A row of CSV data is not saved after checking for its emptiness and existence

Question

Following up this answer and the ensuing discussion, \getRow is expected to check the existence and emptiness of the target row of a CSV data, then save that row columns to another macro.

In other words, the following code snippet of \getRow doesn't process any data of the target row after a successful check for the emptiness and existence of that row.

% star → global assignment, macro, key, label
\NewDocumentCommand \getRow { s m m m }
{
    \IfBooleanTF {#1}
    { \__diaa_get_row:NNnn \cs_gset_protected:Npx }
    { \__diaa_get_row:NNnn \cs_set_protected:Npx }
    #2 {#3} {#4}
}
\cs_new_protected:Npn \__diaa_get_row:NNnn #1 #2 #3 #4
{
    #1 #2 ##1 [ ##2 ]
    { \exp_not:N \msg_expandable_error:nnn { diaa } { improper-row } { \cs_to_str:N #2 } }
    \__diaa_csv_item_exist:nnNT {#4} {#3} \l__diaa_tmpa_tl
    {
        \bool_set_true:N \l__diaa_empty_item_bool
        \tl_map_inline:Nn \l__diaa_tmpa_tl
        {
            \tl_if_empty:nF {##1}
            { \bool_set_false:N \l__diaa_empty_item_bool }
        }
        \bool_if:NTF \l__diaa_empty_item_bool
        { \msg_warning:nnnn { diaa } { row-empty } {#3} {#4} }
        #1 #2 ##1 [ ##2 ]
        {
            \exp_not:N \__diaa_get_column:nnN
            { \exp_not:V \l__diaa_tmpa_tl } {##2} ##1
        }
    }
}
\cs_new_protected:Npn __diaa_get_column:nnN #1 #2 #3
{
    \str_if_eq:nnTF {#2} { non-empty }
    {
        __diaa_nb_nonempty_items_in_row:nn { 0 } #1
        \q_recursion_tail \q_recursion_stop
    }
    { __diaa_check_column_range:nn {#1} {#2} }
}
\cs_new:Npn __diaa_nb_nonempty_items_in_row:nn #1#2
{
    \quark_if_recursion_tail_stop_do:nn {#2} { \int_eval:n {#1} }
    \tl_if_empty:nTF {#2}
    { __diaa_nb_nonempty_items_in_row:nn {#1} }
    { __diaa_nb_nonempty_items_in_row:nn { #1 + 1 } }
}
\msg_new:nnn { diaa } { csv-undefined } { CSV~database~#1'~undefined! } \msg_new:nnn { diaa } { key-undefined } { CSV~#2'~has~no~key~#1'! } \msg_new:nnn { diaa } { out-of-range } { Index~#1~out~of~range! } \msg_new:nnn { diaa } { item-empty } { Item~#1~from~#2'~in~CSV~#3'~is~empty! } \msg_new:nnn { diaa } { row-empty } { Row~#1'~in~CSV~`#2'~is~empty! }
\msg_new:nnn { diaa } { empty-row-item }
{ Empty~item~#1~\msg_line_context:! }
\msg_new:nnn { diaa } { improper-row }
{
    Improper~row~macro~\iow_char:N \#1~\msg_line_context:.\
    The~\iow_char:N \getRow~command~did~not~succeed.
}

So, how to make \getRow process the row data after passing the checks for emptiness and existence for the following full MWE?

P.S. I included the full expl3 code since I don't know if there are dependencies somewhere or not

