tikz/pgf low-level path drawing from list of coordinates and aesthetics

Question

I'm trying to output low-level tikz commands from another program, as a generic way to draw arbitrary lines (and points etc.). I am not familiar enough with the syntax to know where to look for good examples (and the manual is, well, a bit overwhelming at first).

In pseudo-code, I'm hoping to achieve the following:

% define some arrays of numbers and parameters
x = {x1, x2, x3, ..., xn}
y = {y1, y2, y3, ..., yn}
c = {c1, c2, c3, ..., cn}
w = {w1, w2, w3, ..., wn}

% set the start coordinate
current_point = (x(1), y(1))
% loop over all arrays to draw a path
for each (i = 1:n){
 draw [colour = c(i), line width = w(i)] current_point -- ++ (x(i), y(i)) coordinate current_point;
}

I think that would be a sufficiently general syntax, yet still readable. I am however stuck with the syntax to define multiple arrays and reuse them in a foreach loop. Here's my failed attempt (should show a sine wave),

\documentclass[tikz]{standalone}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}[x=1cm,y=0.4cm]
    \def\tabx{{0,0.06,0.13,0.19,0.25,0.32,0.38,0.44,0.51,0.57,0.63,0.7,0.76,0.83,0.89,0.95,1.02,1.08,1.14,1.21,1.27,1.33,1.4,1.46,1.52,1.59,1.65,1.71,1.78,1.84,1.9,1.97,2.03,2.09,2.16,2.22,2.28,2.35,2.41,2.48,2.54,2.6,2.67,2.73,2.79,2.86,2.92,2.98,3.05,3.11,3.17,3.24,3.3,3.36,3.43,3.49,3.55,3.62,3.68,3.74,3.81,3.87,3.93,4,4.06,4.13,4.19,4.25,4.32,4.38,4.44,4.51,4.57,4.63,4.7,4.76,4.82,4.89,4.95,5.01,5.08,5.14,5.2,5.27,5.33,5.39,5.46,5.52,5.59,5.65,5.71,5.78,5.84,5.9,5.97,6.03,6.09,6.16,6.22,6.28}};

\def\taby{{0,0.95,1.86,2.7,3.45,4.07,4.55,4.86,4.99,4.95,4.73,4.33,3.78,3.09,2.29,1.41,0.48,-0.48,-1.41,-2.29,-3.09,-3.78,-4.33,-4.73,-4.95,-4.99,-4.86,-4.55,-4.07,-3.45,-2.7,-1.86,-0.95,0,0.95,1.86,2.7,3.45,4.07,4.55,4.86,4.99,4.95,4.73,4.33,3.78,3.09,2.29,1.41,0.48,-0.48,-1.41,-2.29,-3.09,-3.78,-4.33,-4.73,-4.95,-4.99,-4.86,-4.55,-4.07,-3.45,-2.7,-1.86,-0.95,0,0.95,1.86,2.7,3.45,4.07,4.55,4.86,4.99,4.95,4.73,4.33,3.78,3.09,2.29,1.41,0.48,-0.48,-1.41,-2.29,-3.09,-3.78,-4.33,-4.73,-4.95,-4.99,-4.86,-4.55,-4.07,-3.45,-2.7,-1.86,-0.95,0}};
    \coordinate (current point) at (0,0);
    \foreach[count=\i, evaluate=\i as \y using \taby(\i), evaluate=\i as \lw using \i/10] \x in \tabx
    {
    \draw[red!\i,line width=\lw] (current point) -- ++ (\x,\y) coordinate (current point);
    }

\end{tikzpicture}
\end{document}

My preference would be to keep the array variables self-contained in the tex file, rather than a separate text file, but if there's a clear advantage to doing that it could also be an option.

Note: I am aware of pgfplots and other high-level libraries, but would rather avoid them here because I want to keep a lower-level control to add arbitrary elements, rather than relying on the design of pre-built plotting commands, symbols, etc.

Happy to be directed at an existing question or a specific section of the manual; I found quite a few related references but couldn't place all the pieces together.

Answer 1

You need something more like the following.

• You need to pass a simple list to the \foreach loop to parse. You can't pass an array here because the list will only contain a single item: the array. Instead, just pass a list of integers to be used as indexes for accessing the arrays.

• You need to use square brackets - not round - to access an indexed element of an array.

