Consider this minimal working example. I have two functions, f(x) = x
and g(x) = x+sin(x). I want to draw them with PGF and at the intersection
points, I want put black dots. Here is the TikZ code,

\documentclass{standalone}
\usepackage{tikz, amsmath}
\usetikzlibrary{intersections}
\begin{document}\begin{tikzpicture}

% Plots
\draw[very thick, smooth, samples=20, domain=-6.28:6.28]
[red, name path=line] (0,0) plot (\x, \x);
\draw[very thick, smooth, samples=20, domain=-6.28:6.28]
[blue, name path=sine] (0,0) plot (\x, {\x + sin(\x r)});

% Drawing the dots
\fill[name intersections={of=line and sine, name=i, total=\t}, black]
\foreach \s in {1,...,\t} {(i-\s) circle (2pt)};

\end{tikzpicture}\end{document}

Now this code works as expected. But I also wanted to draw dashed lines from
the intersections to each axes.

% Axes
\draw [<->](-6.28, 0) -- (6.28, 0);
\draw [<->](0, -6.28) -- (0, 6.28);

% Mark intersection points and draw dashed lines
\foreach \n in {1,...,\t} {
\path ({i-\n}) coordinate (i\n); % <--- Error Here
\fill[black] (i\n) circle (2pt);
\draw[dashed] (i\n) -- (i\n |- 0,0);
\draw[dashed] (i\n) -- (0,0 -| i\n);
}

In this part, pdflatex (my distro is TeX Live 2024) throws an error what
I don't understand,

! Undefined control sequence.
\UseTextAccent ...p \@firstofone \let \@curr@enc
\cf@encoding \@use@text@en...
l.28 }

What is undefined? I am pretty sure the name=i in \fill command populates
the namespace with i-1, 1-2 and 1-3, because \fill has drawn them. I
have even tried to help the parser with extra braces, {i-\n} and {i\n}
but that also doesn't help. Anybody know the reason why I get the error
and how to fix them?

Annada Behera <segfault@tilde.green> wrote or quoted:
> \foreach \n in {1,...,\t} {
> \path ({i-\n}) coordinate (i\n); % <--- Error Here
> \fill[black] (i\n) circle (2pt);
> \draw[dashed] (i\n) -- (i\n |- 0,0);
> \draw[dashed] (i\n) -- (0,0 -| i\n);
> }

I'm about as familiar with LaTeX as I am with surfing Pipeline
- which is to say, not at all. And this tikz thing? Totally
off my radar. But after messing around like a bear in a honey
factory, I stumbled onto something. Turns out, if you unravel
that last loop and use the following code, it's all gravy here.

\path ({i-1}) coordinate (i1);
\fill[black] (i1) circle (2pt);
\draw[dashed] (i1) -- (i1 |- 0,0);
\draw[dashed] (i1) -- (0,0 -| i1);
\path ({i-2}) coordinate (i2);
\fill[black] (i2) circle (2pt);
\draw[dashed] (i2) -- (i2 |- 0,0);
\draw[dashed] (i2) -- (0,0 -| i2);
\path ({i-3}) coordinate (i3);
\fill[black] (i3) circle (2pt);
\draw[dashed] (i3) -- (i3 |- 0,0);
\draw[dashed] (i3) -- (0,0 -| i3);
\path ({i-4}) coordinate (i4);
\fill[black] (i4) circle (2pt);
\draw[dashed] (i4) -- (i4 |- 0,0);
\draw[dashed] (i4) -- (0,0 -| i4);

