How to pass a variable to a command in LaTeX?

Question

I am trying to make a tax calculator for an invoice template and the idea is to pass the variable subcost to the command \taxcalc.

\newcommand*{\taxcalc}[2]{%
  \pgfmathparse{#1/(1.0-#2)-#1}%
  \pgfmathprintnumber{\pgfmathresult}%
}

\newcommand{\vatTotal}{
     & & & {\bf BTW 21\%} & {\bf {\bf \euro\taxcalc{subcost}{0.21}}}
    \\*[1.5ex]
}

The problem is, no matter what I try to do to subcost, the compiler keeps complaining:

Package PGF Math Error: Unknown function `subcost' (in 'subcost/(1.0-0.21)-subcost').'

It works perfectly for the following:

\newcommand*{\total}[1]{\FPdiv{\t}{\arabic{#1}}{1000}\formatNumber{\t}}

\newcommand{\subTotal}{
    & & & {\bf Subtotaal (excl. btw)} & {\bf \euro\total{subcost}}
    \\*[1.5ex]
}

Can anyone help me? I've been googling for hours now in order to find how to correctly pass variables to functions in this language but no matter what I try, it only works if I hardcode a number.

subcost is defined as follows:

\newcounter{hours} \newcounter{subhours}
\newcounter{cost} \newcounter{subcost}
\newcounter{vat}

Answer 1

Be aware that the concept of a LaTeX-counter differs from the concept of a mathematical function in PGF-math.

Seems with your first snippet the problem is:

\taxcalc's first and second argument must be numbers or calls to defined PGF-math-functions—i.e., things which can be used for calculations with PGF-math-things.

But by \vatTotal as first argument the sequence subcost is passed to \taxcalc. That sequence does neither denote such a number nor denote a call to such a defined PGF-math-function.

PGF-math-things cannot/do not "know" that the sequence subcost denotes the name of a LaTeX-counter whose value is to be obtained.

PGF-math-things "assume" that subcost denotes the name of a PGF-math-function implemented by means of PGF-math-things for doing PGF-math-things and therefore "complains" that the function "subcost" is undefined.

Another question is: Why \bf twice, opening up two nested local scopes? Seems one local scope and one \bf is sufficient. :-)

You can modify \vatTotal to make sure that the value of the subcost-LaTeX-counter gets expanded "into" \taxcalc's first argument before \taxcalc gets carried out. This way at the time of carrying out \taxcalc the first argument of \taxcalc will not be a character-sequence denoting the name of a LaTeX-counter but will be a number and thus something that is "understood" by the PGF-math-things.

Does

\newcommand*{\taxcalc}[2]{%
  \pgfmathparse{#1/(1.0-#2)-#1}%
  \pgfmathprintnumber{\pgfmathresult}%
}

\newcommand{\vatTotal}{%
    &&&{\bf BTW 21\%}%
    &{\bf\euro\expandafter\taxcalc\expandafter{\number\value{subcost}}{0.21}}%
    \\*[1.5ex]%
}

work?

Perhaps \textbf{..} instead of {\bf...}:

\newcommand*{\taxcalc}[2]{%
  \pgfmathparse{#1/(1.0-#2)-#1}%
  \pgfmathprintnumber{\pgfmathresult}%
}

\newcommand{\vatTotal}{%
    &&&\textbf{BTW 21\%}%
    &\textbf{\euro\expandafter\taxcalc\expandafter{\number\value{subcost}}{0.21}}%
    \\*[1.5ex]%
}

---

Your second snippet works because here the sequence subcost is wrapped into \arabic{...} by the routine \total while the argument of \arabic must denote the name of a LaTeX-counter.
In other words: Here \total takes into account that subcost denotes the name of a LaTeX-counter by applying \arabic to that phrase for obtaining a number which FP-math-stuff can calculate with.

---

Summa summarum:

The routine \taxcalc from your first snippet processes two arguments whereof each must denote a number or a call to a defined PGF-math-function which in turn delivers a number.

The name of a LaTeX-counter neither denotes such a number nor denotes the call to a defined PGF-math-function.
Therefore you need steps for obtaining the value/number from the name of the LaTeX-counter.

The routine \total processes one argument which must denote the name of a LaTeX-counter.

\total internally applies \arabic for obtaining a number from that which FP-math-thingies can use for calculating.

---

By the way:

If PGF-math is in use, then I prefer Schrödinger's cat's approach of not using LaTeX-counters at all for holding values of "variables" but using PGF-math-functions that don't process arguments but always deliver the same number. (Reason: Each LaTeX-counter internally requires allocation of a count-register while the amount of count-registers is limited in LaTeX. Afaik declaring a new PGF-math-function doesn't require allocating another count-register.)

In case a PGF-math-function does not process arguments and always delivers the same number, the name of that PGF-math-function within the context of PGF-math can be taken for a placeholder for that number/for a variable whose value is that number.

Answer 2

Your approach works. You can declare a variable, or parameterless function, with pgf which you seem to be using. In order to be able to redefine the function, you need the starred version, \pgfmathdeclarefunction*. This allows you to define a variable and assign it a value which you can use in any expression that you parse with pgf. For your convenience, I packed the \pgfmathdeclarefunction* stuff in a macro \SetPgfVariable.

\documentclass{article}
\usepackage{pgf}
\newcommand*{\taxcalc}[2]{%
  \pgfmathparse{#1/(1.0-#2)-#1}%
  \pgfmathprintnumber{\pgfmathresult}%
}
\newcommand{\SetPgfVariable}[2]{%
\pgfmathdeclarefunction*{#1}{0}{\pgfmathparse{#2}}}
\begin{document}
\SetPgfVariable{subcost}{0.4}
\taxcalc{subcost}{0.21}

\SetPgfVariable{subcost}{0.5}
\taxcalc{subcost}{0.21}
\end{document}