Odd results using FP package inside table environment

Question

I'm trying to create a number of running totals using the FP package. I define a macro for adding numbers to a running total and then I define a macro to output the final value.

The problem I'm having is that I receive a different output from the totals macro depending on where I call it. If it's inside the table I receive the correct value however if it's outside then the value doubles.

Any ideas?

\documentclass[11pt]{report}

\usepackage{fp} 
\usepackage{tabu,longtable}

\FPset\totalone{0}
\FPset\totaltwo{0}

\def\add#1{%
  \FPadd\0\totalone{#1}
  \global\let\totalone\0
  \FPadd\0\totaltwo{#1}  
  \global\let\totaltwo\0
  #1
}

\def\ptotalone{%
  \totalone
  \FPset\0{0}\global\let\totalone\0
}

\def\ptotaltwo{%
  \totaltwo
  \FPset\0{0}\global\let\totaltwo\0
}

\begin{document}

  \begin{table}[htbp]
    \begin{longtabu}{c}
      \add{0.25} \\
      \add{0.5}  \\
      \add{1}    \\
      total one: \ptotalone
    \end{longtabu}
  \end{table}
  total two: \ptotaltwo

\end{document}

Example output:

0.25
0.5
1
total one: 1.75
total two: 3.5

Answer 1

The tabu (and so also longtabu) environments do two passes over the material, so your operations are performed twice. Unfortunately there doesn't seem to exist a conditional that's set in only one of the passes in order to do register settings or assignments only once (similar to \ifmeasuring@ of amsmath).

So the answer seems to be: don't use tabu. Note that, as said by the author, the next version of tabu will not guarantee back compatibility, so I can't recommend using it (the author answered me saying, about back compatibility, "I don't care").