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关于METAPOST中drawarrow的一个简单问题

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关于METAPOST中drawarrow的一个简单问题

我是 METAPOST 的初学者。我只是想画一个箭头。我在 TeXstudio 2.11.0 中输入以下代码：

% This program uses the METAPOST language to produce the eps file of {fig:R_5}.
% In TeXstudio, click Tools->Commands->MetaPost to compile this file.

prologues:=3;
filenametemplate "%j.eps";

input boxes
beginfig(1);
    % The upper bound of $z_1$.
    z1UB=8.6cm;

    % Draw the $z_1$-axis.
    drawarrow (0,0)--(z1UB,0);  
endfig;
end;

然后我点击 Tools->Commands->MetaPost 来编译上述文件。出乎意料的是，我收到了以下错误消息：

Process started: mpost.exe -interaction=nonstopmode R_5.mp

Process exited with error(s)

日志文件是：

This is MetaPost, version 1.803 (MiKTeX 2.9 64-bit)  2 SEP 2016 16:49
**R_5.mp
(mpost.mp (C:/Program Files/MiKTeX 2.9/metapost/base/plain.mp
Preloading the plain mem file, version 1.004) ) (./R_5.mp
(C:/Program Files/MiKTeX 2.9/metapost/base/boxes.mp)
>> (x1UB,y1UB)
>> 243.77965
! Equation cannot be performed (pair=numeric).
<to be read again> 
                   ;
l.10 ^^Iz1UB=8.6cm;

I'm sorry, but I don't know how to make such things equal.
(See the two expressions just above the error message.)

! Missing `)' has been inserted.
<to be read again> 
                   ,
l.13     drawarrow (0,0)--(z1UB,
                                0);  
I found no right delimiter to match a left one. So I've
put one in, behind the scenes; this may fix the problem.

>> x1UB
! Undefined x coordinate has been replaced by 0.
<to be read again> 
                   ,
l.13     drawarrow (0,0)--(z1UB,
                                0);  
I need a `known' x value for this part of the path.
The value I found (see above) was no good;
so I'll try to keep going by using zero instead.
(Chapter 27 of The METAFONTbook explains that
you might want to type `I ???' now.)

>> y1UB
! Undefined y coordinate has been replaced by 0.
<to be read again> 
                   ,
l.13     drawarrow (0,0)--(z1UB,
                                0);  
I need a `known' y value for this part of the path.
The value I found (see above) was no good;
so I'll try to keep going by using zero instead.
(Chapter 27 of The METAFONTbook explains that
you might want to type `I ???' now.)

! Extra tokens will be flushed.
<to be read again> 
                   ,
<argument> ,
            0)
_finarr->draw._apth(TEXT0)
                          ;filldraw.arrowhead._apth(TEXT0)
<to be read again> 
                   ;
l.13     drawarrow (0,0)--(z1UB,0);

I've just read as much of that statement as I could fathom,
so a semicolon should have been next. It's very puzzling...
but I'll try to get myself back together, by ignoring
everything up to the next `;'. Please insert a semicolon
now in front of anything that you don't want me to delete.
(See Chapter 27 of The METAFONTbook for an example.)

! Extra tokens will be flushed.
<to be read again> 
                   ,
<argument> ,
            0)
_finarr->...TEXT0);filldraw.arrowhead._apth(TEXT0)

<to be read again> 
                   ;
l.13     drawarrow (0,0)--(z1UB,0);

I've just read as much of that statement as I could fathom,
so a semicolon should have been next. It's very puzzling...
but I'll try to get myself back together, by ignoring
everything up to the next `;'. Please insert a semicolon
now in front of anything that you don't want me to delete.
(See Chapter 27 of The METAFONTbook for an example.)

[1] )
1 output file written: R_5.eps

您能告诉我如何编写正确的代码吗？非常感谢。

答案1

Metafont/Metapost 中的标准约定是z（带下标）表示一对。因此您无法设置z1UB=8.6cm。

当你这样做

z1=(1,2);

你基本上是在做

x1=1;y1=2;

对变量使用不同的名称。

或者，

prologues:=3;
filenametemplate "%j.eps";

input boxes
beginfig(1);
    % The upper bound of $z_1$.
    z1UB=(8.6cm,0);

    % Draw the $z_1$-axis.
    drawarrow (0,0)--z1UB;
endfig;
end;

您还可以使用

    drawarrow origin--z1UB;

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