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in this picture: there are 6 intersection points. I need 6 auto change when number of intersection points change. Can you help me? enter image description here

my code: 

\documentclass[tikz,border=5mm]{standalone} \usepackage[utf8]{vietnam} \usetikzlibrary{intersections} \begin{document} \begin{tikzpicture}[smooth] \draw[gray!30](-3,-3) grid (3,3); \draw[->](-3,0)--(3,0) node[below]{$x$}; \draw[->](0,-3)--(0,3) node[right]{$y$}; \draw[blue,name path=hamso] plot[domain=-2.1:2.1] (\x,{(\x)^3-3*(\x)}) node[right, red] {$y=x^3-3x$}; \draw[red,name path=tron] (0,0) circle(2); \fill[violet,name intersections={of=hamso and tron,name=A,total=\t}] \foreach \i in {1,...,\t} {(A-\i) circle (2pt) node[above]{\i}}; \path (current bounding box.south) node[below]{There are: \fbox{6} intersection points}; \end{tikzpicture} \end{document}
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    It's already available. You're doing total=\t, so just write \fbox{\t}. Commented Feb 21, 2020 at 5:07
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    @HenriMenke \fbox{\t} do not work. May be total=\t is locally deffined only. Commented Feb 21, 2020 at 5:28

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I believe Henri wanted to tell you that you only make \t global (or smuggle it out of the path). One way of accomplishing this is to add \pgfextra{\xdef\myt{\t}} to the path, which stores \t in the global macro \myt. (Advanced possibilities of smuggling can be found e.g. here, but I think here you almost necessarily need to globalize the macro.) EDIT: Made this handle the case of 0 intersections, too.

\documentclass[tikz,border=5mm]{standalone} \usepackage[utf8]{vietnam} \usetikzlibrary{intersections} \begin{document} \begin{tikzpicture}[smooth] \draw[gray!30](-3,-3) grid (3,3); \draw[->](-3,0)--(3,0) node[below]{$x$}; \draw[->](0,-3)--(0,3) node[right]{$y$}; \draw[blue,name path=hamso] plot[domain=-2.1:2.1] (\x,{(\x)^3-3*(\x)}) node[right, red] {$y=x^3-3x$}; \draw[red,name path=tron] (0,0) circle(2); \fill[violet,name intersections={of=hamso and tron,name=A,total=\t}] \ifnum\t>0 foreach \i in {1,...,\t} {(A-\i) circle (2pt) node[above]{\i}} \fi \pgfextra{\xdef\myt{\t}}; \path (current bounding box.south) node[below]{There are: \fbox{\myt} intersection points}; \end{tikzpicture} \end{document}

enter image description here

If you change the cubic curve to

\draw[blue,name path=hamso] plot[domain=-2.1:2.1] (\x,{7+(\x)^3-3*(\x)}) node[right, red] {$y=7+x^3-3x$};

you'll get

enter image description here

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  • page 145 of the manual states -- /tikz/intersection/total=(macro) This key means that the total number of intersections found will be stored in macro --- could you please tell how to print the output of this macro Commented Feb 21, 2020 at 6:19
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    @jsbibra Yes, this is what the macro says, but the macro is a local macro, i.e. it is only "known" inside the path. You can print it out by adding \pgfextra{\typeout{I found \t\space intersections}}; to the path. But this literally prints it it out. Commented Feb 21, 2020 at 6:27
  • thank you so much. @Schrödinger'scat You can tell me when they do not have intersection, how to handle errors? Commented Feb 21, 2020 at 6:36
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    @JackNguyễn Good point! Added. Commented Feb 21, 2020 at 6:42

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