| /H H
| :$ .;XM
| =$ +%$@=
| =$ .+$//H:
| =$ .+$+ +H=
| =$ ,+%+ XH
| =$ ,+$/ /H$
| =$ .+%; /XX=
| =$ .--,+HX$/+$X$:
| =$ ;$X$%X@#M+++/:
| =$ %HX/.-+%;-
| =$ :HX, ,+%/ ;: .
| =$ $H: -%%; ;=
| =$ HX -%%; ;=
| =$ .H$-%%; ;=
| :$ HX+$; ;=
| %@H+: ;=
|;/::::::::::::::::;:::::::::::::;:-X#@+:=;:::::::::::%+:::::::::::::::;
|X/---------------=%------------,,/X#M ---------------------------------
|$, / :%$M;/
|$- / ;%%%H:-$
|$- / ;%%-/H= :$
|$- / ;%%- $H, =$
|$- + ;%%- =HX =$
|$- .+ ;%+, ,XH/ =$
|$- ;;%+=-+XX% =$
|$- .;%%%$@#@X%$$%: =$
|$- :XX$;:%$%: =$
|$- -XH/ ;%+, =$
|$- +H+ /$+, =$
|$- XH +%+, =$
|$- H$ /$+, =$
|$-.H$/$+. =$
|+,@H%+. =$
|@#H;. :$
|##;/+++++++++++++++++++++++++++++++X@++++++++++++++++++++++++++++++++++

Annada Behera <segfault@tilde.green> wrote or quoted:
>% Mark intersection points and draw dashed lines
>\foreach \n in {1,...,\t} {
> \path ({i-\n}) coordinate (i\n); % <--- Error Here
> \fill[black] (i\n) circle (2pt);
> \draw[dashed] (i\n) -- (i\n |- 0,0);
> \draw[dashed] (i\n) -- (0,0 -| i\n);
>}

Plain TeX to the rescue! What also works here is:

% Mark intersection points and draw dashed lines
\newcount\n
\n=1
\loop
\path ({i-\the\n}) coordinate (i\the\n);
\fill[black] (i\the\n) circle (2pt);
\draw[dashed] (i\the\n) -- (i\the\n |- 0,0);
\draw[dashed] (i\the\n) -- (0,0 -| i\the\n);
\advance\n by 1
\ifnum\n<5
\repeat

Annada Behera <segfault@tilde.green> wrote or quoted:
>% Mark intersection points and draw dashed lines
>\foreach \n in {1,...,\t} {
.. . .
>What is undefined?

Looks like "\t" is on the fritz or not pulling its weight,
since the loop seems to be cruising when we swap "\t" for "4":

\foreach \n in {1,...,4} {

. That error message was probably about as clear as LA smog.

On Sat, 2024-08-31 at 18:14 +0000, Stefan Ram wrote:
> Annada Behera <segfault@tilde.green> wrote or quoted:
> > % Mark intersection points and draw dashed lines
> > \foreach \n in {1,...,\t} {
> . . .
> > What is undefined?
>
>   Looks like "\t" is on the fritz or not pulling its weight,
>   since the loop seems to be cruising when we swap "\t" for "4":
>
> \foreach \n in {1,...,4} {
>
>   . That error message was probably about as clear as LA smog.

In this line, \t is definitely "pulling its weight,"

\fill[name intersections={of=line and sine, name=i, total=\t}, black]
\foreach \s in {1,...,\t} {(i-\s) circle (2pt)};

It's just looks like \t's scope is limited to the commands where it is defined.
Don't know whether this is expected behavior or a bug.

Annada Behera <segfault@tilde.green> wrote or quoted:
>Don't know whether this is expected behavior or a bug.

The TikZ & PGF manual (for Version 3.1.5b) has got this entry
"/tikz/intersection/total=<macro>" in the "13.3.2 Intersections
of Arbitrary Paths" section. It's got this example where
the "\foreach \s in {1,...,\t}" is all indented and stuff.

|\usetikzlibrary {intersections}
|\begin{tikzpicture}
| \clip (-2,-2) rectangle (2,2);
| \draw [name path=curve 1] (-2,-1) .. controls (8,-1) and (-8,1) .. (2,1);
| \draw [name path=curve 2] (-1,-2) .. controls (-1,8) and (1,-8) .. (1,2);
| | \fill [name intersections={of=curve 1 and curve 2, name=i, total=\t}]
| [red, opacity=0.5, every node/.style={above left, black, opacity=1}]
| \foreach \s in {1,...,\t}{(i-\s) circle (2pt) node {\footnotesize\s}};
|\end{tikzpicture}
| from "TikZ & PGF" "Manual for Version 3.1.5b".