\begin{filecontents*}{test.csv}
    Third Parameter , 7 , 9 ,
    First Parameter , 5 , {foo, bar} ,
    Second Parameter , 3 , 6 , 44
    Empty Parameter , , ,
\end{filecontents*}
\documentclass{article}
\usepackage{xparse,siunitx,xfp}
\ExplSyntaxOn
% Step 1: reading the file
\ior_new:N \l__diaa_csv_ior
\bool_new:N \l__diaa_csv_str_bool
\bool_new:N \l__diaa_empty_item_bool
\seq_new:N \l__diaa_csv_tmp_seq
\tl_new:N \l__diaa_tmpa_tl
% str mode (bool/star), key column, label, value columns, file
\NewDocumentCommand \ReadCSV { s O{1} m O{} m }
{
    \IfBooleanTF {#1}
    { \bool_set_true:N \l__diaa_csv_str_bool }
    { \bool_set_false:N \l__diaa_csv_str_bool }
    \diaa_csv_read:nnnn {#3} {#2} {#4} {#5}
}
% label, key column, value columns, file
\cs_new_protected:Npn \diaa_csv_read:nnnn #1 #2 #3 #4
{
    \tl_if_blank:nTF {#3} % Detect number of columns and use 2 to last
    {
        \ior_open:NnTF \l__diaa_csv_ior {#4}
        {
            \bool_if:NTF \l__diaa_csv_str_bool
            { \ior_str_get:NN }
            { \ior_get:NN }
            \l__diaa_csv_ior \l_tmpa_tl
            \ior_close:N \l__diaa_csv_ior
            \seq_set_split:NnV \l_tmpa_seq { , } \l_tmpa_tl
            \seq_clear:N \l__diaa_csv_tmp_seq
            \int_step_inline:nnn { 2 } { \seq_count:N \l_tmpa_seq }
            { \seq_put_right:Nn \l__diaa_csv_tmp_seq {##1} }
        }
        { \msg_expandable_error:nnn { diaa } { file-not-found } {#4} }
    }
    { \seq_set_split:Nnn \l__diaa_csv_tmp_seq { , } {#3} } % explicit columns
    \ior_open:NnTF \l__diaa_csv_ior {#4}
    {
        \prop_new:c { g__diaa_csv_#1_prop }
        __diaa_csv_read:nn {#1} {#2}
        \ior_close:N \l__diaa_csv_ior
    }
    { \msg_expandable_error:nnn { diaa } { file-not-found } {#4} }
}
\msg_new:nnn { diaa } { file-not-found }
{ File~`#1'~not~found. }
\cs_generate_variant:Nn \prop_put:Nnn { cxV }
% label, key column
\cs_new_protected:Npn __diaa_csv_read:nn #1 #2
{
    \bool_if:NTF \l__diaa_csv_str_bool
    { \ior_str_map_inline:Nn }
    { \ior_map_inline:Nn }
    \l__diaa_csv_ior
    {
        \seq_set_split:Nnn \l_tmpa_seq { , } {##1} % split one CSV row
        \tl_clear:N \l_tmpa_tl
        \seq_map_inline:Nn \l__diaa_csv_tmp_seq
        { \tl_put_right:Nx \l_tmpa_tl { { \seq_item:Nn \l_tmpa_seq {####1} } } }
        \prop_put:cxV { g__diaa_csv_#1_prop }
        { \seq_item:Nn \l_tmpa_seq {#2} }
        \l_tmpa_tl
    }
}
% Step 2: getting the values
% star → global assignment, macro or tl var, value column, key, label
\NewDocumentCommand \getValue { s m O{1} m m }
{
    \tl_clear:N \l__diaa_tmpa_tl
    \diaa_csv_item:nnnN {#4} {#3} {#5} \l__diaa_tmpa_tl
    \IfBooleanTF {#1}
    { \tl_gset_eq:NN } { \tl_set_eq:NN }
    #2 \l__diaa_tmpa_tl
    \tl_if_empty:NT #2
    { \msg_warning:nnnnn { diaa } { item-empty } {#3} {#4} {#5} }
}
% key, value column, label
\cs_new_protected:Npn \diaa_csv_item:nnnN #1 #2 #3 #4
{
    __diaa_csv_item_exist:nnNT {#3} {#1} #4
    { \exp_args:NV __diaa_check_column_range:nn #4 {#2} }
