如何仅显示段落的某些行？

Question 1

缺少 Lua(La)TeX 解决方案。不再有：

 \documentclass{article}
 \usepackage{luatexbase}
 \usepackage{luacode}
 \begin{luacode}
 twolines = function ( head )
   local orig_head = head
   local linecounter = 0

   while head do
     if head.id == 0 then
       linecounter = linecounter + 1
     end

     if linecounter > 1 then
       head.next = nil
       return true
     end

     head = head.next
   end
   return true
 end


 luatexbase.add_to_callback("post_linebreak_filter",twolines,"twolines")
 \end{luacode}

 \begin{document}
 \hsize 4in
      A wonderful serenity has taken possession of my entire soul, like these sweet mornings of spring which I enjoy with my
 whole heart. I am alone, and feel the charm of existence in this spot, which was created for the bliss of souls like mine. I am
 so happy, my dear friend, so absorbed in the exquisite sense of mere tranquil existence, that I neglect my talents. I should be
 incapable of drawing a single stroke at the present moment; and yet I feel that I never was a greater artist than now.
 \end{document}

这添加了一个回调函数（twolines），它post_linebreak_filter计算在段落中找到的（节点 id 0）的数量hlists，并删除hlist段落中第二个节点之后的所有内容（a vlist- 这就是orig_head指向的 - 在此示例中不需要）。

缺点：

它适用于每个段落。为了避免这种情况，请仅在需要时激活回调。请参阅chickenize 包作者：Arno Trautmann。
它存在内存泄漏。第二个节点之后的节点hlist仍在结构中。需要调用node.free(...)以从内存中删除节点。请参阅参考手册（第 4 章，节点库）。
它不适用于 iTeX [叮咚]。

编辑：这是一个完整的解决方案，包括环境：

\documentclass{article}
\usepackage{luatexbase}
\usepackage{luacode}
\begin{luacode}
twolines = function ( head )
  local linecounter = 0

  while head do
    if head.id == 0 then
      linecounter = linecounter + 1
    end

    if linecounter >= showlinescount then
      local lastline=head

      head = head.next
      while head do
        local head_next = head.next
        node.free(head)
        head = head_next
      end

      lastline.next = nil
      return true
    end

    head = head.next
  end
  return true
end
\end{luacode}

\newenvironment{showlines}[1]
  {\directlua{showlinescount = tonumber('#1') 
              luatexbase.add_to_callback("post_linebreak_filter",twolines,"twolines")}}
  {\par
   \directlua{luatexbase.remove_from_callback("post_linebreak_filter","twolines")}}

\begin{document}
\hsize 4in

\newcommand\sample{A wonderful serenity has taken possession of my entire soul, like these sweet mornings
of spring which I enjoy with my whole heart. I am alone, and feel the charm of existence in this spot,
which was created for the bliss of souls like mine. I am so happy, my dear friend, so absorbed in the exquisite
sense of mere tranquil existence, that I neglect my talents. I should be incapable of drawing a single
stroke at the present moment; and yet I feel that I never was a greater artist than now.}

\begin{showlines}{3}
\sample
\end{showlines}
\sample
\end{document}

这仍然无法与 iTeX [叮铃铃] 一起使用。但它不再有内存泄漏，并且它提供了一个showlines环境。

Answer

缺少 Lua(La)TeX 解决方案。不再有：

 \documentclass{article}
 \usepackage{luatexbase}
 \usepackage{luacode}
 \begin{luacode}
 twolines = function ( head )
   local orig_head = head
   local linecounter = 0

   while head do
     if head.id == 0 then
       linecounter = linecounter + 1
     end

     if linecounter > 1 then
       head.next = nil
       return true
     end

     head = head.next
   end
   return true
 end


 luatexbase.add_to_callback("post_linebreak_filter",twolines,"twolines")
 \end{luacode}

 \begin{document}
 \hsize 4in
      A wonderful serenity has taken possession of my entire soul, like these sweet mornings of spring which I enjoy with my
 whole heart. I am alone, and feel the charm of existence in this spot, which was created for the bliss of souls like mine. I am
 so happy, my dear friend, so absorbed in the exquisite sense of mere tranquil existence, that I neglect my talents. I should be
 incapable of drawing a single stroke at the present moment; and yet I feel that I never was a greater artist than now.
 \end{document}