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}[x=1cm,y=0.4cm]
  \def\tabx{%
    {%
      0,0.01057775,0.02115551,0.03173326,0.04231101,0.05288877,0.06346652,0.07404427,%
      0.08462202,0.09519978,0.1057775,0.1163553,0.126933,0.1375108,0.1480885,0.1586663,0.169244,%
      0.1798218,0.1903996,0.2009773,0.2115551,0.2221328,0.2327106,0.2432883,0.2538661,0.2644438,%
      0.2750216,0.2855993,0.2961771,0.3067548,0.3173326,0.3279103,0.3384881,0.3490659,0.3596436,%
      0.3702214,0.3807991,0.3913769,0.4019546,0.4125324,0.4231101,0.4336879,0.4442656,0.4548434,%
      0.4654211,0.4759989,%
      0.4865766,0.4971544,0.5077321,0.5183099,0.5288877,0.5394654,0.5500432,0.5606209,0.5711987,0.5817764,%
      0.5923542,0.6029319,0.6135097,0.6240874,0.6346652,0.6452429,0.6558207,0.6663984,0.6769762,%
      0.6875539,0.6981317,0.7087095,0.7192872,0.729865,0.7404427,0.7510205,0.7615982,0.772176,%
      0.7827537,0.7933315,0.8039092,0.814487,0.8250647,0.8356425,0.8462202,0.856798,0.8673757,0.8779535,%
      0.8885313,0.899109,0.9096868,0.9202645,0.9308423,0.94142,0.9519978,0.9625755,0.9731533,0.983731,%
      0.9943088,1.004887,1.015464,1.026042,1.03662,1.047198%
    }%
  }
  \def\taby{%
    {%
      0,0.06342392,0.1265925,0.1892512,0.251148,0.3120334,0.3716625,0.4297949,0.4861967,%
      0.5406408,0.5929079,0.6427876,0.690079,0.7345917,0.7761465,0.814576,0.8497254,0.8814534,0.909632,%
      0.9341479,0.9549022,0.9718116,0.9848078,0.9938385,0.9988673,0.9998741,0.9968548,0.9898214,0.9788024,%
      0.9638422,0.9450008,0.9223543,0.8959938,0.8660254,0.8325699,0.7957618,0.7557496,0.7126942,0.666769,%
      0.618159,0.5670599,0.5136774,0.4582265,0.4009305,0.3420201,0.2817326,0.2203105,0.1580014,0.09505604,%
      0.03172793,-0.03172793,-0.09505604,-0.1580014,-0.2203105,-0.2817326,-0.3420201,-0.4009305,-0.4582265,%
      -0.5136774,-0.5670599,-0.618159,-0.666769,-0.7126942,-0.7557496,-0.7957618,-0.8325699,-0.8660254,%
      -0.8959938,-0.9223543,-0.9450008,-0.9638422,-0.9788024,-0.9898214,-0.9968548,-0.9998741,-0.9988673,%
      -0.9938385,-0.9848078,-0.9718116,-0.9549022,-0.9341479,-0.909632,-0.8814534,-0.8497254,-0.814576,%
      -0.7761465,-0.7345917,-0.690079,-0.6427876,-0.5929079,-0.5406408,-0.4861967,-0.4297949,-0.3716625,%
      -0.3120334,-0.251148,-0.1892512,-0.1265925,-0.06342392,-0.000000027563173%
    }%
  }
  \coordinate (p O) at (0,0);
  \foreach \i [remember=\i as \ilast (initially O), evaluate=\i as \y using {\taby[\i]}, evaluate=\i as \x using {\tabx[\i]}, evaluate=\i as \lw using \i/10] in {0,...,99}
  {
    \draw [red!\i, line width=\lw] (p \ilast) -- ++(\x,\y) coordinate (p \i);
  }
\end{tikzpicture}
\end{document}

Note, however, that there is no point giving TikZ data to this number of decimal places. It will just increase parsing time, without improving accuracy.

EDIT

Here's an extended example in response to the request for one showing how to use a loop within a path.

