# Tikz picture using two “foreach” loops

I am making a picture using foreach. MWE is appended below

\documentclass{article}

\usepackage[svgnames]{xcolor}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}

\foreach \x in {0,...,5}
\draw [DodgerBlue](\x,-1) circle (0.25);

\foreach \x in {0,...,5}
\draw [-latex,red](\x,-1) -- (\x,-1.5);

\foreach \x in {0,...,5}
\draw [-latex,DarkGreen](\x,-1) -- (\x,-0.5);

\draw [ultra thin] (5,-0.5) -- (0,-1);

\end{tikzpicture}

\end{document}


The resulting picture is attached below

Now what I want is that the green arrows on the top should be limited by the black line diagonal line. That is for the first circle there should be no green arrow, and its length should increase until it becomes equal to the red arrow in the last circle. How can this be done using foreach using the variable \y. Is there a way such that the intersection of the black diagonal line with the green arrows can be used to limit the length of the green arrows to the desired values? Or is there any other way to achieve this?

In case you do not want to do analytic computations (or if you do not have a simple parametrization for the line).

\documentclass{article}

\usepackage[svgnames]{xcolor}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

\begin{tikzpicture}

\foreach \x in {0,...,5}
\draw [DodgerBlue](\x,-1) circle (0.25);

\foreach \x in {0,...,5}
\draw [-latex,red](\x,-1) -- (\x,-1.5) ;

\draw [ultra thin] (5,-0.5) coordinate(aux0)  -- (0,-1) coordinate(aux1);

\foreach \x in {1,...,5}
\draw [-latex,DarkGreen] (\x,-1) coordinate(aux2) (\x,-0.5) coordinate(aux3)
(aux2) -- (intersection cs:first line={(aux2)--(aux3)},
second line={(aux0)--(aux1)});

\end{tikzpicture}

\end{document}


To make @ArtificialStupidity happy (?) one loop...

\documentclass[border=3.14mm]{standalone}
\usepackage[svgnames]{xcolor}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}
\draw [ultra thin] (5,-0.5) coordinate(aux0)  -- (0,-1) coordinate(aux1);
\foreach \x in {0,...,5}
{ \draw [DodgerBlue](\x,-1) circle (0.25);
\draw [-latex,red](\x,-1) -- (\x,-1.5) ;
\unless\ifnum\x=0%
\draw [-latex,DarkGreen] (\x,-1) coordinate(aux2) (\x,-0.5) coordinate(aux3)
(aux2) -- (intersection cs:first line={(aux2)--(aux3)},
second line={(aux0)--(aux1)});
\fi
}
\end{tikzpicture}
\end{document}

• Using intersections is an interesting idea :) – TeXnician Dec 12 '18 at 11:11
• @TeXnician These are actually those intersections that come with calc and do not require the intersections library. They are restricted to intersections of straight lines, though. (+1 to your analytic solution ;-) – user121799 Dec 12 '18 at 11:13
• I have used these "calc intersections" before, but I would not have thought of them in this case… – TeXnician Dec 12 '18 at 11:22
• I am almost happy. The code is not so short and cryptic as mentioned before. :-) – Money Oriented Programmer Dec 12 '18 at 16:07
• @ArtificialStupidity Yes, but of course now it is no longer a valid answer to the question which in its title asks for two loops. (3 loops includes 2, of course ;-) – user121799 Dec 12 '18 at 16:12

You can use a simple linear function. Please note that your arrow heads are quite large in your scale. It might be a good idea to choose a larger scale or use another arrow tip to have a better representation of the second case.

\documentclass{article}

\usepackage[svgnames]{xcolor}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}

\foreach \x in {0,...,5}{
\draw [DodgerBlue](\x,0) circle (0.25);
\draw [-latex,red](\x,0) -- (\x,-.5);
}
\foreach \x in {1,...,5}{
\draw [-latex,DarkGreen](\x,0) -- (\x,{0.1*\x});
}

\draw [ultra thin] (5,.5) -- (0,0);

\end{tikzpicture}

\end{document}


A PSTricks solution with just one loop only for comparison purposes.

\documentclass[pstricks]{standalone}
\usepackage{pst-calculate}
\begin{document}
\begin{pspicture}[arrowsize=.2,arrowinset=0,linewidth=1pt](8.5,\pscalculate{8/3})
\foreach \i in {0,1,...,5}{%
\pstVerb{/x {\i\space  1.5 mul .5 add} def}%
\pscircle[linecolor=blue](!x 1){.5}
\psline[linecolor=red]{->}(!x 1)(!x 0)
\ifnum\i=0\relax\else\psline[linecolor=green]{->}(!x 1)(!x \i\space 3 div 1 add)\fi
}
\psline[linestyle=dashed](.5,1)(!8 dup 3 div)
\end{pspicture}
\end{document}


one more with use of lines intersections, but defined with help of the package intersections:

\documentclass[tikz, svgnames, margin=3]{standalone}
\usetikzlibrary{intersections}

\begin{document}
\begin{tikzpicture}
\foreach \x in {0,...,5}{
\draw [DodgerBlue](\x,0) circle (0.25);
\draw [-latex,red](\x,0) -- (\x,-.5);
}
\draw [ultra thin, name path=A] (5,.5) -- (0,0);
\foreach \x in {1,...,5}
{
\path[name path=B\x]   (\x,0) -- ++ (0,0.5);
\draw[-latex,DarkGreen,
name intersections={of=A and B\x,by={B\x}}] (\x,0) -- (B\x);
}
\end{tikzpicture}
\end{document}


Like this (please observe \foreach \x in {1,...,5} instead of \foreach \x in {0,...,5}?

\documentclass{article}

\usepackage[svgnames]{xcolor}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}

\foreach \x in {0,...,5}
\draw [DodgerBlue](\x,-1) circle (0.25);

\foreach \x in {0,...,5}
\draw [-latex,red](\x,-1) -- (\x,-1.5);

%\foreach \x in {0,...,5}
%   \draw [-latex,DarkGreen](\x,-1) -- (\x,-0.5);
\foreach \x in {1,...,5}
\draw [-latex,DarkGreen](\x,-1) -- (\x,-1+0.1*\x);

\draw [ultra thin] (5,-0.5) -- (0,-1);

\end{tikzpicture}

\end{document}