Using \foreach inside a draw command using TIKZ

Question

I'm brand new to using \foreach but not coding in general. Following the code in answer number 40 of this example: How to draw tangent line of an arbitrary point on a path in TikZ

How can I properly condense the tangent=0.x, list in the draw statement?

I've tried a few ideas with no success:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\begin{document}

\begin{tikzpicture}[
tangent/.style={
    decoration={
        markings,% switch on markings
        mark=
            at position #1
            with
            {
                \coordinate (tangent point-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (0pt,0pt);
                \coordinate (tangent unit vector-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (1,0pt);
                \coordinate (tangent orthogonal unit vector-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (0pt,1);
            }
    },
    postaction=decorate
},
use tangent/.style={
    shift=(tangent point-#1),
    x=(tangent unit vector-#1),
    y=(tangent orthogonal unit vector-#1)
},
use tangent/.default=1
]
\draw [
tangent=0.1,
tangent=0.2,
tangent=0.3,
tangent=0.4,
tangent=0.5,
tangent=0.6,
tangent=0.7,
tangent=0.8,
tangent=0.9,
%%%% attempt 1
%tangent={0.05,0.01,...0.95}
%%%%% attempt 2
%\foreach \y in {0.1,0.2,...,0.9}
%{
%tangent=\y,
%}
] (0,0) arc (180:0:10);

\foreach \x [count=\xi] in {1,1,2,1,2,1,4,3,1.5}
{
\draw [red, thick,->, use tangent=\xi] (0,0) -- (0,-1*\x);
}
\end{tikzpicture}
\end{document}

to get a result like this:

How can I properly condense the tangent=0.x, list in the draw statement?

Thanks everyone for your input. Interesting alternate approaches. — user3780514, Feb 20 '16 at 15:13

score 8 · Accepted Answer · answered Feb 19 '16 at 19:59

When you need to call the same key repeatedly, you can use <key>/.list={<foreach expression>}, where <foreach expression> can be a simple list, or a list with ellipses. So in this case, you can use \draw [tangent/.list={0.1,0.2,...,0.9}](0,0) arc (180:0:10);

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\begin{document}

\begin{tikzpicture}[
tangent/.style={
    decoration={
        markings,% switch on markings
        mark=
            at position #1
            with
            {
                \coordinate (tangent point-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (0pt,0pt);
                \coordinate (tangent unit vector-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (1,0pt);
                \coordinate (tangent orthogonal unit vector-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (0pt,1);
            }
    },
    postaction=decorate
},
use tangent/.style={
    shift=(tangent point-#1),
    x=(tangent unit vector-#1),
    y=(tangent orthogonal unit vector-#1)
},
use tangent/.default=1
]
\draw [tangent/.list={0.1,0.2,...,0.9}](0,0) arc (180:0:10);

\foreach \x [count=\xi] in {1,1,2,1,2,1,4,3,1.5}
{
\draw [red, thick,->, use tangent=\xi] (0,0) -- (0,-1*\x);
}
\end{tikzpicture}
\end{document}

What about if I have two arguments in my style and I want both to run through a list? Specifically, my style reads loop/.style={decoration={markings,mark=at position {#1} with {\arrow{>},\node[anchor=\pgfdecoratedangle-90,rotate=-\pgfdecoratedangle] {#2};}},postaction={decorate}} and I'd like to give pairwise values to #1 and #2. — Janosh, Sep 27 '17 at 10:54

Ondrian · Answer 2 · 2016-02-19T20:16:05.460

This is a slightly different approach (I admit this is a workaround), however, it works for this scenario just fine.

\foreach [count=\y] \x in {1,1,2,1,2,1,4,3,1.5}
{
    \path [tangent=\y/10] (0,0) arc (180:0:10);
    \draw [red, thick, ->, use tangent=1] (0,0) -- (0,-1*\x);
}   
\draw (0,0) arc (180:0:10);

Just replace the tikzpicture body by this code.

Final code:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}

    \begin{tikzpicture}[
    tangent/.style={
        decoration={
            markings,% switch on markings
            mark=
            at position #1
            with
            {
                \coordinate (tangent point-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (0pt,0pt);
                \coordinate (tangent unit vector-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (1,0pt);
                \coordinate (tangent orthogonal unit vector-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (0pt,1);
            }
        },
        postaction=decorate
    },
    use tangent/.style={
        shift=(tangent point-#1),
        x=(tangent unit vector-#1),
        y=(tangent orthogonal unit vector-#1)
    },
    use tangent/.default=1
    ]

    \foreach [count=\y] \x in {1,1,2,1,2,1,4,3,1.5}
    {
        \path [tangent=\y/10] (0,0) arc (180:0:10);
        \draw [red, thick,->, use tangent=1] (0,0) -- (0,-1*\x);
    }   
    \draw (0,0) arc (180:0:10); 

    \end{tikzpicture}
\end{document}

CroCo · Answer 3 · 2016-02-20T02:16:36.167

This can be done easily with foreach. I use simpler approach than yours. The result is

\documentclass[border={10}]{standalone}
\usepackage{tikz}  

\begin{document}

\def \r {8}

\begin{tikzpicture}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\draw[very thick] (\r,0) arc (0:180:\r);

\foreach \angle / \y in {18/6, 36/7, 54/6, 72/7,90/6, 108/7,126/6,144/7,162/6}
\draw [<-, very thick,red]
   ( {  (\y)*cos(\angle)}, { (\y)*sin(\angle)} ) --
   ( {  (\r)*cos(\angle)}, { (\r)*sin(\angle)} );
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\end{tikzpicture}

\end{document}

Using \foreach inside a draw command using TIKZ

3 Answers3