Wsrtjtyk

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]  (xy) at (1.5*x,1.5*y) {$U_{label}$};} 



  foreach x in {1,2,3}

    foreach y  [count=yi] in {1,2} 

      draw (xy)--(xyi) (yx) --(yix) ;



end{tikzpicture}

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]  (xy) at (1.5*x,1.5*y) {$U_{x y}$};} 

end{tikzpicture}

end{document}

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

draw (xy)--(xyi)

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}{xy}

       node [darkstyle]  (xy) 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 (xy)edge(xyi)(yx)edge(yix);



end{tikzpicture}

end{document}

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}{xy}

            fill[color=white] (1.5*x,1.5*y) circle (0.5cm);

            draw (1.5*x,1.5*y) circle (0.5cm);

            node (xy) at (1.5*x,1.5*y) {$U_{label}$};

        } 

end{tikzpicture}

end{document}

Hope this will help!