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--- latex:tikz [2022/07/20 11:54] – [How to draw diagrams in LaTeX with TikZ] iwn
+++ latex:tikz [2022/12/02 15:42] (current) – [For-loops: Pseudorapidity 𝜂 and polar angle 𝜃] iwn
@@ Line 389: / Line 389: @@
 </WRAP></WRAP>
+Using a for-loop over two variables:
+<WRAP group><WRAP half column lo 65%>
+<code latex>
+\begin{tikzpicture}[scale=3]
+  % limits
+  \def\R{1.2} % radius/length of lines
+  % axis labels
+  \node[scale=1,below left=1] at (0,\R) {$y$}; % y axis
+  \node[scale=1,below left=1] at (\R,0) {$z$}; % z axis
+  % lines
+  \foreach \t/\e in {90/0,60/0.55,45/0.88,30/1.32,10/2.43,0/+\infty}{ % loop over theta/eta
+    \pgfkeys{/pgf/number format/precision=2}
+    \draw[->,black!60!red,thick,line cap=round] % eta lines
+      (0,0) -- (\t:\R) node[anchor=180+\t,black] {$\eta=\e$}
+      node[black,pos=0.7,fill=white,scale=0.8,inner sep=1.5pt] {$\theta=\t^\circ$};
+  }
+\end{tikzpicture}
+</code>
+</WRAP><WRAP half column 30%>
+\\
+{{ latex:tikz:axis2d_pseudorapidity.png ? 2000 }}
+\\
+</WRAP></WRAP>
+Using for-loop and calculation + rounding:
 <WRAP group><WRAP half column lo 65%>
 <code latex>
@@ Line 395: / Line 423: @@
   % limits
-  \def\R{1.2}
+  \def\R{1.2} % radius/length of lines
   % axis labels
-  \node[scale=0.9,below=5pt,left=2pt] at (0,\R) {$y$};
+  \node[scale=1,below left=1] at (0,\R) {$y$}; % y axis
-  \node[scale=0.9,left=5pt,below=2pt] at (\R,0) {$z$};
+  \node[scale=1,below left=1] at (\R,0) {$z$}; % z axis
   % lines
-  \foreach \t/\e in {90/0,60/0.55,45/0.88,30/1.32,10/2.44,0/\infty}{
+  \pgfkeys{/pgf/number format/precision=2} % two decimals
-    \draw[->,black!60!red,thick]
+  \foreach \t in {90,60,45,30,10,0}{ % loop over theta
-      (0,0) -- ({\R*cos(\t)},{\R*sin(\t)})
+    \ifnum \t = 0
-      node[anchor=180+\t,black] {$\eta=\e$};
+      \def\e{+\infty} % infinity symbol
-    \node[fill=white,scale=0.8] at ({0.8*cos(\t)},{0.8*sin(\t)}) {$\theta=\t^\circ$};
+    \else
+      \pgfmathparse{-ln(tan(\t/2))} % pseudorapidity
+      %\pgfmathroundto{\pgfmathresult} % round without traling zeroes
+      \pgfmathroundtozerofill{\pgfmathresult} % round with trailing zeroes
+      \pgfmathsetmacro\e{\t==90?0:\pgfmathresult} % no trailing zeroes for theta = 0
+    \fi
+    \draw[->,black!60!red,thick,line cap=round] % eta lines
+      (0,0) -- (\t:\R) node[anchor=180+\t,black] {$\eta=\e$}
+      node[black,pos=0.7,fill=white,scale=0.8,inner sep=1.5pt] {$\theta=\t^\circ$};
   }
 \end{tikzpicture}
 </code>
 </WRAP><WRAP half column 30%>
 \\
-{{ latex:theta-eta.png ? 2000 }}
+{{ latex:tikz:axis2d_pseudorapidity.png ? 2000 }}
 \\
 </WRAP></WRAP>

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