I need to draw something like this (forgive me, it is very crude), to denote the dependencies in my algorithm. Basically, I want to say that the circle (l,r) has incoming arrows from the 3 other circles (l,r-1), (l-1,r-1) and (l-1,r). I could only come up with a grid, using Tikz. how does one draw the dependency arrows and nodes as in the below image?



\draw[step=0.5cm,color=gray] (-1,-1) grid (1,1);
\foreach \x/\y/\m in {+0.75/+0.75/,-0.75/-0.75/} % etc
    \node at (\x,\y) {\m};


  • 1
    Here it would probably make sense to use a matrix of nodes and interconnecting those rather than a foreach approach. – Claudio Fiandrino Apr 4 '19 at 12:27
  • @ClaudioFiandrino Any references are welcome. – GermanShepherd Apr 4 '19 at 12:29
  • @ClaudioFiandrino Hiiiiiiiiiii :-). My affectionate regards. – Sebastiano Apr 4 '19 at 12:30

I would draw your diagram like this

\foreach \i in {0,1,2,3.5,4.5,5.5,7,8} {
    \draw[gray,very thin] (0,\i) -- (8.5,\i);
    \draw[gray,very thin] (\i,0) -- (\i,8.5);
\draw[<->] (8.5,0) node[below] {$n$} -- (0,0) node[below left] {$(1,1)$} -- (0,8.5) node[left] {$n$};
\draw[->] (2,-.5) -- (3,-.5) node[above,near start] {$r$};
\foreach \i in {2.75,6.25} {
    \foreach \j in {.5,1.5,4,5,7.5} {
        \node at (\i,\j) {$\cdots$};
        \node at (\j,\i) {$\vdots$};
    \foreach \j in {2.75,6.25}
        \node at (\i,\j) {\reflectbox{$\ddots$}};
\begin{scope}[every node/.style={
    minimum size=.8cm,
    inner sep=0pt
    \node at (4,4) (11) {$\scriptstyle l-1$\\[-1ex]$\scriptstyle r-1$};
    \node at (5,4) (21) {$\scriptstyle l-1$\\[-1ex]$\scriptstyle r$};
    \node at (4,5) (12) {$\scriptstyle l$\\[-1ex]$\scriptstyle r-1$};
    \node at (5,5) (22) {$\scriptstyle l$\\[-1ex]$\scriptstyle r$};
\draw[thick,red,->] (11) -- (22);
\draw[thick,red,->] (12) to[bend left=45] (22);
\draw[thick,red,->] (21) to[bend right=45] (22);

enter image description here


Using a loop here is probably over-killing, but anyway, here is a modification of your code. Single draw command to do this ;)

    \draw[nodes={inner sep=1pt},color=gray]
      (0,0) grid[shift={(-.5,-.5)}] (4,4)
      foreach[count=\i] \x/\y/\m in {2/3/a,2/2/b,3/3/c,}{(\x,\y) node(n\i) {\m}}
      (n1) edge[red,-latex] (n3)
      (n2) edge[red,-latex] (n3)

enter image description here

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