yabasic.htm

一个小巧的BASIC解释器的源代码很小可它的确做到了

        face="Courier New"><strong>clear window</strong></font>-statement.
        If you have your printer open (i.e. if you have issued
        the <font size="2" face="Courier New"><strong>open
        printer</strong></font>-command) <font size="2"
        face="Courier New"><strong>clear window</strong></font>
        finishes the current page, sends it to the printer and
        starts a new one.<br>
        <br>
        The next command in the example is the <font
        face="Courier New"><strong>text</strong></font>-statement,
        which writes its text at the specified position. Aligned
        with this position is the left lower corner of the text.
        To change the alignement, you can add as a third argument
        a two character string; The first one specifies the
        horizontal alignement and can be <font face="Courier New"><strong>&quot;l&quot;</strong></font>
        (text is left aligned), <font face="Courier New"><strong>&quot;r&quot;
        </strong></font>(right aligned) or <font
        face="Courier New"><strong>&quot;c&quot;</strong></font>
        (centered); the second character specifies the vertical
        alignement and can be <font face="Courier New"><strong>&quot;t&quot;</strong></font>
        (top of text is aligned), <font face="Courier New"><strong>&quot;b&quot;</strong></font>
        (bottom) or <font face="Courier New"><strong>&quot;c&quot;</strong></font>
        (center). Some valid arguments would be <font
        face="Courier New"><strong>&quot;ct&quot;</strong></font>,
        <font face="Courier New"><strong>&quot;rb&quot;</strong></font>,
        <font face="Courier New"><strong>&quot;lc&quot;</strong></font>,
        ... By the way: Textalignement can also be changed by <a
        href="#Peek and Poke"><font face="Courier New"><strong>poking</strong></font></a><font
        face="Courier New"> </font>into <font face="Courier New"><strong>&quot;textalign&quot;</strong></font>.<br>
        <br>
        Finally <font face="Courier New"><strong>close window</strong></font>
        closes the graphics-window.</dd>
    <dt><a name="Getting a key from the keyboard"><strong>Getting
        a key from the keyboard</strong></a></dt>
    <dd>But before the window is closed, the <a
        name="inkey$-statement"><font face="Courier New"><strong>inkey$</strong></font>-statement</a>
        waits, until the user presses any key and returns this
        key as a string. In this example the key, which is
        actually pressed is not important, so you may just write <font
        face="Courier New"><strong>inkey$</strong></font>
        (without assignment). Some important nonprintable keys
        (e.g. the function or cursor keys) are returned as
        strings: <strong>up, down, left, right, f1, f2, f3, f4,
        f5, f6, f7, f8, f9, f10, f11, f12, esc, ins, del, home,
        end, scrnup, scrndown, enter, tab, backspace. </strong>If
        your keyboard gives other keycodes than mine, or if you
        press a key, which is unknown to yabasic, you will
        receive a rather lengthy string (e.g. key1b5b31317e).<br>
        Normally yabasic's <font face="Courier New"><strong>inkey$</strong></font>
        waits until the user presses a key; but if you want the
        inkey$-function to return even if no key has been
        pressed, you may add a timeout (in seconds) argument.
        E.g.<font face="Courier New"> <strong>inkey$(2)</strong></font>
        returns immediately, if the user hits a key and after 2
        seconds (returning an empty string) if not; note that a
        timeout of 0 seconds is possible, which is the normal
        behaviour of other basics.</dd>
    <dt><a name="printing"><strong>Printing</strong></a></dt>
    <dd>Getting a hardcopy of your graphics involves two new
        commands:</dd>
</dl>

<blockquote>
    <blockquote>
        <pre><strong>open window 200,200
open printer
circle 100,100,80
close printer
close window</strong></pre>
    </blockquote>
</blockquote>

<dl>
    <dd>Everything between <font face="Courier New"><strong>open
        printer</strong></font> and <font face="Courier New"><strong>close
        printer</strong></font> appears on paper. If you prefer
        sending your hardcopy to a file, you may add a filename,
        e.g. <font face="Courier New"><strong>open printer
        &quot;foo&quot;</strong></font> sends the output to the
        file <font face="Courier New"><strong>foo</strong></font>.
        Note that the <font face="Courier New"><strong>open
        printer</strong></font> statement has to appear after the
        window has been opened. <font face="Courier New"><strong>close
        printer</strong></font> can be omitted; it is done
        automatically, if the window is closed.</dd>
</dl>

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

<hr>

<h2><a name="Plotting">Plotting</a></h2>

<p>Yabasics grafic is rather simple: It does not support color,
nor animation; but it <strong>does</strong> support plotting.
Have a look at the following program, which plots the sine
function, complete with axis and errors:</p>