}
\cs_new_protected:Npn __diaa_check_column_range:nn #1 #2
{
    \bool_lazy_or:nnTF
    { \int_compare_p:nNn {#2} = { 0 } }
    { \int_compare_p:nNn { \tl_count:n {#1} } < { \int_abs:n {#2} } }
    { \msg_expandable_error:nnn { diaa } { out-of-range } {#2} }
    { \tl_set:Nx \l__diaa_tmpa_tl { \tl_item:nn {#1} {#2} } }
}
\prg_new_protected_conditional:Npnn __diaa_csv_item_exist:nnN #1 #2 #3 { T }
{
    \prop_if_exist:cTF { g__diaa_csv_#1_prop }
    {
        \prop_get:cnNTF { g__diaa_csv_#1_prop } {#2} #3
        { \prg_return_true: }
        {
            \msg_expandable_error:nnnn { diaa } { key-undefined } {#2} {#1}
            \prg_return_false:
        }
    }
    {
        \msg_expandable_error:nnn { diaa } { csv-undefined } {#1}
        \prg_return_false:
    }
}
\cs_new_protected:Npn __diaa_check_empty:nn #1 #2
{
    \tl_if_empty:nT {#1}
    { \msg_warning:nnn { diaa } { empty-row-item } {#2} }
    #1
}
% star → global assignment, macro, key, label
\NewDocumentCommand \getRow { s m m m }
{
    \IfBooleanTF {#1}
    { __diaa_get_row:NNnn \cs_gset_protected:Npx }
    { __diaa_get_row:NNnn \cs_set_protected:Npx }
    #2 {#3} {#4}
}
\cs_new_protected:Npn __diaa_get_row:NNnn #1 #2 #3 #4
{
    #1 #2 ##1 [ ##2 ]
    { \exp_not:N \msg_expandable_error:nnn { diaa } { improper-row } { \cs_to_str:N #2 } }
    __diaa_csv_item_exist:nnNT {#4} {#3} \l__diaa_tmpa_tl
    {
        \bool_set_true:N \l__diaa_empty_item_bool
        \tl_map_inline:Nn \l__diaa_tmpa_tl
        {
            \tl_if_empty:nF {##1}
            { \bool_set_false:N \l__diaa_empty_item_bool }
        }
        \bool_if:NTF \l__diaa_empty_item_bool
        { \msg_warning:nnnn { diaa } { row-empty } {#3} {#4} }
        #1 #2 ##1 [ ##2 ]
        {
            \exp_not:N __diaa_get_column:nnN
            { \exp_not:V \l__diaa_tmpa_tl } {##2} ##1
        }
    }
}
\cs_new_protected:Npn __diaa_get_column:nnN #1 #2 #3
{
    \str_if_eq:nnTF {#2} { non-empty }
    {
        __diaa_nb_nonempty_items_in_row:nn { 0 } #1
        \q_recursion_tail \q_recursion_stop
    }
    { __diaa_check_column_range:nn {#1} {#2} }
}
\cs_new:Npn __diaa_nb_nonempty_items_in_row:nn #1#2
{
    \quark_if_recursion_tail_stop_do:nn {#2} { \int_eval:n {#1} }
    \tl_if_empty:nTF {#2}
    { __diaa_nb_nonempty_items_in_row:nn {#1} }
    { __diaa_nb_nonempty_items_in_row:nn { #1 + 1 } }
}
\msg_new:nnn { diaa } { csv-undefined } { CSV~database~#1'~undefined! } \msg_new:nnn { diaa } { key-undefined } { CSV~#2'~has~no~key~#1'! } \msg_new:nnn { diaa } { out-of-range } { Index~#1~out~of~range! } \msg_new:nnn { diaa } { item-empty } { Item~#1~from~#2'~in~CSV~#3'~is~empty! } \msg_new:nnn { diaa } { row-empty } { Row~#1'~in~CSV~`#2'~is~empty! }
\msg_new:nnn { diaa } { empty-row-item }
{ Empty~item~#1~\msg_line_context:! }
\msg_new:nnn { diaa } { improper-row }
{
    Improper~row~macro~\iow_char:N \#1~\msg_line_context:.\
    The~\iow_char:N \getRow~command~did~not~succeed.
}
\ExplSyntaxOff
\parindent0pt
\begin{document}
\ReadCSV{mydata}{test.csv}

