font face="Courier New"><strong>gosub</strong></font>-statement
        then it searches for the matching label (<font
        face="Courier New"><strong>label thanx</strong></font> in
        the example) and proceeds with execution at the position
        of the label, until it finds a <font face="Courier New"><strong>return</strong></font>-statement.
        <font face="Courier New"><strong>return</strong></font>
        makes yabasic return to the position of the original
        gosub and proceed from there.<br>
        <br>
        Note that both <font face="Courier New"><strong>goto</strong></font>
        and <font face="Courier New"><strong>gosub</strong></font>
        can be used as <a href="#on gosub"><font
        face="Courier New"><strong>on goto</strong></font></a>
        and <a href="#on gosub"><font face="Courier New"><strong>on
        gosub</strong></font></a>.</dd>
    <dt><a name="Finding strings in strings"><strong>Finding
        strings in strings</strong></a></dt>
    <dd>The example program above checks whether the user input
        contains the string <font face="Courier New"><strong>&quot;yabasic&quot;</strong></font>;
        this is done with the help of the <font
        face="Courier New"><strong>instr()</strong></font>-function;
        <font face="Courier New"><strong>instr()</strong></font>
        gives back the position of its second string-argument
        within the first or zero, if it can't be found. E.g. <font
        face="Courier New"><strong>instr(&quot;Hallo&quot;,&quot;al&quot;)</strong></font>
        gives back 2, because <font face="Courier New"><strong>&quot;al&quot;</strong></font>
        appears at position 2 within <font face="Courier New"><strong>&quot;Hallo&quot;</strong></font>;
        whereas <font face="Courier New"><strong>instr(&quot;Hallo&quot;,&quot;Al&quot;)</strong></font>
        returns 0, because <font face="Courier New"><strong>&quot;Al&quot;</strong></font>
        is not contained in <font face="Courier New"><strong>&quot;Hallo&quot;</strong></font>
        (the case doesn't match).</dd>
    <dt><a name="Changing the case of strings"><strong>Changing
        the case of strings</strong></a></dt>
    <dd>The sample-program contains some further
        string-functions: <font face="Courier New"><strong>lower$()</strong></font>
        and its counterpart <font face="Courier New"><strong>upper$()</strong></font>
        convert their string-argument to all lower or all upper
        case characters respectively, i.e. <font
        face="Courier New"><strong>lower$(&quot;aBcD12fG&quot;)</strong></font>
        gives back <font face="Courier New"><strong>&quot;abcd12fg&quot;</strong></font>.</dd>
    <dt><a name="Removing spaces"><strong>Removing spaces</strong></a></dt>
    <dd><font face="Courier New"><strong>ltrim$()</strong></font>
        and <font face="Courier New"><strong>rtrim$()</strong></font>
        are two functions to remove leading or trailing spaces
        from a string, e.g. <font face="Courier New"><strong>ltrim$(&quot;
        foo &quot;) </strong></font>gives<font face="Courier New"><strong>
        &quot;foo &quot; </strong></font>and<font
        face="Courier New"><strong> rtrim$(&quot; foo &quot;) </strong></font>gives<font
        face="Courier New"><strong> &quot; foo&quot;</strong></font>.
        Finally<font face="Courier New"><strong>, trim$() </strong></font>is
        the same as<font face="Courier New"><strong>
        rtrim$(ltrime$())</strong></font>.</dd>
    <dt><a name="Splitting astring into tokens"><strong>Splitting
        a string into tokens</strong></a></dt>
    <dd>There is a handy function to split a string into tokens;
        an example would be:</dd>
    <dt>&nbsp;</dt>
    <dd><blockquote>
            <p><font size="3" face="Courier New"><strong>l$=&quot;
            one two three &quot;<br>
            label loop:print token$(a$):if
            (a$&lt;&gt;&quot;&quot;) then goto loop:fi</strong></font></p>
        </blockquote>
        <p>Running this program gives:</p>
    </dd>
</dl>

<blockquote>
    <blockquote>
        <p><font size="3" face="Courier New"><strong>one<br>
        two<br>
        three</strong></font></p>
    </blockquote>
    <p>Note, that <font face="Courier New"><strong>token$()</strong></font>
    automatically removes the tokens from <font
    face="Courier New"><strong>a$</strong></font>. If you supply
    a second argument (e.g. <font face="Courier New"><strong>token$(a$,&quot;:&quot;)</strong></font>),
    the tokens are split at the characters given in the second
    string.</p>
</blockquote>

<p><a href="#Table of contents">Back to table of contents ...</a></p>

<hr>

<h2><a name="Graphics and printing">Graphics and printing</a></h2>

<p>Yabasic provides some simple, general purpose
graphic-commands:</p>

<blockquote>
    <pre><strong>open window 400,400
line 0,0 to 400,400
circle 200,200,150
dot 200,200
a$=inkey$
clear window
text 100,200,&quot;Hello !&quot;
print &quot;Press any key to close the window&quot;
inkey$
close window</strong></pre>
</blockquote>

<dl>
    <dt><strong>Drawing</strong></dt>
    <dd>If you run this program, you will see a window with size
        of 400 pixels in x- and y-direction (the window size is
        given along with the <font face="Courier New"><strong>open
        window</strong></font>-statement). To specify a certain
        font for the text within this window, you may add a third
        argument, e.g. <font face="Courier New"><strong>open
        window 400,400,&quot;swiss&quot;</strong></font>.</dd>
    <dd><br>
        Not surprising: The <font face="Courier New"><strong>line</strong></font>-command
        draws a line, the <font face="Courier New"><strong>circle</strong></font>-command
        draws a circle (the arguments determine x- and y-position
        of the center and the radius of the circle) and the <font
        face="Courier New"><strong>dot</strong></font>-command
        draws a single dot at the specified location.<br>
        <br>
        After the user has pressed a key (see <a
        href="#Getting a key from the keyboard">below</a>) the
        window contents is cleared with the <font