这添加了一个回调函数（twolines），它post_linebreak_filter计算在段落中找到的（节点 id 0）的数量hlists，并删除hlist段落中第二个节点之后的所有内容（a vlist- 这就是orig_head指向的 - 在此示例中不需要）。

缺点：

它适用于每个段落。为了避免这种情况，请仅在需要时激活回调。请参阅chickenize 包作者：Arno Trautmann。
它存在内存泄漏。第二个节点之后的节点hlist仍在结构中。需要调用node.free(...)以从内存中删除节点。请参阅参考手册（第 4 章，节点库）。
它不适用于 iTeX [叮咚]。

编辑：这是一个完整的解决方案，包括环境：

\documentclass{article}
\usepackage{luatexbase}
\usepackage{luacode}
\begin{luacode}
twolines = function ( head )
  local linecounter = 0

  while head do
    if head.id == 0 then
      linecounter = linecounter + 1
    end

    if linecounter >= showlinescount then
      local lastline=head

      head = head.next
      while head do
        local head_next = head.next
        node.free(head)
        head = head_next
      end

      lastline.next = nil
      return true
    end

    head = head.next
  end
  return true
end
\end{luacode}

\newenvironment{showlines}[1]
  {\directlua{showlinescount = tonumber('#1') 
              luatexbase.add_to_callback("post_linebreak_filter",twolines,"twolines")}}
  {\par
   \directlua{luatexbase.remove_from_callback("post_linebreak_filter","twolines")}}

\begin{document}
\hsize 4in

\newcommand\sample{A wonderful serenity has taken possession of my entire soul, like these sweet mornings
of spring which I enjoy with my whole heart. I am alone, and feel the charm of existence in this spot,
which was created for the bliss of souls like mine. I am so happy, my dear friend, so absorbed in the exquisite
sense of mere tranquil existence, that I neglect my talents. I should be incapable of drawing a single
stroke at the present moment; and yet I feel that I never was a greater artist than now.}

\begin{showlines}{3}
\sample
\end{showlines}
\sample
\end{document}

这仍然无法与 iTeX [叮铃铃] 一起使用。但它不再有内存泄漏，并且它提供了一个showlines环境。

Question 2

当 TeX 将段落分成几行时，它会为每一行构建一个水平盒子。这堆水平盒子可以放在一个垂直盒子中（这样实际上就有了第一个水平盒子），然后使用该操作一次分析一个水平盒子\lastbox。

下面的代码深受以下影响TeX 按主题分类，第 5.9.6 节。

\documentclass{article}

\usepackage{lipsum}

\newbox\linebox

\newcount\mycount
\def\processline{% Underline every second line
  \global\advance\mycount by 1
  \ifodd\mycount
    \underline{\box\linebox}%
  \else
    \box\linebox
  \fi}

\def\splitlines{%
  \setbox\linebox\lastbox
  \ifvoid\linebox
    \noindent\ignorespaces
  \else
    \unskip\unpenalty
    \begingroup\splitlines\space\endgroup
    \processline
  \fi}

\long\def\doit#1{\vbox{#1\par\splitlines}}

\begin{document}

\doit{\lipsum[1]}

\end{document}

您可以重新定义\processline以满足您的需要。例如，它可能会根据条件丢弃行或\unhbox进行进一步更改。按数字选择行现在只需遍历水平框并比较数字即可。可能需要两次剖析垂直框。

Answer

当 TeX 将段落分成几行时，它会为每一行构建一个水平盒子。这堆水平盒子可以放在一个垂直盒子中（这样实际上就有了第一个水平盒子），然后使用该操作一次分析一个水平盒子\lastbox。

下面的代码深受以下影响TeX 按主题分类，第 5.9.6 节。

\documentclass{article}

\usepackage{lipsum}

\newbox\linebox

\newcount\mycount
\def\processline{% Underline every second line
  \global\advance\mycount by 1
  \ifodd\mycount
    \underline{\box\linebox}%
  \else
    \box\linebox
  \fi}

\def\splitlines{%
  \setbox\linebox\lastbox
  \ifvoid\linebox
    \noindent\ignorespaces
  \else
    \unskip\unpenalty
    \begingroup\splitlines\space\endgroup
    \processline
  \fi}