\getRow\myRow{Second Parameter}{mydata}
\num{\myRow[2]}\\
\num{\myRow[1]}\\
\edef\Value{\fpeval{\myRow[2]-\myRow[1]}}
\num[\Value]

\end{document}

Answer 1

Here is a fix for \__diaa_get_column:nnN (which didn't assign to its third argument with the original code), along with an example using the results inside siunitx's \num macro.

For your convenience, I also added a \getNbNonEmpty macro to find the number of non-empty items in a row and store it in a macro:

\getNbNonEmpty [*] \⟨result⟩ {⟨key⟩} {⟨CSV file label⟩}

This macro checks if the ⟨CSV file label⟩ is valid and if the ⟨key⟩ identifies an existing row. If not, you'll get the same error messages as for \getRow. However, \getNbNonEmpty doesn't warn if the row is empty because I believe this wouldn't be very helpful here; this would be easy to change if you wish, though. As for \getValue and \getRow, the star form performs a global assignment to \⟨result⟩.

% Based on Phelype Oleinik's answer at
% <https://tex.stackexchange.com/a/575687/73317>
\begin{filecontents*}{test.csv}
Third Parameter  , 7 , 9          ,
First Parameter  , 5 , {foo, bar} ,
Second Parameter , 3 , 6          , 44
Empty Parameter  ,   ,            ,
\end{filecontents*}
\documentclass{article}
\usepackage{xparse}
\usepackage{siunitx}            % for the test using \num
\ExplSyntaxOn
% Step 1: reading the file
\ior_new:N \l__diaa_csv_ior
\bool_new:N \l__diaa_csv_str_bool
\bool_new:N \l__diaa_empty_item_bool
\seq_new:N \l__diaa_csv_tmp_seq
\tl_new:N \l__diaa_tmpa_tl
% str mode (bool/star), key column, label, value columns, file
\NewDocumentCommand \ReadCSV { s O{1} m O{} m }
  {
    \IfBooleanTF {#1}
      { \bool_set_true:N \l__diaa_csv_str_bool }
      { \bool_set_false:N \l__diaa_csv_str_bool }
    \diaa_csv_read:nnnn {#3} {#2} {#4} {#5}
  }
% label, key column, value columns, file
\cs_new_protected:Npn \diaa_csv_read:nnnn #1 #2 #3 #4
  {
    \tl_if_blank:nTF {#3}       % Detect number of columns and use 2 to last
      {
        \ior_open:NnTF \l__diaa_csv_ior {#4}
          {
            \bool_if:NTF \l__diaa_csv_str_bool
              { \ior_str_get:NN }
              { \ior_get:NN }
              \l__diaa_csv_ior \l_tmpa_tl
            \ior_close:N \l__diaa_csv_ior
            \seq_set_split:NnV \l_tmpa_seq { , } \l_tmpa_tl
            \seq_clear:N \l__diaa_csv_tmp_seq
            \int_step_inline:nnn { 2 } { \seq_count:N \l_tmpa_seq }
              { \seq_put_right:Nn \l__diaa_csv_tmp_seq {##1} }
          }
          { \msg_error:nnn { diaa } { file-not-found } {#4} }
      }
      { \seq_set_split:Nnn \l__diaa_csv_tmp_seq { , } {#3} } % explicit columns
    \ior_open:NnTF \l__diaa_csv_ior {#4}
      {
        \prop_new:c { g__diaa_csv_#1_prop }
        __diaa_csv_read:nn {#1} {#2}
        \ior_close:N \l__diaa_csv_ior
      }
      { \msg_error:nnn { diaa } { file-not-found } {#4} }
  }
\msg_new:nnn { diaa } { file-not-found }
  { File~`#1'~not~found. }
\cs_generate_variant:Nn \prop_put:Nnn { cxV }
% label, key column
\cs_new_protected:Npn __diaa_csv_read:nn #1 #2
  {
    \bool_if:NTF \l__diaa_csv_str_bool
      { \ior_str_map_inline:Nn }
      { \ior_map_inline:Nn }
        \l__diaa_csv_ior
        {
          \seq_set_split:Nnn \l_tmpa_seq { , } {##1} % split one CSV row
          \tl_clear:N \l_tmpa_tl
          \seq_map_inline:Nn \l__diaa_csv_tmp_seq
            { \tl_put_right:Nx \l_tmpa_tl { { \seq_item:Nn \l_tmpa_seq {####1} } } }
          \prop_put:cxV { g__diaa_csv_#1_prop }
            { \seq_item:Nn \l_tmpa_seq {#2} }
            \l_tmpa_tl
        }
  }
% Step 2: getting the values
% star → global assignment, macro or tl var, value column, key, label
\NewDocumentCommand \getValue { s m O{1} m m }
  {
    \tl_clear:N \l__diaa_tmpa_tl
    \diaa_csv_item:nnnN {#4} {#3} {#5} \l__diaa_tmpa_tl
    \IfBooleanTF {#1}