<blockquote>
    <p><font size="2" face="Courier New"><strong>open window
    600,400<br>
    open printer<br>
    </strong></font><a href="#window origin"><font size="2"
    face="Courier New"><strong>window origin &quot;lb&quot;</strong></font></a><font
    size="2" face="Courier New"><strong><br>
    </strong></font><a href="#rectangle"><font size="2"
    face="Courier New"><strong>rectangle 10,10 to 590,390</strong></font></a><font
    size="2" face="Courier New"><strong><br>
    <br>
    </strong></font><a href="#map"><font size="2"
    face="Courier New"><strong>map -pi,-1,pi,1 to 40,60,560,330</strong></font></a><font
    size="2" face="Courier New"><strong><br>
    arrow </strong></font><a href="#map"><font size="2"
    face="Courier New"><strong>map(-3.2,0)</strong></font></a><font
    size="2" face="Courier New"><strong> to map(3.2,0): rem this
    draws the x-axis<br>
    </strong></font><a href="#arrow"><font size="2"
    face="Courier New"><strong>arrow map(0,-1.2) to map(0,1.2)</strong></font></a><font
    size="2" face="Courier New"><strong>: rem this is the y-axis<br>
    for x=-3 to 3<br>
    if (x&lt;&gt;0) then </strong></font><a href="#xtick"><font
    size="2" face="Courier New"><strong>xtick map(x,0),str$(x)</strong></font></a><font
    size="2" face="Courier New"><strong> fi: rem these are ticks<br>
    if (x&lt;3) then xtick map(x+0.5,0) fi<br>
    next x<br>
    </strong></font><a href="#map"><font size="2"
    face="Courier New"><strong>ytick map(0,-1),&quot;-1&quot;</strong></font></a><font
    size="2" face="Courier New"><strong><br>
    ytick map(0,1),&quot;+1&quot;<br>
    <br>
    </strong></font><a href="#new line"><font size="2"
    face="Courier New"><strong>new curve</strong></font></a><font
    size="2" face="Courier New"><strong><br>
    for x=-3 to 3 step 0.3: rem this loop actually draws the
    function<br>
    y=sin(x)<br>
    </strong></font><a href="#new line"><font size="2"
    face="Courier New"><strong>line to map(x,y)</strong></font></a><font
    size="2" face="Courier New"><strong><br>
    </strong></font><a href="#marker"><font size="2"
    face="Courier New"><strong>marker map(x,y),&quot;dot&quot;</strong></font></a><font
    size="2" face="Courier New"><strong>: rem mark the function
    points with &quot;dots&quot;<br>
    rem even plot some made-up errors<br>
    marker map(x,y),&quot;err&quot;,ran(0.3),ran(0.5)<br>
    next x<br>
    <br>
    map 0,0,100,4 to 400,250,590,320<br>
    </strong></font><a href="#rectangle"><font size="2"
    face="Courier New"><strong>box map(0,2) to map(90,4)</strong></font></a><font
    size="2" face="Courier New"><strong>: rem draw the legend<br>
    </strong></font><font color="#000000" size="2"
    face="Courier New"><strong>marker</strong></font><font
    size="2" face="Courier New"><strong>
    map(10,3),&quot;dot&quot;<br>
    marker map(10,3),&quot;err&quot;,0.5<br>
    text map(20,3),&quot;Sine with errors&quot;,&quot;lc&quot;<br>
    <br>
    inkey$<br>
    close printer<br>
    close window</strong></font></p>
</blockquote>

<p>First thing you will notice: The program is rather long;
plotting with yabasic still requires a lot of code, but at least
there are some commands, which reduce the work; these are
explained below:</p>

