# Computing coordinates in a tikz figure

I am trying to draw a figure using tikz. As the figure involves graph of a trigonometric functions, I was hoping that Latex/Tikz could compute coordinates directly.

I tried the following code:

\documentclass{article}

\usepackage{pgfplots}
\pgfplotsset{compat=1.3}

\begin{document}

\begin{tikzpicture}[scale=1, every node/.style={scale=1.4}]

%axes
\draw[->, >=stealth] (-1,0) -- (7,0);
\draw (7,0) node[right] {$x$};
\draw[->, >=stealth] (0,-1) -- (0,5);
\draw (0,5) node[above] {$y$};

\draw[blue, very thick] plot[domain=1:6.28] ( {\x} , {1+\x/2+sin(\x r)} );

\draw[black, dashed] (1,0) -- (1,1+1/2+sin(1 r));
\draw[black, dashed] (6.28,0) -- (6.28,1+3.14+sin(6.28 r));
\end{tikzpicture}

\end{document}


My main problem is when drawing the two vertical dashed line. Latex returns

Package tikz Error: Giving up on this path. Did you forget a semicolon?. ...black, dashed] (1,0) -- (1,1+1/2+sin(1 r))


and similarly

Package tikz Error: Giving up on this path. Did you forget a semicolon?. ...hed] (6.28,0) -- (6.28,1+3.14+sin(6.28 r))


as errors. The figure appears as I want though (because the error arises at the end of the path).

Why did I get this error? My code includes a semicolon at the end of every line.

If the two lines in question are problematic, is there a way I can get Tikz to compute numerical values?

• What's the r in sin(1 r)? – Herr K. Feb 25 '18 at 6:27
• @HerrK. Don't I need it for radians? sin(\x) is for sin using \x in degrees, while sin(\x r) is for radians. I simply substituted the value 1 in the expression of my function. – Taladris Feb 25 '18 at 6:29
• I tried the following and it worked: 1. comment out the last two lines (that draw the dashed lines); 2. delete the r in sin(\x r) and compile --- this should produce a blue straight line; 3. add the r back and compile again --- this should produce the correct sine curve; 4. uncomment the last two lines, and also "brace" the y-coordinates in both commands (the part that involve sin's); 5. compile one last time --- this should produce the desired result. I don't know why it works, but it seems to do the trick. – Herr K. Feb 25 '18 at 7:02

Just put the coordinates into a group.

\documentclass{article}

\usepackage{pgfplots}
\pgfplotsset{compat=1.3}

\begin{document}

\begin{tikzpicture}[scale=1, every node/.style={scale=1.4}]

%axes
\draw[->, >=stealth] (-1,0) -- (7,0);
\draw (7,0) node[right] {$x$};
\draw[->, >=stealth] (0,-1) -- (0,5);
\draw (0,5) node[above] {$y$};

\draw[blue, very thick] plot[domain=1:6.28] ( {\x} , {1+\x/2+sin(\x r)} );

\draw[black, dashed] (1,0) -- (1,{1+1/2+sin(1 r)});
\draw[black, dashed] (6.28,0) -- (6.28,{1+3.14+sin(6.28 r)});
\end{tikzpicture}

\end{document}