pgfmathtikz-pgf
How do I use the commands \pgfmathparse {tan (pi/3 r)} \pgfmathresult
to draw the green line in the figure below?
\begin{tikzpicture}[>=latex,scale=2]
\draw circle (1);
\draw[->] (-1.2,0) -- (1.2,0) coordinate (x);
\draw[->] (0,-1.2) -- (0,1.2);
\fill[fill=green!25] (0,0) -- (.2,0) arc (0:60:.2) -- cycle;
\draw (0,0) -- (60:1);
\draw[<-,red,line width=1pt] (60:1) -- (60:1 |- x);
\draw (0,0) -- (60:2);
%tangent
\draw[<-,green,line width=1pt] (60:2) -- (1,0);
\end{tikzpicture}
A solution which allows to draw intersection segments of any two intersections is available as tikz library fillbetween.
This library works as general purpose tikz library, but it is shipped with pgfplots and you need to load pgfplots in order to make it work:
\documentclass{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{fillbetween}
\begin{document}
\begin{tikzpicture}
\draw [name path=red,red] (120:1.06) circle (1.9);
%\draw [name path=yellow,yellow] (0:1.06) circle (2.12);
\draw [name path=green,green!50!black] (0:0.77) circle (2.41);
\draw [name path=blue,blue] (0:0) circle (1.06);
% substitute this temp path by `\path` to make it invisible:
\draw[name path=temp1, intersection segments={of=red and blue,sequence=L1}];
\draw[red,-stealth,ultra thick, intersection segments={of=temp1 and green,sequence=L3}];
\end{tikzpicture}
\end{document}
The key intersection segments is described in all detail in the pgfplots reference manual section "5.6.6 Intersection Segment Recombination"; the key idea in this case is to
create a temporary path temp1 which is the first intersection segment of red and blue, more precisely, it is the first intersection segment in the Left argument in red and blue : red. This path is drawn as thin black path. Substitute its \draw statement by \path to make it invisible.
Compute the desired intersection segment by intersecting temp1 and green and use the correct intersection segment. By trial and error I figured that it is the third segment of path temp1 which is written as L3 (L = left argument in temp1 and green and 3 means third segment of that path).
The argument involves some trial and error because fillbetween is unaware of the fact that end and startpoint are connected -- and we as end users do not see start and end point.
Note that you can connect these path segments with other paths. If such an intersection segment should be the continuation of another path, use -- as before the first argument in sequence. This allows to fill paths segments:
\documentclass{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{fillbetween}
\begin{document}
\begin{tikzpicture}
\draw [name path=red,red] (120:1.06) circle (1.9);
%\draw [name path=yellow,yellow] (0:1.06) circle (2.12);
\draw [name path=green,green!50!black] (0:0.77) circle (2.41);
\draw [name path=blue,blue] (0:0) circle (1.06);
% substitute this temp path by `\path` to make it invisible:
\draw[name path=temp1, intersection segments={of=red and blue,sequence=L1}];
\draw[red,fill=blue,-stealth,ultra thick, intersection segments={of=temp1 and green,sequence=L3}]
[intersection segments={of=temp1 and green, sequence={--R2}}]
;
\end{tikzpicture}
\end{document}
The \pgfmathresult is often set and used by other stuff, in this case probably the \node (the y-coordinate is 0.3, so seems a likely candidate). Moving the \pgfmathparse to immediately before the \pgfmathresult solves this.
Another option is to save the result to a macro with \pgfmathsetmacro{\<macro name>}{<calculation>}.
One final point: If you know that the numerator in your cases will always become an integer (as here, with 3*10/3), then you can use for example the int function to avoid getting 10.0 as output. All of this is demonstrated in the example below.
\documentclass{article}
\usepackage{tikz}
\newcommand{\NLdotA}[5] %{xscale}{yscale}{xmax}{denom}{xval of dot}
{\tikz[xscale=#1,yscale=#2]
{
\draw (0,0)--(#3,0);
\pgfmathsetmacro{\EndOfForLoop}{#3*#4}
\foreach \x in {0,...,\EndOfForLoop}
\draw (\x/#4,-0.2)--(\x/#4,0.2);
\foreach \x in {0,...,#3}
\node[below] at (\x,-0.3) {\x};
\fill[gray,opacity=0.5] (#5,0) circle [x radius = 0.2/#1, y radius = 0.2/#2];
\pgfmathsetmacro{\TheNumerator}{int(#4*#5)}
\node[above] at (#5,0.3) {$\frac{\TheNumerator}{#4}$}
}
}
\newcommand{\NLdotB}[5] %{xscale}{yscale}{xmax}{denom}{xval of dot}
{\tikz[xscale=#1,yscale=#2]
{
\draw (0,0)--(#3,0);
\pgfmathparse{#3*#4}
\foreach \x in {0,...,\pgfmathresult}
\draw (\x/#4,-0.2)--(\x/#4,0.2);
\foreach \x in {0,...,#3}
\node[below] at (\x,-0.3) {\x};
\fill[gray,opacity=0.5] (#5,0) circle [x radius = 0.2/#1, y radius = 0.2/#2];
\node[above] at (#5,0.3) {$\frac{\pgfmathparse{#4*#5}\pgfmathresult}{#4}$}
}
\begin{document}
\noindent\NLdotA{2}{1.25}{5}{3}{10/3}
\noindent\NLdotB{2}{1.25}{5}{3}{10/3}
\end{document}
Best Answer
Not exactly sure what you are asking or what is wrong with what you have, but here is how I would do it: Use
\pgfmathsetmacrowhich invokes\pgfmathparseand uses the result from\pgfmathresult.I also removed the redundant
\draw (0,0) -- (60:1);as that line gets overwritten by\draw (0,0) -- (60:2);.