Control height of longtable's last page

Question

I want to control table, created by longtable enwironment such way that it would have pagebreak before last two lines if the rest of the table would be longer than some height (it depends on height of text, that follows the table and should be fully on last page with at least two last columns of table). The last line and text after table is static block, I put it into lastfoot. But line before last should be always on the same page with last line.

Here is the shortest working example:

\documentclass[12pt]{letter}
\usepackage[english]{babel}
\usepackage{longtable}
\usepackage{array}

\makeatletter
\newtoks\f@st
\f@st={}
\def\pr@line#1{#1\global\f@st={}}
\def\RPOline#1#2{\pr@line{\the\f@st}\global\f@st={#1&#2\cr\hline}}
\newcolumntype{C}[1]{>{\centering}m{#1}}
\newsavebox\l@stb@x
\def\RPOb@g{
    \global\savebox{\l@stb@x}{\hbox to\textwidth{\vbox to 10cm{some text\vfill bottom}}}
    \begin{longtable}{|C{1cm}|C{5cm}|}
    \hline N& Name\cr\hline\endfirsthead
    \endhead\endfoot
    \multicolumn{2}{l}{\usebox{\l@stb@x}}\endlastfoot
}
\newtoks\old@utput
\newdimen\texth@
\old@utput=\expandafter{\the\output}
\def\ts@{\texth@=\textheight\advance\texth@ by -\ht\l@stb@x\advance\texth@ by -5mm\relax
    \setbox0=\vbox{\unvbox255}\ifdim\ht0<\texth@
    \setbox255=\vbox{\unvbox0\pr@line{\the\f@st}}\else
    \setbox255=\vbox{\unvbox0}\fi
    \the\old@utput
    \global\output=\expandafter{\the\old@utput}}
\def\PRO@nd{
    \global\output={\ts@}\cr\pagebreak
    \pr@line{\the\f@st}
    \end{longtable}
}
\newenvironment{RPOtable}{\RPOb@g}{\PRO@nd}
\makeatother
\newcount\tstcnt
\def\fillt#1{\ifnum\tstcnt>0\global\advance\tstcnt by -1\relax #1\fillt{#1}\fi}
\def\filltbl#1#2{\global\tstcnt=#1\relax\fillt{#2}}

\begin{document}
\begin{RPOtable}
    \filltbl{31}{\RPOline{a}{Sample line}}
    \RPOline{L}{Last line}
\end{RPOtable}
\end{document}

When I put \cr\pagebreak into \PRO@nd (like written above) I get no errors but have empty lines in table and the table always breaks on \pagebreak. If I omit \pagebreak, line marked as "Last line" stays on previous page, and there's still an empty table line after it.

Question

How can I make "unbreaking" of page with longtable? I can't do \vsplit bacause it is already a box. Also I always have error messages Misplased \noalign if I don't put \cr before of \pagebreak.

Answer 1

As documented in the longtable documentation the way to keep the final rows together is to put them into the last foot, or to use \\*.

The code as posted is in the wrong place so producing spurious cells hence the extra vertical line segments, and is missing % at ends of lines so introducing white space. If I understand the problem there should be no need to switch output routines. Certainly switching output routine inside the table seems brave.

You need \arraybackslash after \centering to use \\ in this context (or use \tabularnewline never \cr except in internal code.

This version puts the last 4 rows in the last foot below there is a version that uses nopagebreak penalties (which as noted in comments probably need some further tweaks).

\documentclass[12pt]{letter}
\usepackage[english]{babel}
\usepackage{longtable}
\usepackage{array}

\raggedbottom
\newcolumntype{C}[1]{>{\centering\arraybackslash}m{#1}}
\setlength\extrarowheight{2pt}

\def\samplerow#1#2{%
\ifnum#1>0
#2\\\hline
\afterfi
\expandafter\samplerow\expandafter{\the\numexpr#1-1\relax}{#2}%
\fi}
\def\afterfi#1\fi{\fi#1}


\raggedbottom
\begin{document}


\makeatother
\begin{longtable}{@{}|C{1cm}|C{5cm}|@{}}
\hline
N&Name\\\hline\endhead
\samplerow{4}{a&sample line}
\noalign{\nobreak\smallskip\noindent\parbox\textwidth{Last line}}\endlastfoot
\samplerow{25}{a&Sample Line}
\end{longtable}
\end{document}

---

The following generates 31 lines plus a head and following paragraph, ensuring at least 4 rows are kept with the end of the table.

You could normally just use \nopagebreak but \hline allows page breaking at \hline so this substitutes a non page break version for the last few rows.

\documentclass[12pt]{letter}
\usepackage[english]{babel}
\usepackage{longtable}
\usepackage{array}

\raggedbottom
\newcolumntype{C}[1]{>{\centering\arraybackslash}m{#1}}
\setlength\extrarowheight{2pt}

\makeatletter
\def\samplerow#1#2{%
\ifnum#1>0
#2\\
\ifnum#1<5 
\nopagebreak
  \multispan\LT@cols
     \unskip\leaders\hrule\@height\arrayrulewidth\hfill\cr
\nopagebreak
\else
\hline
\fi
\afterfi
\expandafter\samplerow\expandafter{\the\numexpr#1-1\relax}{#2}%
\fi}
\def\afterfi#1\fi{\fi#1}
\makeatother

\raggedbottom
\begin{document}


\makeatother
\begin{longtable}{@{}|C{1cm}|C{5cm}|@{}}
\hline
N&Name\\\hline\endhead
\noalign{\nobreak\smallskip\noindent\parbox\textwidth{Last line}}\endlastfoot
\samplerow{31}{a&Sample Line}
\end{longtable}
\end{document}