Ygtjki

I am attempting to create a triangular grid of numbers like shown below:

This was created in TikZ by hard-coding each point, but I would like to generate the grid procedurally so I can make adjustments easily with a large number of points. The following code is how I have approached the problem:

documentclassarticle
usepackagetikz

begintikzpicture[scale=1.5]
 foreach s in 2,...,4 
 foreach m in 1,...,numexprs-1 
 defns-m
 draw (n,-m) node $a_mn$;
 
 
endtikzpicture

However this produces the following undesired result:

My thought to resolve this issue was to add numexpr in the n definition or prior to n in the nodes. However this produces the error you can't use 'numexpr' in math mode.

I'm at a loss as to what to do next. How can I evaluate variables like this in math mode in a tikzpicture?

As a further note, if I for testing purposes ignore my need for math mode and simply put numexprn in the node text I instead receive the similar error message you can't use 'numexpr' in restricted horizontal mode.

Also, if possible I would like to preserve this foreach iteration structure as it is ideal for other annotations I am using in my full application.

You could use edefnthenumexprs-m, or just use it in the subscript:

documentclassarticle
usepackagetikz

begindocument

begintikzpicture[scale=1.5]
 foreach s in 2,...,4
 foreach m in 1,...,numexprs-1
 draw (s-m,-m) node $a_mthenumexprs-m$;
 

endtikzpicture

enddocument

Avoid additional calculations. Try to think of another way of using nested foreachs.

documentclass[tikz]standalone
begindocument
begintikzpicture[scale=1.5]
 foreach y in 1,...,3 
 foreach x in 1,...,numexpr4-y 
 draw (x,-y) node $a_yx$;
 
 
endtikzpicture
enddocument

There is already one answer how to use numexpr, one how to rewrite your loop to avoid it, and here is another possibility : you can use the tools provided by foreach to make calculations. You can use count and evaluate:

documentclass[tikz,border=7pt]standalone
begindocument
 begintikzpicture[scale=1.5]
 foreach[count=t from 1] s in 2,...,4 
 foreach[evaluate=n=int(s-m)] m in 1,...,t 
 draw (n,-m) node $a_mn$;
 
 
 endtikzpicture
enddocument