      { \tl_gset_eq:NN } { \tl_set_eq:NN }
        #2 \l__diaa_tmpa_tl
    \tl_if_empty:NT #2
      { \msg_warning:nnnnn { diaa } { item-empty } {#3} {#4} {#5} }
  }
% key, value column, label
\cs_new_protected:Npn \diaa_csv_item:nnnN #1 #2 #3 #4
  {
    __diaa_csv_item_exist:nnNT {#3} {#1} #4
      { \exp_args:NV __diaa_check_column_range:nn #4 {#2} }
  }
\cs_new_protected:Npn __diaa_check_column_range:nn #1 #2
  {
    \bool_lazy_or:nnTF
        { \int_compare_p:nNn {#2} = { 0 } }
        { \int_compare_p:nNn { \tl_count:n {#1} } < { \int_abs:n {#2} } }
      { \msg_error:nnn { diaa } { out-of-range } {#2} }
      { \tl_set:Nx \l__diaa_tmpa_tl { \tl_item:nn {#1} {#2} } }
  }
\prg_new_protected_conditional:Npnn __diaa_csv_item_exist:nnN #1 #2 #3 { T }
  {
    \prop_if_exist:cTF { g__diaa_csv_#1_prop }
      {
        \prop_get:cnNTF { g__diaa_csv_#1_prop } {#2} #3
          { \prg_return_true: }
          {
            \msg_error:nnnn { diaa } { key-undefined } {#2} {#1}
            \prg_return_false:
          }
      }
      {
        \msg_error:nnn { diaa } { csv-undefined } {#1}
        \prg_return_false:
      }
  }
\cs_new_protected:Npn __diaa_check_empty:nn #1 #2
  {
    \tl_if_empty:nT {#1}
      { \msg_warning:nnn { diaa } { empty-row-item } {#2} }
    #1
  }
% star → global assignment, macro, key, label
\NewDocumentCommand \getRow { s m m m }
  {
    \IfBooleanTF {#1}
      { __diaa_get_row:NNnn \cs_gset_protected:Npx }
      { __diaa_get_row:NNnn \cs_set_protected:Npx }
        #2 {#3} {#4}
  }
% #1: \cs_set_protected:Npx or \cs_gset_protected:Npx
% #2: macro for the result
% #3: key
% #4: label
\cs_new_protected:Npn __diaa_get_row:NNnn #1 #2 #3 #4
  {
    #1 #2 ##1 [ ##2 ]
      { \msg_error:nnx { diaa } { improper-row } { \cs_to_str:N #2 } }
    __diaa_csv_item_exist:nnNT {#4} {#3} \l__diaa_tmpa_tl
      {
        \bool_set_true:N \l__diaa_empty_item_bool
        \tl_map_inline:Nn \l__diaa_tmpa_tl
          {
            \tl_if_empty:nF {##1}
              { \bool_set_false:N \l__diaa_empty_item_bool }
          }
        \bool_if:NT \l__diaa_empty_item_bool
          { \msg_warning:nnnn { diaa } { row-empty } {#3} {#4} }
        #1 #2 ##1 [ ##2 ]
          {
            \exp_not:N __diaa_get_column:nnN
              { \exp_not:V \l__diaa_tmpa_tl } {##2} ##1
          }
      }
  }
% #1: token list {1st item}{2nd item} ... {nth item}
% #2: index (integer expression) or 'non-empty'
% #3: macro for the result
\cs_new_protected:Npn __diaa_get_column:nnN #1 #2 #3
  {
    \str_if_eq:nnTF {#2} { non-empty }
      {
        \tl_set:Nx #3
          {
            __diaa_nb_nonempty_items_in_row:nw { 0 } #1
            \q_recursion_tail \q_recursion_stop
          }
      }
      {
        __diaa_check_column_range:nn {#1} {#2}
        \tl_set_eq:NN #3 \l__diaa_tmpa_tl
      }
  }
\cs_new:Npn __diaa_nb_nonempty_items_in_row:nw #1#2
  {
    \quark_if_recursion_tail_stop_do:nn {#2} { \int_eval:n {#1} }
    \tl_if_empty:nTF {#2}
      { __diaa_nb_nonempty_items_in_row:nw {#1} }
      { __diaa_nb_nonempty_items_in_row:nw { #1 + 1 } }
  }
% star → global assignment, macro, key, label
\NewDocumentCommand \getNbNonEmpty { s m m m }
  {
    \IfBooleanTF {#1}
      { __diaa_get_nb_nonempty_items:NNnn \cs_gset_nopar:Npx }
      { __diaa_get_nb_nonempty_items:NNnn \cs_set_nopar:Npx }
        #2 {#3} {#4}
  }
% #1: \cs_set_nopar:Npx or \cs_gset_nopar:Npx
% #2: macro
% #3: key
% #4: label
\cs_new_protected:Npn __diaa_get_nb_nonempty_items:NNnn #1 #2 #3 #4
  {
    #1 #2
      {
        \msg_error:nnx { diaa } { improper-non-empty-macro }
          { \cs_to_str:N #2 }
      }
\__diaa_csv_item_exist:nnNT {#4} {#3} \l__diaa_tmpa_tl
  {
    #1 #2
      {
        \exp_last_unbraced:Nno \__diaa_nb_nonempty_items_in_row:nw { 0 }
          \l__diaa_tmpa_tl \q_recursion_tail \q_recursion_stop
      }
  }