If you drop a semicolon in front of the "\foreach" (right after
"opacity=1}]"), it's going to wig out on you with that error message.

Well, "\fill" is shorthand for "\path[fill]", and the 411 on
"\path" syntax is dropped in "14 Syntax for Path Specifications",
which lays it down like this:

|\path<specification>;

. So, there's that semicolon hanging ten, but I'm not stoked on where
its implications for the scope of "name intersections" is posted up.

Annada Behera wrote:

> \documentclass{standalone}
> \usepackage{tikz, amsmath}
> \usetikzlibrary{intersections}
> \begin{document}\begin{tikzpicture}
>
> % Plots
> \draw[very thick, smooth, samples=20, domain=-6.28:6.28]
> [red, name path=line] (0,0) plot (\x, \x);
> \draw[very thick, smooth, samples=20, domain=-6.28:6.28]
> [blue, name path=sine] (0,0) plot (\x, {\x + sin(\x r)});
>
> % Drawing the dots
> \fill[name intersections={of=line and sine, name=i, total=\t}, black]
> \foreach \s in {1,...,\t} {(i-\s) circle (2pt)};
>
> \end{tikzpicture}\end{document}
>
> Now this code works as expected. But I also wanted to draw dashed lines from
> the intersections to each axes.
>
> % Axes
> \draw [<->](-6.28, 0) -- (6.28, 0);
> \draw [<->](0, -6.28) -- (0, 6.28);
>
> % Mark intersection points and draw dashed lines
> \foreach \n in {1,...,\t} {
> \path ({i-\n}) coordinate (i\n); % <--- Error Here
> \fill[black] (i\n) circle (2pt);
> \draw[dashed] (i\n) -- (i\n |- 0,0);
> \draw[dashed] (i\n) -- (0,0 -| i\n);
> }
>
> In this part, pdflatex (my distro is TeX Live 2024) throws an error what
> I don't understand,
>
> ! Undefined control sequence.
> \UseTextAccent ...p \@firstofone \let \@curr@enc
> \cf@encoding \@use@text@en...
> l.28 }
>
> What is undefined?

When you say

\errorcontextlines=10000
\documentclass{...

, the error-message is:

! Undefined control sequence.
\UseTextAccent ...p \@firstofone \let \@curr@enc
\cf@encoding
\@use@text@en...

\?-cmd ...sname \csname ?\string #1\endcsname \fi
\csname \cf@encoding
\stri...

\pgffor@dots@charcheck ...@dots@charcheck@temp {#1
}\expandafter
\expandafter...

\pgffor@dots@value@process ...value \pgffor@@stop

\ifpgffor@alphabeticsequen...

\pgffor@dotsscanend ...@process {\pgffor@dotsend }

\pgffor@dots@value@process...

\pgffor@values ->1,...,\t ,
\pgffor@stop ,
l.27 }

and you see that \t in "\foreach \n in {1,...,\t} {...}" is undefined,

The problem is that the macro \t comes into being while a TikZ-path is
evaluated - \fill is a macro which expands to "\path..." - while TikZ
does not do control-sequence-evaluation/macro-expansion as usual while
parsing/evaluating/carrying out a TikZ-path-directive.

So you face the nice problem that \t being defined is restricted to the
scope of that TikZ-path-directive while inside TikZ-path-directives you
cannot easily use macros/control-sequences as directives for saving \t
away as a global macro which is available outside the scope of the
\fill-path-directive also.

