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After receiving a nice answer to a previous question, I got enthousiastic and tried to create a macro \sum such that \sum<1+2+3> would be expanded into 6.

My idea was to work recursively, using the parsing functionality of \def. For instance, the following code outputs "7+6+5"

\documentclass{article} \def\sumtestX<#1+#2>{\ifx\empty#2{#1}\else{\sumtestX<#2>+#1 }\fi} \def\sumtest<#1>{\sumtestX<#1+\empty>} \begin{document} \sumtest<5+6+7> \end{document}

So one would expect that turning the "+" into a real "sum" would calculate the actual sum. Nevertheless, the following code doe not work.

\documentclass{article} \def\sum#1+#2{\the\numexpr#1+#2\relax} \def\sumtestX<#1+#2>{\ifx\empty#2{#1}\else{\sum\sumtestX<#2>+#1 }\fi} \def\sumtest<#1>{\sumtestX<#1+\empty>} \begin{document} \sumtest<5+6+7> \end{document}

"Missing number, treated as zero" !? What's wrong?

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Actually some curly braces were wrong and some curly braces are missing around arguments. For analyzing/debugging the setting \tracingmacros=1 is helpful.

\documentclass{article} \def\sum#1+#2{% \the\numexpr#1+#2\relax } \def\sumtestX<#1+#2>{% \ifx\empty#2% #1% \else \sum{\sumtestX<#2>}+{#1}% argument braces \fi } \def\sumtest<#1>{\sumtestX<#1+\empty>} \begin{document} \sumtest<5+6+7> \end{document}

Of course, the recursive definitions are only an exercise? \sumtest could be defined much easier:

\def\sumtest<#1>{\the\numexpr(#1)\relax}
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  • Thank you! (Actually I was seriously going to use this >_> ... as part of my 'solution' for the problem that occurs when trying to calculate sums of "empty" strings, e.g. 5++5=10 or 4+3+=7. But I just found out that appending a +0 always solves that problem.) Commented Nov 17, 2012 at 22:35
  • @Archibald You want more exercises? Strings starting with + as in +1+2, supporting - as binary (and unary) operator, parentheses, ... Commented Nov 17, 2012 at 22:38
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LuaTeX is an ideal choice for such macros. For example, in ConTeXt, you can define:

\def\simplemath<#1>{\ctxlua{context(#1)}} \starttext \simplemath<1+2+3> \simplemath<2*4+5> \stoptext

and in (Lua)LaTeX you can define:

\documentclass{minimal} \def\simplemath<#1>{\directlua{tex.print(#1)}} \begin{document} \simplemath<1+2+3> \simplemath<2*4+5> \end{document}

Note that this works for all types of mathematical expressions, and not just addition.

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