Retain coordinate value in tikz foreach loop

Question

I would like to retain a computed coordinate value from one step of a tikz foreach loop to the next. In the code below, I would like the increment in the y-value to depend on the previous y-value, which I am denoting \lasty. Is there a way to retain this value? Thanks for the help. The example below errors out including the - \lasty; the example should work otherwise.

\documentclass{beamer}
\usepackage{tikz}

\begin{document}
\begin{frame}[fragile]
\newcommand{\Emmett}[6]{% points, advance, rand factor, options, end label
\draw[#4] (0,0)
\foreach \x [remember=\x as \lastx] in {1,...,749}
{   -- ++(#2,{rand*#3 + #2*(#6-\lasty)/(#1-\x)})
}
node[right] {#5};
}
\scalebox{0.5}{   
\begin{tikzpicture}
\draw[help lines] (0,-5) grid (15,5);
\Emmett{750}{0.02}{0.2}{red}{first one}{2.0}
\Emmett{750}{0.02}{0.2}{green}{second one}{-1.0}
\Emmett{750}{0.02}{0.2}{blue}{third one}{-2.0}
\end{tikzpicture}
}
%\pgfmathsetseed{1337}
\end{frame}
\end{document}

Not sure about understanding your problem but may be this answer to How to fix the trajectory of Brownian motions which generated by the “rand” function with tikz in beamer frames could help you. It proposes to save all random generated values into data files and use them later without having to compute (or remeber) them again. — Ignasi, Apr 09 '14 at 07:06
Yes, I think that might work. I'm trying to plot several Brownian bridges. — Kevin, Apr 09 '14 at 14:21
Is it \lastx or \lasty? If all are \lasty try [remember=\x as \lasty (initially 0)] or define \lasty before the loop as zero — percusse, Apr 09 '14 at 15:18

A.Ellett · Answer 1 · 2014-04-09T14:59:06.267

Here's a solution which remembers previous y values.

\documentclass{article}
\usepackage{tikz}
\usepackage{etoolbox}

\makeatletter
\def\ae@path{}
\newcommand{\Emmett}[6]{% points, advance, rand factor, options, end label
  \def\ae@last@y{0}
  \def\ae@initial@portion{\draw[#4] (0,0)}
  \xdef\ae@path{\expandonce\ae@initial@portion}
  \foreach \x in {1,...,749}
  {   
    \pgfmathparse{rand*#3 + #2*(#6-\ae@last@y)/(#1-\x)}
    \xdef\ae@last@y{\pgfmathresult}
    \xdef\ae@path{\expandonce\ae@path -- ++(#2,\expandonce\ae@last@y)}
  }
  \ae@path node[right] {#5};
}

\makeatother

\begin{document}

\scalebox{0.5}{   
\begin{tikzpicture}
\draw[help lines] (0,-5) grid (15,5);
\Emmett{750}{0.02}{0.2}{red}{first one}{2.0}
\Emmett{750}{0.02}{0.2}{green}{second one}{-1.0}
\Emmett{750}{0.02}{0.2}{blue}{third one}{-2.0}
\end{tikzpicture}
}
%\pgfmathsetseed{1337}

\end{document}

enter image description here

\typeout for debugging

Here for debugging purposes, you can see that they y's are being properly updated.

\documentclass{article}
\usepackage{tikz}
\usepackage{etoolbox}

\makeatletter
\def\ae@path{}
\newcommand{\Emmett}[6]{% points, advance, rand factor, options, end label
  \def\ae@last@y{0}
  \def\ae@initial@portion{\draw[#4] (0,0)}
  \xdef\ae@path{\expandonce\ae@initial@portion}
  \foreach \x [remember=\x as \lastx (initially 0)]in {1,...,749}
  {   
    \typeout{==>--------------------------------------------------}%%
    \typeout{==>(x=\x) ==> (old y=\ae@last@y)}%%
    \pgfmathparse{rand*#3 + #2*(#6-\ae@last@y)/(#1-\x)}
    \xdef\ae@last@y{\pgfmathresult}
    \typeout{==>(x=\x) ==> (new y=\ae@last@y)}%%
    \xdef\ae@path{\expandonce\ae@path -- ++(#2,\expandonce\ae@last@y)}
  }
  \ae@path node[right] {#5};
}
\makeatother

\begin{document}

    \scalebox{0.5}{   
    \begin{tikzpicture}
      \draw[help lines] (0,-5) grid (15,5);
      \Emmett{750}{0.02}{0.2}{red}{first one}{2.0}
      \Emmett{750}{0.02}{0.2}{green}{second one}{-1.0}
      \Emmett{750}{0.02}{0.2}{blue}{third one}{-2.0}
    \end{tikzpicture}
    }
    %\pgfmathsetseed{1337}

\end{document}

Thanks for the response. I do want the previous y coordinate rather than the previous x coordinate, as it looks like you are trying to do in the second part of your answer. The second code provided still does not retain the y-value. For example, consider starting the line at (0,5) and incrementing by -#2\ae@last@y (`\pgfmathparse{-#2\ae@last@y))}`). This plots a flat line rather than a declining one. — Kevin, Apr 09 '14 at 14:15
@kevin It's remembering things correctly. I've added some more lines to type out the values so you can see how they change. — A.Ellett, Apr 09 '14 at 14:58
@kevin, \xdef<cmd>{<definition>} fully expands the definition for <cmd> before the assignment takes place. It also does so globally. \def would be completely ineffective, and \edef would not do so globally. — A.Ellett, Apr 09 '14 at 15:01

Retain coordinate value in tikz foreach loop

1 Answers1

\typeout for debugging