mjhjmtu

I want to draw a graph like this:

Which I can do well enough with the following code:

node[circle,draw] (v) at (0,0) $v$;
foreach i in 1,...,6
 node[circle,draw] (u_i) at (180 - 360/6 * i:2cm) $u_i$;
 draw (u_i) -- (v);

draw (u_1) -- (u_2);
draw (u_2) -- (u_3);
draw (u_3) -- (u_4);
draw (u_4) -- (u_5);
draw (u_5) -- (u_6);
draw (u_6) -- (u_1);

But I want to be able to draw it with a foreach loop for all of the outer edges instead of doing them one by one, like this:

foreach i in 1,...,6
 draw let nj = Mod(i,6)+1
 in (u_i) -- (u_nj);

However, when I try and do it this way, I end up with a graph looking like this:

MWE:

documentclassarticle
usepackagetikz
usetikzlibrarycalc

begindocument
tikz
 node[circle,draw] (v) at (0,0) $v$;
 foreach i in 1,...,6
 node[circle,draw] (u_i) at (180 - 360/6 * i:2cm) $u_i$;
 draw (u_i) -- (v);
 
 foreach i in 1,...,6
 draw let nj = Mod(i,6)+1
 in (u_i) -- (u_nj);
 

enddocument

Is there any way to do this with a foreach loop, without all the edges getting messed up?

Welcome to TeX.SE! You get floating point numbers like 5.0 from your computation, where .0 gets interpreted as the east anchor. So you basically need to wrap the result in int, and you can do this without calc like this.

documentclass[tikz,border=3.14mm]standalone
begindocument
begintikzpicture
node[circle,draw] (v) at (0,0) $v$;
foreach i in 1,...,6
 node[circle,draw] (u_i) at (180 - 360/6 * i:2cm) $u_i$;
 draw (u_i) -- (v);

foreach i [evaluate=i as j using int(mod(i,6)+1)] in 1,...,6

 draw (u_i) -- (u_j);

endtikzpicture
enddocument

This also results from you MWE if you add int:

documentclassarticle
usepackagetikz
usetikzlibrarycalc

begindocument
tikz
 node[circle,draw] (v) at (0,0) $v$;
 foreach i in 1,...,6
 node[circle,draw] (u_i) at (180 - 360/6 * i:2cm) $u_i$;
 draw (u_i) -- (v);
 
 foreach i in 1,...,6
 draw let nj = int(Mod(i,6)+1)
 in (u_i) -- (u_nj);
 

enddocument

However, IMHO this is slightly more complicated than the version without calc.