<dl>
    <dt><a name="window origin"><font size="2" face="Courier New"><strong>window
        origin &quot;lb&quot;</strong></font></a></dt>
    <dd>In normal cases the coordinate origin (i.e. the point
        0,0) of any window lies in the upper left corner; the
        command <font size="2" face="Courier New"><strong>window
        origin</strong></font> can move this origin to any of the
        four corners of a window. The string argument
        (&quot;lb&quot; in the example) consists of two chars;
        the first one can be &quot;l&quot; (for left) or
        &quot;r&quot; (for right); the second char can be
        &quot;t&quot; (for top) or &quot;b&quot; (for bottom).
        This gives you a total of four variants &quot;lb&quot;,
        &quot;lt&quot;, &quot;rb&quot; and &quot;rt&quot;, which
        correspond with the four corners of a window.<br>
        Once the origin has been moved, it applies to any grafic
        operation whatsoever.</dd>
    <dt><a name="rectangle"><font size="2" face="Courier New"><strong>rectangle
        10,10 to 590,390</strong></font></a></dt>
    <dd>This command simply draws a rectangle, defined by any two
        opposite corners. Closely related with <font size="2"
        face="Courier New"><strong>rectangle</strong></font> is
        the <font size="2" face="Courier New"><strong>box</strong></font>-command,
        which draws a rectangle too, but additionally clears its
        interior.</dd>
    <dt><a name="map"><font size="2" face="Courier New"><strong>map
        -pi,-1,pi,1 to 40,60,560,330</strong></font></a><font
        size="2" face="Courier New"><strong> </strong></font>and<font
        size="2" face="Courier New"><strong> map(10,3)</strong></font></dt>
    <dd>The <font size="2" face="Courier New"><strong>map</strong></font>-command
        comes in two variants: The first one (<font size="2"
        face="Courier New"><strong>map -pi,-1,pi,1 to
        40,60,560,330</strong></font> in the example) defines a
        mapping, the second one (<font size="2"
        face="Courier New"><strong>map(10,3)</strong></font> in
        the example) may be used <strong>at any point</strong>,
        where a coordinate pair is required (e.g. you may write <font
        size="2" face="Courier New"><strong>dot map(10,10)</strong></font>
        instead of <font size="2" face="Courier New"><strong>dot
        10,10</strong></font>). <font size="2" face="Courier New"><strong>map(x,y)</strong></font>
        transforms a coordinate pair, depending on the previously
        defined mapping; As an example: After <font size="2"
        face="Courier New"><strong>map p1,q1,p2,q2 to x1,y1,x2,y2</strong></font>
        the commands <font size="2" face="Courier New"><strong>dot
        map(p1,q1)</strong></font> and <font size="2"
        face="Courier New"><strong>dot x1,y1</strong></font>
        would draw the same point; also <font size="2"
        face="Courier New"><strong>map(p2,q2)</strong></font> and
        <font size="2" face="Courier New"><strong>x2,y2</strong></font>
        designate the same point. Any intermediate point would be
        interpolated linearly.<br>
        As can be seen in the example, the map-command is used to
        map the x and y-range (-1 ... +1 and -pi ... +pi in the
        example) of a function to the part of the window reserved
        for the plot. This mapping saves a lot of trivial
        calculations.<br>
        Finally note, that there are two additional functions <font
        size="2" face="Courier New"><strong>mapx()</strong></font>
        and <font size="2" face="Courier New"><strong>mapy()</strong></font>,
        both with a single argument, which transform just one
        coordinate.</dd>
    <dt><a name="arrow"><font size="2" face="Courier New"><strong>arrow
        map(0,-1.2) to map(0,1.2)</strong></font></a></dt>
    <dd>The <font size="2" face="Courier New"><strong>arrow</strong></font>-command
        draws an arrow between the specified points; the example
        employs the <font size="2" face="Courier New"><strong>map(,)</strong></font>-function,
        but you could just as well specify the coordiantes
        explicitly (e.g. <font size="2" face="Courier New"><strong>arrrow
        10,10 to 100,100</strong></font>).</dd>
    <dt><a name="xtick"><font size="2" face="Courier New"><strong>xtick
        map(x,0),str$(x)</strong></font></a><font size="2"
        face="Courier New"><strong> </strong></font>and<font
        size="2" face="Courier New"><strong> ytick
        map(0,-1),&quot;-1&quot;</strong></font></dt>
    <dd>The xtick (and ytick) function simply draw a tick at the
        specified location (e.g. <font size="2"
        face="Courier New"><strong>map(x,0)</strong></font> or <font
        size="2" face="Courier New"><strong>100,100</strong></font>);
        if you add a string as a third argument (<font size="2"
        face="Courier New"><strong>str$(x)</strong></font> in the
        example), the tick is drawn somewhat longer and the
        string is written near its end.</dd>
    <dt><a name="new line"><font size="2" face="Courier New"><strong>new
        </strong></font></a><font size="2" face="Courier New"><strong>curve
        </strong></font>and<font size="2" face="Courier New"><strong>
        line to map(x,y)</strong></font></dt>
    <dd>These functions help to plot a curve as a sequence of
        many lines. Let's look at an example: <font size="2"
        face="Courier New"><strong>line to 10,10:line to 100,100</strong></font>;
        the <strong>second </strong><font size="2"
        face="Courier New"><strong>line to</strong></font>-command
        just draws a line from the point specified in the first <font
        size="2" face="Courier New"><strong>line to</strong></font>-command
        (i.e. 10,10) to the point specified in the command itself
        (i.e. 100,100); to add more line segments, you just have
        to specify further <font size="2" face="Courier New"><strong>line
        to</strong></font>-commands. If you want to start with a
        new curve (i.e. a new sequence of lines) just issue the <font
        size="2" face="Courier New"><strong>new curve