\pdfminorversion=7
% ateb: http://tex.stackexchange.com/a/354419/ addaswyd o gwestiwn baptiste: http://tex.stackexchange.com/q/354406/
\documentclass[tikz,border=10pt]{standalone}
\begin{document}
\begin{tikzpicture}[x=1cm,y=0.4cm]
  \def\tabx{%
    {%
      0,0.01057775,0.02115551,0.03173326,0.04231101,0.05288877,0.06346652,0.07404427,%
      0.08462202,0.09519978,0.1057775,0.1163553,0.126933,0.1375108,0.1480885,0.1586663,0.169244,%
      0.1798218,0.1903996,0.2009773,0.2115551,0.2221328,0.2327106,0.2432883,0.2538661,0.2644438,%
      0.2750216,0.2855993,0.2961771,0.3067548,0.3173326,0.3279103,0.3384881,0.3490659,0.3596436,%
      0.3702214,0.3807991,0.3913769,0.4019546,0.4125324,0.4231101,0.4336879,0.4442656,0.4548434,%
      0.4654211,0.4759989,%
      0.4865766,0.4971544,0.5077321,0.5183099,0.5288877,0.5394654,0.5500432,0.5606209,0.5711987,0.5817764,%
      0.5923542,0.6029319,0.6135097,0.6240874,0.6346652,0.6452429,0.6558207,0.6663984,0.6769762,%
      0.6875539,0.6981317,0.7087095,0.7192872,0.729865,0.7404427,0.7510205,0.7615982,0.772176,%
      0.7827537,0.7933315,0.8039092,0.814487,0.8250647,0.8356425,0.8462202,0.856798,0.8673757,0.8779535,%
      0.8885313,0.899109,0.9096868,0.9202645,0.9308423,0.94142,0.9519978,0.9625755,0.9731533,0.983731,%
      0.9943088,1.004887,1.015464,1.026042,1.03662,1.047198%
    }%
  }
  \def\taby{%
    {%
      0,0.06342392,0.1265925,0.1892512,0.251148,0.3120334,0.3716625,0.4297949,0.4861967,%
      0.5406408,0.5929079,0.6427876,0.690079,0.7345917,0.7761465,0.814576,0.8497254,0.8814534,0.909632,%
      0.9341479,0.9549022,0.9718116,0.9848078,0.9938385,0.9988673,0.9998741,0.9968548,0.9898214,0.9788024,%
      0.9638422,0.9450008,0.9223543,0.8959938,0.8660254,0.8325699,0.7957618,0.7557496,0.7126942,0.666769,%
      0.618159,0.5670599,0.5136774,0.4582265,0.4009305,0.3420201,0.2817326,0.2203105,0.1580014,0.09505604,%
      0.03172793,-0.03172793,-0.09505604,-0.1580014,-0.2203105,-0.2817326,-0.3420201,-0.4009305,-0.4582265,%
      -0.5136774,-0.5670599,-0.618159,-0.666769,-0.7126942,-0.7557496,-0.7957618,-0.8325699,-0.8660254,%
      -0.8959938,-0.9223543,-0.9450008,-0.9638422,-0.9788024,-0.9898214,-0.9968548,-0.9998741,-0.9988673,%
      -0.9938385,-0.9848078,-0.9718116,-0.9549022,-0.9341479,-0.909632,-0.8814534,-0.8497254,-0.814576,%
      -0.7761465,-0.7345917,-0.690079,-0.6427876,-0.5929079,-0.5406408,-0.4861967,-0.4297949,-0.3716625,%
      -0.3120334,-0.251148,-0.1892512,-0.1265925,-0.06342392,-0.000000027563173%
    }%
  }

  \coordinate (p O) at (0,0);
  \foreach \i [remember=\i as \ilast (initially O), evaluate=\i as \y using {\taby[\i]}, evaluate=\i as \x using {\tabx[\i]}, evaluate=\i as \lw using \i/10] in {0,...,99}
  {
    \draw [red!\i, line width=\lw] (p \ilast) -- ++(\x,\y) coordinate (p \i);
  }

  \draw [green!75!black] (0,-1) \foreach \i [evaluate=\i as \y using {\taby[\i]}, evaluate=\i as \x using {\tabx[\i]}] in {0,...,99} { -- ++(\x,\y) };

  \draw  (0,-2) coordinate (p O) \foreach \i [remember=\i as \ilast (initially O), evaluate=\i as \y using {\taby[\i]}, evaluate=\i as \x using {\tabx[\i]}, evaluate=\i as \lw using \i/10] in {0,...,99}
  {
    (p \ilast) edge [blue!\i, line width=\lw] coordinate [pos=1] (p \i) ++(\x,\y)
  };
\end{tikzpicture}
\end{document}

The red curve uses a loop over paths, one per iteration, as before. The lower green and blue curves use loops within paths, one per coordinate.

The green curve has to use the same options for all parts of the path, so the colour and width are static. In this case, we don't need to remember coordinates by naming them, because we are drawing a continuous path and are wherever we've got to.

The blue curve works around the same-options restriction by using the edge operation, which breaks off the current path to make a diversion. In this case, however, we are back to needing to remember the coordinates by naming them, as the path resumes from where it began when the edge is complete.