How to eliminate the \\ at the end of a longtable

Question

I define a marco \mycmd like this

\newcommand\mycmd[2]{{some complicated format #1}&{some complicated format #2}\\}

to simply the input. So I can just write:

sometext

\begin{longtable}{cc}
  \mycmd{contentA1}{contentA2}
  \mycmd{contentB1}{contentB2}
  \mycmd{contentC1}{contentC2}
  ...

But, at the end of the longtable, as

  \mycmd{contentZ1}{contentZ2}
\end{longtable}

some text

there is a white space between longtable and text. How can I eliminate the space by just setting a command at the end of the last \mycmd like

  \mycmd{contentZ1}{contentZ2}\avalidmacro
\end{longtable}

some text

to let the table near to the text?

---Resupply MWE for comparison---

\setlength{\LTpre}{0pt}\setlength{\LTpost}{0pt}%
\begin{longtable}{cc}
  sample table text&sample table text\\
\end{longtable}

main text main text main text main text

\begin{longtable}{cc}
  sample table text&sample table text
\end{longtable}

main text main text main text main text

main text main text main text main text

---Edit for Yiannis---

\documentclass{article}
\usepackage{longtable,xcolor}
\def\fqdi{fqd}
\def\fqdv{\fqdi\ \fqdi\ \fqdi\ \fqdi\ \fqdi\ }

\begin{document}
\setlength{\LTpre}{0pt}\setlength{\LTpost}{0pt}%
\itshape 
\fqdv\fqdv

\begin{longtable}{cc}
\color{red}\fqdv\fqdv&\color{red}\fqdv\fqdv\\
\end{longtable}

\fqdv\fqdv

\begin{longtable}{cc}
\color{red}\fqdv\fqdv&\color{red}\fqdv\fqdv
\end{longtable}

\fqdv\fqdv

\fqdv\fqdv

\begin{tabular}{cc}
\color{blue}\fqdv\fqdv&\color{blue}\fqdv\fqdv
\end{tabular}

\fqdv\fqdv
\end{document}

The red is the text in longtable, the blue is the text in tabular, black the main text.

• You will find a slightly space added to the first line's red and the second line's black.
• The red is not centered between lines, while the blue is.

Queation:

1. eliminate the effect of \\;
2. make longtable's text same as tabular's text, centering between lines.

Answer 1

It is not your command, but rather the amount of glue allowed at the end of the table. See this minimal and adjust slightly the LTpost value.

\documentclass{article}
\usepackage{lipsum,longtable}
\newcommand\addrow[2]{{some complicated format #1}&{some complicated format #2}\\}
\setlength{\LTpre}{0pt}\setlength{\LTpost}{-1pt}%
\begin{document}
\lipsum[3]
\begin{longtable}{cc}
%\hline
  \addrow{contentA1}{contentA2}
  \addrow{contentB1}{contentB2}
  \addrow{contentC1}{contentC2}
  \addrow{contentZ1}{contentZ2}
%\hline
\end{longtable}
\lipsum[1]
\end{document}

It is always better to fully provide a working minimal, as I have done here. Uncomment the \hline to see the space between the rows. Also it is better to use "semantic" commands, for example instead of \mycmd rather use \addrow.