Ttdfjt

I am trying to print a number at the beginning of my document, say x. The problem is, that this number gets computed at a later part of my document, for example x = 2 + 3.

How do I use the result of this calculation at the beginning of my document? Commands like tableofcontents are also able to do similar things by multiple compile-steps, so i think this should be possible, but I am stuck here. Do you have any advice?

as in classic tex, you can use the aux file

documentclassarticle


gdefmynumber0% first time
begindocument

The number will be mynumber.

makeatletter
immediatewrite@auxoutgdefstringmynumberdirectlua
n=1+2+3
tex.print(n)
makeatother

enddocument

Welcome to TeX.SX! Here is a LaTeX3 implementation. It is independent of the TeX engine (LuaTeX, pdfTeX, XeTeX...). Moreover, as for LaTeX references, you will be warned if the result has changed since the last compilation run (meaning you should recompile to get correct output). The arithmetic expression passed to writeresult uses the int_eval:n function described in interface3.pdf (cf. part titled The l3int package).

documentclassarticle
usepackagexparse

ExplSyntaxOn

str_new:N g_lemonbonbon_result_str
str_new:N g_lemonbonbon_previous_result_str

msg_new:nnn lemonbonbon additional-compilation-run-may-be-needed 
 The~computed~result~has~changed;~another~compilation~run~may~be~needed. 

cs_new_protected:Npn lemonbonbon_write_result_to_aux_file:n #1
 
 cs_set_eq:cN lemonbonbon@set@result relax % will not expand

 iow_now:cx @auxout 
 use:c lemonbonbon@set@result int_eval:n #1 
 

NewDocumentCommand writeresult m 
 
 lemonbonbon_write_result_to_aux_file:n #1
 

NewDocumentCommand useresult 
 
 str_use:N g_lemonbonbon_result_str
 

AtBeginDocument
 
 % Save the previous result (this is run after the .aux file has been read)
 str_gset_eq:NN g_lemonbonbon_previous_result_str g_lemonbonbon_result_str
 

% Command run when the .aux file is reread, after the AtEndDocument hook.
cs_new_protected:Npn lemonbonbon_warn_if_result_changed:n #1
 
 str_if_eq:VnF g_lemonbonbon_previous_result_str #1
 
 msg_warning:nn lemonbonbon 
 additional-compilation-run-may-be-needed 
 
 

% Reading the .aux file at end of document triggers comparison with the
% previous result in order to warn the user if it has changed.
AtEndDocument
 
 cs_gset_eq:cN lemonbonbon@set@result 
 lemonbonbon_warn_if_result_changed:n
 

makeatletter
% Command used in the .aux file (uses LaTeX2e naming conventions)
NewDocumentCommand lemonbonbon@set@result m 
 
 str_gset:Nn g_lemonbonbon_result_str #1
 
makeatother

ExplSyntaxOff

begindocument
 The result is useresult.

 writeresult3+4*(5-3)
enddocument

Output after two compilation runs: