LaTeX complaining about illegal parameter number

Question

Inspired by a question yesterday, I thought I'd try my hand at a LaTeX3 solution which would show how to convert base 10 numbers into any other base of the user's choice.

Yesterday I wrote a nice simple version that worked. But today, I decided I wanted to separate the building of the steps from the process of outputting and formatting those steps. I also thought it would be interesting to explore the possibilities of creating the effect of an array within an array using sequences and token lists.

But now, I'm getting an error.

\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage{xparse}
\pagestyle{empty}
\ExplSyntaxOn
%% setting and getting the base
\int_new:N \g__new_base_int
\NewDocumentCommand{\setbase}{ m }{
    \int_gset:Nn \g__new_base_int { #1 }
}
\NewDocumentCommand{\getbase}{}{
    \int_use:N \g__new_base_int
}
%% setting and getting the number to be converted
\int_new:N \g__number_to_convert_int
\NewDocumentCommand{\setnumber}{ m }{
    \int_gset:Nn \g__number_to_convert_int {#1}
}
\NewDocumentCommand{\getnumber}{ }{
    \int_use:N \g__number_to_convert_int
}

%%---internal function to build each step in the conversion process
%%---and the parameters to be used for this function
\int_new:N \l__quotient_int
\int_new:N \l__tmp_new_quotient_int
\int_new:N \l__remainder_int
\seq_new:N \g__steps_of_conversion
\cs_new:Nn \__build_steps_of_conversion: {
    \int_set_eq:NN \l__quotient_int  \g__number_to_convert_int
    \int_do_while:nn { \l__quotient_int >= \g__new_base_int }
        {
            \int_set:Nn \l__remainder_int {  \int_mod:nn { \l__quotient_int } { \g__new_base_int } }
            \int_set:Nn \l__tmp_new_quotient_int { \int_div_truncate:nn { \l__quotient_int } {\g__new_base_int }}
            \exp_args:NNx \seq_gput_right:Nn \g__steps_of_conversion 
                {  
                    {\int_use:N \g__new_base_int  }
                    {\int_use:N \l__quotient_int  }
                    {\int_use:N \l__remainder_int }
                    {\int_use:N \l__tmp_new_quotient_int}
                }
            \int_set_eq:NN \l__quotient_int \l__tmp_new_quotient_int
        }
}
%%---internal function to construct the display of the conversion process
\bool_new:N \l__first_item_on_line_bool
\cs_new:Nn \__show_steps_of_conversion: {
    \__build_steps_of_conversion:
    \seq_map_inline:Nn \g__steps_of_conversion 
        {
            \bool_set_true:N \l__first_item_on_line_bool
            \tl_map_inline:nn { ##1 } {
                \bool_if:NF \l__first_item_on_line_bool { & }
                ###1 
                \bool_set_false:N \l__first_item_on_line_bool
            }
            \\
        }
}
%%---user interface to displaying the conversion
\NewDocumentCommand{\showstepsofconversion}{ o }{
    \[
        \begin{array}{crc|r}
            \text{Base} & & \text{R} & \text{Q} \\\hline
            \__show_steps_of_conversion: 
        \end{array}
    \]
}
%% accessing and using LaTeX3's internal base conversion
\NewDocumentCommand{\converttobase}{m}{
    \int_to_base:nn {#1} \g__new_base_int
}
%%
\ExplSyntaxOff
\setlength{\parindent}{0pt}
\setbase{21}
\setnumber{2680010}
\begin{document}

The base is \getbase.  \getnumber{} becomes $\converttobase{\getnumber}_{\getbase}$.

\showstepsofconversion

\end{document}

The problem seems to be ###1. Oddly, the output is as I desire. The suggested correction to ##1 is clearly not correct and when I rewrite my code to ##1 I get the wrong value.

Answer 1

you want 4 # to refer to #1 in a macro definition. (This is primitive TeX parsing, not a LaTeX3 feature)

            ####1 

not

            ###1