[Tex/LaTex] PGF: Varied Number of Decimal Places

pgfmath

I am making a macro to help show kids how to estimate, say, 34.1% of 5.

I just can't get the calculations to output exact numbers with appropriate (minimal) decimal places. Instead, PGF is showing me that 40% of 5 is 1.99997

How do I fix that? And what about in the future when I may want to specify a maximum number of decimal places?

\documentclass{article}

\usepackage{tikz}
\usepackage{pgf}
\usepackage[margin=1.5cm]{geometry}

\title{Percents of Whole Numbers on a Number Line}
\date{}
\author{}

\setlength{\parindent}{0pt}

\newcommand{\percentA}[2]
{
\begin{tikzpicture}[xscale=18]
 \foreach \x in {0,1,...,#1}
  {
   \node at (\x/#1,0.45) {\x}; % Whole numbers
   \draw [very thick] (\x/#1,0)--(\x/#1,0.25);  % Whole number tick marks
  }

 \draw[very thick] (0,0)--(1,0); % x axis

 \foreach \x in {0,...,100} % Hundredths tick marks
  \draw (\x/100,-0.1)--(\x/100,0);

 \foreach \x in {0,1,2,...,10}
  {
   \draw[thick] (\x/10,-0.18)--(\x/10,0); % Tenths tick marks
   \pgfmathsetmacro{\tenspercents}{int(\x*10)}   
   \node at (\x/10,-0.6) {\tenspercents \%};
   \node at (\x/10,-1) {of #1};
   \pgfmathsetmacro{\result}{\tenspercents/100*#1,}   
   \node at (\x/10,-1.4) {is \result};
  }  
\end{tikzpicture}
}

\begin{document}

\pagestyle{empty}

\maketitle

\percentA{5}

\end{document}

That code yields this:

Percents of 5

Best Answer

As I said in response to your other question, you want to format the number to pretty-print it. In this case, I'm guessing the default format will be suitable, but you can obviously tune it according to your particular needs.

pretty printed numberline

\documentclass[border=10pt,multi,tikz]{standalone}
\newcommand{\percentA}[2]
{%
\begin{tikzpicture}[xscale=18]
 \foreach \x in {0,1,...,#2}
  {
   \node at (\x/#2,0.45) {\x}; % Whole numbers
   \draw [very thick] (\x/#2,0)--(\x/#2,0.25);  % Whole number tick marks
  }

 \draw[very thick] (0,0)--(1,0); % x axis

 \foreach \x in {0,...,100} % Hundredths tick marks
  \draw (\x/100,-0.1)--(\x/100,0);

 \foreach \x in {0,1,2,...,10}
  {
   \draw[thick] (\x/10,-0.18)--(\x/10,0); % Tenths tick marks
   \pgfmathsetmacro{\tenspercents}{int(\x*10)}
   \node at (\x/10,-0.6) {\tenspercents \%};
   \node at (\x/10,-1) {of #2};
   \pgfmathsetmacro{\myresult}{\tenspercents/100*#2}
   \node at (\x/10,-1.4) {is \pgfmathprintnumber{\myresult}};
  }
\end{tikzpicture}%
}
\begin{document}
\percentA{20}{5}
\end{document}

The default maths stuff is never going to be accurate. If you need accuracy, you can use floating point, for example. But I doubt that is really needed for grade 2 maths (assuming this is still for your second graders). Just printing the numbers to make them prettier is all that's really required.

EDIT

This part of my answer uses a corrected version of the alternative version of \percentA which you posted later. Unlike the first version, this only uses 1 argument and your example only gave 1 argument. Unfortunately, like the first version, it still demanded 2.

Also note that both versions introduce spurious spaces. I corrected this above and have annotated the corrections for the alternative version below.

Also also note that you should not include a comma in the argument you expect pgfmath to parse as a number. This will create problems. For example, the number cannot be formatted correctly afterwards because it contains alien stuff. Again, I corrected that above and repeat the correction with an annotation below.

The code below is designed to demonstrate a few of the various most-relevant-to-second-grader ways in which a number can be formatted by pgfmath. For other options, such as scientific formats, see TikkZ's manual where the options are extensively described.

The following gives the output of 

\percentA{5}

for a single definition of \percentA{}. That is, the numbers being printed are the same in every case. All that changes are relevant settings for /pgf/number format, as given in the code which follows.

various number formats of possible interest to second graders

Remember: the series of values held by \result are identical for each of the number lines, because the definition of \percentA{} is held constant. What changes is the effect of the macro \pgfmathprintnumber{} when fed the series of values in \result as arguments.

\documentclass[border=10pt,multi,tikz]{standalone}
\newcommand{\percentA}[1]% if you are only going to give one argument, don't say you'll give two
{% avoid introducing spurious spaces
\begin{tikzpicture}[xscale=18]
 \foreach \x in {0,1,...,#1}
  {
   \node at (\x/#1,0.45) {\x}; % Whole numbers
   \draw [very thick] (\x/#1,0)--(\x/#1,0.25);  % Whole number tick marks
  }

 \draw[very thick] (0,0)--(1,0); % x axis

 \foreach \x in {0,...,100} % Hundredths tick marks
  \draw (\x/100,-0.1)--(\x/100,0);

 \foreach \x in {0,1,2,...,10}
  {
   \draw[thick] (\x/10,-0.18)--(\x/10,0); % Tenths tick marks
   \pgfmathsetmacro{\tenspercents}{int(\x*10)}
   \node at (\x/10,-0.6) {\tenspercents \%};
   \node at (\x/10,-1) {of #1};
   \pgfmathsetmacro{\result}{\tenspercents/100*#1}% comma is not part of number!!
   \node at (\x/10,-1.4) {is \pgfmathprintnumber{\result}};
  }
\end{tikzpicture}% avoid introducing spurious spaces
}
\begin{document}
\percentA{5}
\tikzset{%
  /pgf/number format/.cd,
  fixed,
  precision=2,
}
\percentA{5}
\tikzset{%
  /pgf/number format/.cd,
  fixed zerofill,
}
\percentA{5}
\tikzset{%
  /pgf/number format/.cd,
  precision=0,
}
\percentA{5}
\tikzset{%
  /pgf/number format/.cd,
  int trunc,
}
\percentA{5}
\end{document}
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