\long\def\doit#1{\vbox{#1\par\splitlines}}

\begin{document}

\doit{\lipsum[1]}

\end{document}

您可以重新定义\processline以满足您的需要。例如，它可能会根据条件丢弃行或\unhbox进行进一步更改。按数字选择行现在只需遍历水平框并比较数字即可。可能需要两次剖析垂直框。

Question 3

以下代码输出参数中指定的行数，前提是所有行具有相同的高度（更准确地说，基线是均匀分布的）：

\documentclass[a4paper]{article}

\makeatletter
\newbox\wernerboxa
\newbox\wernerboxb

\newenvironment{showlines}[1]
  {\vbadness=\@M\def\how@many{#1}\par
   \setbox\wernerboxa=\vbox\bgroup}
  {\egroup\setbox\wernerboxb=\vsplit\wernerboxa to\how@many\baselineskip
   \vbox{\unvbox\wernerboxb}}
\makeatother

\usepackage{lipsum}

\begin{document}
\begin{showlines}{5}
\lipsum
\end{showlines}
\end{document}

递归地重复该\vsplit操作可以解决显示指定行列表的问题。

让我们看看另一种可以应对一行中有更高物体的情况的方法。

如果段落中没有花哨的东西，只有纯文本，则修改我的答案会为段落的每一行添加下划线；实际上，规则是由宏添加的\add@rule，因此可以对其进行修改，使其仅在满足某些条件时才绘制规则；最终的行数可在宏中找到\how@many。这是自下而上的；添加检查某行是否必须加下划线不应该太复杂。

\documentclass[a4paper]{article}
\usepackage{lipsum}

\makeatletter

\newif\ifboxended
\newbox\wernerbox
\def\add@rule{\kern0.8\p@\hrule\kern-1.2\p@}
\newenvironment{werner}
  {\par\setbox\wernerbox=\vbox\bgroup}
  {\par\xdef\how@many{\the\prevgraf}\egroup
   \message{The box has \how@many\space lines}%
   \underlinevbox}

\def\underlinevbox{\begingroup\global\setbox\@ne=\box\voidb@x
  \global\boxendedfalse
  \setbox\z@=\copy\wernerbox\relax\dounderlinevbox}
\def\dounderlinevbox{%
  \setbox\z@=\vbox{\unvbox\z@ 
    \ifcase\lastnodetype
% char node (can't remove)
    \or
% hlist node
    \setbox\tw@=\lastbox
    \def\next{\global\setbox\@ne=\vbox{\box\tw@\add@rule\unvbox\@ne}}%
    \or
% vlist node
    \setbox\tw@=\lastbox
    \def\next{\global\setbox\@ne=\vbox{\box\tw@\add@rule\unvbox\@ne}}%
    \or
% rule node (can't remove)
    \or
% ins node (can't remove)
    \or
% mark node (can't remove)
    \or
% adjust node (can't remove)
    \or
% ligature node (can't happen)
    \or
% disc node (can't happen)
    \or
% whatsit node (can't remove)
    \or
% math node (can't remove)
    \or
% glue node
    \skip@=\lastskip\unskip
    \def\next{\global\setbox\@ne=\vbox{\vskip\skip@\unvbox\@ne}}%
    \or
% kern node
    \dimen@=\lastkern\unkern
    \def\next{\global\setbox\@ne=\vbox{\kern\dimen@\unvbox\@ne}}%
    \or
% penalty node
    \count@=\lastpenalty\unpenalty
    \def\next{\global\setbox\@ne=\vbox{\penalty\count@\unvbox\@ne}}%
    \or
% unset node (can't happen)
    \or
% math mode node (can't remove)
    \else
% empty list
    \def\next{\global\boxendedtrue}
    \fi
    \next}
  \ifboxended
    \def\next{\unvbox\@ne\endgroup}
  \else
    \let\next\dounderlinevbox
  \fi
  \next}
\makeatother

\begin{document}
\begin{werner}
\lipsum[2]
\end{werner}
\end{document}

可以修改代码以仅输出指定的行。需要注意的是，拆除是从最底下的一行开始的。

Answer