How can I use a counter with a dynamically-created name in Latex Expl3?

Internally, int-type expl3 variables use TeX \count registers, which must be defined before they're used. So something like

\int_use:c { __example_some_int } 

will only work if you have previously ran

\int_new:c { __example_some_int } 

The obvious solution here is to add \int_new:c to the definition of \__strxgen_construct_counter_name:nn, but that won't work, because \int_new:c is unexpandable, and you can only use expandable commands inside of a :c argument.

An alternate solution is to define a \__strxgen_construct_counter:nn that almost works like the current command, except instead of returning the target csname as a string, it defines a control sequence that points at the same underlying register. So whereas before you could use

\int_use:c { \__strxgen_construct_counter_name:nn { #1 } { #2 } } 

you'll now need to use

\__strxgen_construct_counter:nn { #1 } { #2 } \int_use:N \l__strx_last_counter_int 

---

Complete implementation:

\documentclass{article} \ExplSyntaxOn \str_new:N \l__strx_counter_csname_str \cs_new_protected:Npn \__strxgen_construct_counter:nn #1 #2 { \str_set:Nn \l__strx_counter_csname_str { g_strx#1_#2_count_int } \int_if_exist:cF { \l__strx_counter_csname_str } { \int_new:c { \l__strx_counter_csname_str } } \cs_set_eq:Nc \l__strx_last_counter_int { \l__strx_counter_csname_str } } % For user purposes \NewDocumentCommand\GetValue { m m } { \__strxgen_construct_counter:nn { #1 } { #2 } \int_use:N \l__strx_last_counter_int } \NewDocumentCommand\IncrementValue { m m } { \__strxgen_construct_counter:nn { #1 } { #2 } \int_incr:N \l__strx_last_counter_int } \ExplSyntaxOff \pagestyle{empty} \begin{document} \verb|\GetValue{div}{leviv}|: \GetValue{div}{leviv} \verb|\IncrementValue{div}{leviv}|: \IncrementValue{div}{leviv} \verb|\GetValue{div}{leviv}|: \GetValue{div}{leviv} \end{document} 

You never defined \g_strxdiv_leviv_count_int, so your \csname...\endcsname command ends up with \relax and that's the reason for the error message.

If you use the counter name you actually allocated, it works.

\documentclass{article} \ExplSyntaxOn % declare counter \int_new:N \g_strxdiv_leveliv_division_count_int % initialize counter \int_gset:Nn \g_strxdiv_leveliv_division_count_int { 1 } % Construct the counter name dynamically \cs_new:Npn \__strxgen_construct_counter_name:nn #1% first variable part #2% second variable part { g_strx#1_#2_count_int } % For user purposes \NewDocumentCommand\Test { m m } { \int_use:c { \__strxgen_construct_counter_name:nn { #1 } { #2 } } % <other stuff should be placed here using the counter once "reconstructed"> } \ExplSyntaxOff \begin{document} \Test{div}{leveliv_division} \end{document}