I am attempting to write a macro which can remove any extra negative signs in a general equation:

15.2 + -3.1

would be changed to:

15.2 - 3.1

when using the commands:

\[ A \pluss{\B} \]

My problems start when defining the macro \pluss{} for cases such as:

\[ A \pluss{\B} \]


I thought that in the pluss#1{} macro I would just count the number of negative signs in #1. If it is odd then the result is negative but I am having two problems:

  1. How to count negative signs when the input to the macro is \frac{-3}{12}
  2. How to convert the catcode of the negative signs in \frac{-3}{12} so that they don't print...
  • Welcome to TeX.SE! Does the fraction have to be stated as \frac{-3}{4} in the \B macro, or would it be OK to state it as -\frac{3}{4}? Could a minus sign be tucked away in the denominator as well? – Mico Jun 30 '12 at 0:17
  • Unfortunately, minus sign can be anywhere... – JAD Jun 30 '12 at 0:43
  • Is 1 \pplus{1-2} allowed? – Caramdir Jun 30 '12 at 0:54
  • 2
    Can you create a list of test cases that you would like this to work for. I have an idea, but need to know what kind of expressions you are thinking of and to test that it works for the cases you are thinking of. – Peter Grill Jun 30 '12 at 1:21
  • 1
    The assumption is also that you don't want two negatives to turn into a positive "automagically", right? – Werner Jun 30 '12 at 1:25

The argument to \pluss is scanned twice, one for counting the minus signs, the second time ignoring them:


\newcommand{\pluss}{\begingroup\mathcode`-="8000 \plussaux}
  \sbox0{\global\minuses=0 \minuscounts$#1$}%
  \begingroup\lccode`~=`- \lowercase{\endgroup
  \def~}{\global\advance\minuses1 }}
  \begingroup\lccode`~=`- \lowercase{\endgroup















enter image description here

The argument to \pluss should be a monomial expression: something like \pluss{1-2} would produce a weird result.


Here is a solution that redefines the \frac macro within the \pluss macro and evaluates the ratio to test if it is negative.

This seems to work for all the cases I could come up with. I attempted to format the output so that it clearly shows the input and output, but there are a few cases (noted with a red asterix) where you need to refer to the code to actually see what it is.

enter image description here





    \sbox\ExpresionBox{\ensuremath{#1}}% set EncountredFraction
        \renewcommand{\frac}[2]{(##1 / ##2)}%





\newcommand{\Test}[2]{$#1~\text{\texttt{\textbackslash pluss}} \{#2\} \to #1 \pluss{#2}$}%

\section*{These  work}


\Test{\A}{-\PositiveNumber} \Note\quad Subtract a postive number\par
\Test{\A}{\NegativeNumber}  \Note\quad Add a negative number\par
\Test{\A}{-\NegativeNumber} \Note\quad Subtract a negative number\par

\textit{Add a positive frac:}\par

\textit{Add a negative frac:}\par


\textit{Subtract a positive frac:}\par

\textit{Subtract a negative frac:}\par

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