Tikz picture using two "foreach" loops

Question

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?

Answer 1

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}

Answer 2

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}

Answer 3

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}

Answer 4

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}

Answer 5

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}