Ttdfjt

I have an volatile integer variable (counter) and at some point I wish to save its value to a token list (saved_counter).

If I save it with just tl_set, as soon as counter changes the value, this also changes the value of saved_counter, which is undesirable.

Example:

documentclassarticle
usepackage[utf8]inputenc
usepackagexparse

ExplSyntaxOn
int_zero_new:N counter % Set counter to 0.
tl_new:N saved_counter
ExplSyntaxOff

begindocument
ExplSyntaxOn
 % Save C_0 in "saved_counter"
 tl_set:Nn saved_counter C textsubscript thecounter

 % Print the value of saved counter -> C_0
 saved_counter

 % Change the value of counter for whatever the reason.
 int_incr:N counter

 % Print the value of saved counter again -> C_1
 saved_counter
ExplSyntaxOff
enddocument

I understand that there are no real variables in LaTeX and that everything's a macro, therefore, I understand why this is happening.

However, this seems like a common problem and there must be some convenient solution, like expanding/converting the value of counter to string and then assigning it to saved_counter.

Unfortunately, I am new to LaTeX and couldn't figure it out by myself.

The problem with your code is that when you do:

tl_set:Nn saved_counter C textsubscript thecounter

the token list variable contains:

• the C character token;


• the textsubscript control sequence token;


• an opening brace character token;


• the the control sequence token;


• the counter control sequence token;


• a closing brace character token.

In particular, thecounter is not expanded in your saved token list. It gets expanded later, when you use the token list and it is absorbed by TeX token by token. This is why it reflects the value of the counter when your saved_counter is expanded—at the last moment.

The solution is to expand thecounter when you want to save the token list (I used a LaTeX3-ish way to print the counter using decimal notation as with LaTeX2e's arabic or TeX's number):

documentclassarticle
usepackage[utf8]inputenc
usepackagexparse

ExplSyntaxOn

int_zero_new:N g_my_counter_int % Set counter to 0.
tl_new:N g_my_saved_tl

cs_new_protected:Npn my_update_token_list:n #1
 
 tl_gset:Nn g_my_saved_tl C textsubscript #1 
 

cs_generate_variant:Nn my_update_token_list:n x 

NewDocumentCommand myUpdateTokenList m 
 
 % The x-form eXpands the argument first
 my_update_token_list:x int_to_arabic:n #1 
 

ExplSyntaxOff

begindocument
ExplSyntaxOn
 % Save the token list using the current counter value -> C_0
 myUpdateTokenListg_my_counter_int

 % Print the saved token list
 tl_use:N g_my_saved_tl

 % Change the counter value for whatever the reason.
 int_gincr:N g_my_counter_int

 % This is still the token list saved by myUpdateTokenList above, where
 % the value 0 had been used.
 tl_use:N g_my_saved_tl
ExplSyntaxOff
enddocument

I also adapted your code to be more LaTeX3-conventional (variable names) and used global incr/set functions for the counter and token list (depends on your application, but is more straightforward this way). I believe it would be cleaner to put ExplSyntaxOn stuff only inside the preamble. For this, you'd have to wrap all the LaTeX3 code inside user-oriented commands, as I did for myUpdateTokenList. This makes the LaTeX3 things accessible to LaTeX2e: via arguments passed to commands that you implement in the preamble under ExplSyntaxOn regime.

The answer by frogon covers the approach best if you want to save not only the value itself but also additional material. There are however other ways.

I would probably just save the value in a second int:

documentclassarticle
usepackageexpl3
ExplSyntaxOn
int_new:N counter % Set counter to 0.
int_new:N l__iskustvo_saved_counter_int
ExplSyntaxOff

begindocument
ExplSyntaxOn
 % Save C_0 in "saved_counter"
 int_set_eq:NN l__iskustvo_saved_counter_int counter

 % Print the value of saved counter -> C_0
 int_use:N l__iskustvo_saved_counter_int

 % Change the value of counter for whatever the reason.
 int_incr:N counter

 % Print the value of saved counter again -> C_1
 int_use:N l__iskustvo_saved_counter_int
ExplSyntaxOff
enddocument

If you want to use a tl, you can save the value using V-type expansion

documentclassarticle
usepackageexpl3
ExplSyntaxOn
int_new:N counter % Set counter to 0.
tl_new:N l__iskustvo_saved_counter_tl
ExplSyntaxOff

begindocument
ExplSyntaxOn
 % Save C_0 in "saved_counter"
 tl_set:NV l__iskustvo_saved_counter_tl counter

 % Print the value of saved counter -> C_0
 l__iskustvo_saved_counter_tl

 % Change the value of counter for whatever the reason.
 int_incr:N counter

 % Print the value of saved counter again -> C_1
 l__iskustvo_saved_counter_tl
ExplSyntaxOff
enddocument