Grid using TikZ

Question

I'm using the code below to generate the grid, however it's not connecting all points in the grid.

Could anyone help with this? Thank you!

\begin{tikzpicture}[darkstyle/.style={circle,draw,minimum size=9}]
 \foreach \x in {1, 2, 3}
    \foreach \y in {1, 2, 3} 
       {\pgfmathtruncatemacro {\label}{\x \y}
       \node [darkstyle]  (\x\y) at (1.5*\x,1.5*\y) {$U_{\label}$};} 

  \foreach \x in {1,2,3}
    \foreach \y  [count=\yi] in {1,2} 
      \draw (\x\y)--(\x\yi) (\y\x) --(\yi\x) ;

\end{tikzpicture}

Answer 1

Unsurprisingly, a grid can be drawn with grid... (and there is no need to do \pgfmathtruncatemacro {\label}{\x \y}).

\documentclass[tikz,border=3.14mm]{standalone}
\begin{document}
\begin{tikzpicture}[darkstyle/.style={circle,draw,minimum size=9,fill=white}]
 \draw (1.5,1.5) grid[step=1.5] (4.5,4.5);
 \foreach \x in {1, 2, 3}
    \foreach \y in {1, 2, 3} 
       {
       \node [darkstyle]  (\x\y) at (1.5*\x,1.5*\y) {$U_{\x \y}$};} 
\end{tikzpicture}
\end{document}

Answer 2

The problem is that you draw a line from the node to itself, so it doesn't draw anything at all:

\draw (\x\y)--(\x\yi)

Indeed, your loop generates a counter [count=\yi] in {1,2} which starts at 1 and therefore during the first iteration with \x=1 and \y =1, you generate this \x=1 \yi=1, etc.

Assuming you want the same result as @DũngVũ, here is another way to do it:

\documentclass[crop,tikz,border=5mm]{standalone}
\begin{document}

\usetikzlibrary{positioning,calc}
\tikzstyle{block} = [draw, rectangle, minimum height=1cm, minimum width=1cm, outer sep=0pt]

\begin{tikzpicture}[darkstyle/.style={circle,draw,minimum size=9}]
 \foreach \x in {1, 2, 3}
    \foreach \y in {1, 2, 3} 
       {\pgfmathtruncatemacro {\label}{\x\y}
       \node [darkstyle]  (\x\y) at (1.5*\x,1.5*\y) {$U_{\label}$};
       } 

  \foreach \x in {1,2,3}
    \foreach \y  [count=\yi from 2] in {1,2} 
      \path[] (\x\y)edge(\x\yi)(\y\x)edge(\yi\x);

\end{tikzpicture}
\end{document}

Answer 3

Do you mean this?

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
    \foreach \x in {1, 2, 3}
        \foreach \y in {1, 2}
            \draw (1.5*\x,1.5*\y) -- (1.5*\x,2.5*\y);
    \foreach \x in {1, 2}
        \foreach \y in {1, 2, 3}
            \draw (1.5*\x,1.5*\y) -- (2.5*\x,1.5*\y);
    \foreach \x in {1, 2, 3}
        \foreach \y in {1, 2, 3} {
            \pgfmathtruncatemacro {\label}{\x\y}
            \fill[color=white] (1.5*\x,1.5*\y) circle (0.5cm);
            \draw (1.5*\x,1.5*\y) circle (0.5cm);
            \node (\x\y) at (1.5*\x,1.5*\y) {$U_{\label}$};
        } 
\end{tikzpicture}
\end{document}

Hope this will help!