mjhjmtu

I define an array in the following way:

begintikzpicture
 defn10
 pgfmathparseint(n-1)
 foreach x in 0,...,pgfmathresult 
 foreach y in 0,...,pgfmathresult 
 pgfmathtruncatemacronodelabelx + y*n
 node at (x,y) (x) cnodelabel;
 
 
endtikzpicture

How can can I zero-pad the node labels such that I get c001, c010 etc.?

Another solution with siuntix:

documentclassarticle
usepackagesiunitx
usepackagetikz

begindocument
begintikzpicture
 defn10
 pgfmathparseint(n-1)
 foreach x in 0,...,pgfmathresult 
 foreach y [evaluate=y as ni using int(x+y*n)] in 0,...,pgfmathresult 
 node at (x,y) cnum[minimum-integer-digits=3]ni;
 
 
endtikzpicture

enddocument

Adapting the PGF answer given here: How to output a counter with leading zeros? we can use the same approach with your example. Instead of using pgfmathtruncatemacro I've use pgfmathsetcounter and then used the base conversion to pad the zeros.

documentclassarticle
usepackagetikz
newcounternodelabel
begindocument
begintikzpicture
 defn10
 pgfmathsetbasenumberlength3
 pgfmathparseint(n-1)
 foreach x in 0,...,pgfmathresult 
 foreach y in 0,...,pgfmathresult 
 pgfmathsetcounternodelabelx + y*n
 pgfmathbasetodecnodelabelthevaluenodelabel10%
 node at (x,y) (x) cnodelabel;
 
 
endtikzpicture
enddocument

Update:

I may have misunderstood the question, because labels can be written naturally like this:

documentclassarticle
usepackagetikz
begindocument
begintikzpicture
 defn10
 pgfmathparseint(n-1)
 foreach x in 0,...,pgfmathresult 
 foreach y in 0,...,pgfmathresult 
 node at (x,y) (x) c0yx;
 
 
endtikzpicture
enddocument

Old answer:

You can use the macro opprint from the xlop package that prints the numbers as they are written useless zeros included.
For example 00000.000 will be written 00000.000

documentclassarticle
usepackagetikz
usepackagexlop
begindocument
begintikzpicture
 defn10
 pgfmathparseint(n-1)
 foreach x in 0,...,pgfmathresult 
 foreach y in 0,...,pgfmathresult 
 node at (x,y) (x) copprint0yx;
 
 
endtikzpicture
enddocument