[Tex/LaTex] TikZ Coordinate calculation – calculate coordinates separately

calculationstikz-pgf

I'm trying to calculate coordinates based on existing coordinates but each coordinate individually.

The first thing which is unclear to me is how to define scalar variables. I can define coordinates using the \coordinate command, but what about scalars?

Looking through the manual I found the \pgfextractx command, but I don't know how to use it, is it possible to use it within a coordinate calculation like for example

\coordinate(blah) at ($ (\pgfextractx{(centre)} + 2*cos(30), .. )$)

Best Answer

It is possible to do by means of let operation:

\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}
\begin{tikzpicture}
    \fill[blue] (0,0) coordinate (centre) circle[radius=1pt];
    \fill[red] let \p1=(centre) in
        ({\x1 + 2 * cos(30)}, \y1) circle[radius=1pt];
\end{tikzpicture}
\end{document}

Related Solutions

[Tex/LaTex] use trig functions in TikZ within polar coordinates

This is actually a bug that is fixed in the CVS version of PGF/TikZ, see Using math in TikZ

As percusse reports in his answer to that question, you can get around this bug by simply adding a space after the closing brace in the radius calculation:

\draw (0,0) -- (2,0) -- (45:{1/cos(45)} ) -- (0,0);

cjorssen also posted the fix that is now in the CVS version, and which you can use in your document. See his answer to said question.

Original answer

You can get around this by using the let keyword to do the calculation outside the coordinate. This is described in section 14.15 The Let Operation of the manual, which is on page 150 (for version 2.10, dated 25 October 2010).

I used cycle to close the path, as this ensures a proper merging of the lines at that corner.

\documentclass[tikz]{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

\begin{tikzpicture}
\draw let \n1 = {1/cos(45)}  in
   (0,0) -- (2,0) -- (45:\n1) -- cycle; 
\end{tikzpicture}

\end{document}

[Tex/LaTex] tikz – relative coordinates of a second position

You can do this using the calc library (See Section 13.5 Coordinate Calculations of the pgf manual):

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

\begin{tikzpicture}
  \coordinate (a) at (2,2);
  \coordinate (b) at (0,-2);
  \node[draw=red] at (a) {a};
  \node[draw=red] at (b) {b};
  \draw[help lines] (-1,-3) grid (3,3);
  \draw ($ (a) + (0,1) $) -- ($ (b) + (0,1) $);
\end{tikzpicture}
\end{document}

Best Answer

Related Solutions

[Tex/LaTex] use trig functions in TikZ within polar coordinates

Original answer

[Tex/LaTex] tikz – relative coordinates of a second position

Related Question