As \t denotes a natural number, you can work around this problem by as a
component of the TikkZ-path-directive specifying a TikZ-coordinate where
one component (either the X-component or the Y-component) comes from \t,
using the measurement-unit sp (scaled point) and later retrieving that
component of the TikZ-coordinate and using it with \number, hereby
taking into account that using a TeX-\dimen or a TeX-\skip or a
LaTeX-length with \number directly yields the numerical value which
belongs to the quantity in question when it is expressed as a multiple
of the measurement-unit sp (scaled point):

%\errorcontextlines=10000
\documentclass{standalone}
\usepackage{tikz, amsmath}
\usetikzlibrary{intersections}

\newlength\scratchlength

\begin{document}

\begin{tikzpicture}

% Plots
\draw[very thick, smooth, samples=20, domain=-6.28:6.28]
[red, name path=line] (0,0) plot (\x, \x);
\draw[very thick, smooth, samples=20, domain=-6.28:6.28]
[blue, name path=sine] (0,0) plot (\x, {\x + sin(\x r)});

% Draw the dots and use \t for saving a coordinate so that
% \t can later be retrieved outside the scope of the \fill-path
% as well:
\fill [name intersections={of=line and sine, name=i, total=\t},
black]
\foreach \s in {1,...,\t} {(i-\s) circle (2pt)}
% Before ending with a semicolon (;) the path-specifica-
% tion started via \fill, let's save total/\t as the
% y-value of a TikZ-coordinate whose name is "total";
% specify the unit sp (scaled point) as all lengths in
% TeX internally are calculated/rounded to be integer
% multiples of 1sp; thus when specifying sp you don't get
% rounding-errors when later retrieving the value:
coordinate (total) at (0pt, \t sp);

% Extraxt to \scratchlength the y-coordinate of the pgfpoint
% which forms the center-anchor of the coordinate-node whose
% name is "total" :
\pgfextracty{\scratchlength}{\pgfpointanchor{total}{center}}%
% When you use a TeX-\dimen or TeX-\skip/LaTeX-length with
% \number directly, you get the numerical value which belongs
% to the (unstretched and unshrinked) quantity in question
% when it is expressed as a multiple of the measurement-unit
% sp(scaled point).
% So we are lucky as in the begin dimensions for coordinates
% were provided with measurement unit sp:
\edef\t{\number\scratchlength}%
% Now we have \t defined outside the scope of the \fill-path
% and can use it in next \foreach-loop.
%\show\t

% Axes
\draw [<->](-6.28, 0) -- (6.28, 0);
\draw [<->](0, -6.28) -- (0, 6.28);

% Mark intersection points and draw dashed lines
\foreach \n in {1,...,\t} {
\path ({i-\n}) coordinate (i\n); % <--- Error Here
\fill[black] (i\n) circle (2pt);
\draw[dashed] (i\n) -- (i\n |- 0,0);
\draw[dashed] (i\n) -- (0,0 -| i\n);
}

\end{tikzpicture}

\end{document}

Sincerely

Ulrich

Annada Behera wrote:

> Consider this minimal working example. I have two functions, f(x) = x
> and g(x) = x+sin(x). I want to draw them with PGF and at the intersection
> points, I want put black dots. Here is the TikZ code,

I just realized that inside TikZ-paths you can use \pgfextra{...}
for having LaTeX carry out arbitrary LaTeX code.
You can use this for making \t a global macro:

%\errorcontextlines=10000
\documentclass{standalone}
\usepackage{tikz, amsmath}
\usetikzlibrary{intersections}

\begin{document}

\begin{tikzpicture}

% Plots
\draw[very thick, smooth, samples=20, domain=-6.28:6.28]
[red, name path=line] (0,0) plot (\x, \x);
\draw[very thick, smooth, samples=20, domain=-6.28:6.28]
[blue, name path=sine] (0,0) plot (\x, {\x + sin(\x r)});

% Draw the dots and use \pgfextra for globally saving \t:
\fill [name intersections={of=line and sine, name=i, total=\t},
fill=green, draw=black, thin]
foreach \n in {1,...,\t} {(i-\n) circle (2pt)}
\pgfextra{\global\let\t\t};

% Draw dashed lines
\foreach \n in {1,...,\t} {
\draw[dashed] ({i-\n}) -- (0,0 -| {i-\n});
}

% Axes
\draw [<->](-6.28, 0) -- (6.28, 0);
\draw [<->](0, -6.28) -- (0, 6.28);

\end{tikzpicture}

\end{document}

Sincerely

Ulrich