}
\msg_new:nnn { diaa } { csv-undefined } { CSV~database~#1'~undefined! } \msg_new:nnn { diaa } { key-undefined } { CSV~#2'~has~no~key~#1'! } \msg_new:nnn { diaa } { out-of-range } { Index~#1~out~of~range! } \msg_new:nnn { diaa } { item-empty } { Item~#1~from~#2'~in~CSV~#3'~is~empty! } \msg_new:nnn { diaa } { row-empty } { Row~#1'~in~CSV~`#2'~is~empty! }
\msg_new:nnn { diaa } { empty-row-item }
  { Empty~item~#1~\msg_line_context:! }
\msg_new:nnn { diaa } { improper-row }
  {
    Improper~row~macro~\iow_char:N \#1~\msg_line_context:.\
    The~\iow_char:N \getRow~command~did~not~succeed.
  }
\msg_new:nnn { diaa } { improper-non-empty-macro }
  {
    Improper~non-empty~macro~\iow_char:N \#1~\msg_line_context:.\
    The~\iow_char:N \getNbNonEmpty~command~did~not~succeed.
  }
\ExplSyntaxOff
\parindent0pt
\begin{document}
\ReadCSV{mydata}{test.csv}
% Get the second item of the row identified by Second Parameter
\getRow\myRow{Second Parameter}{mydata}
\myRow\myValue[2]
% Using the result:
\num{\myValue}
% Get the number of non-empty items in the same row
\myRow\nbNonEmpty[non-empty]
% Using the result:
\num{\nbNonEmpty}
% Same thing in two steps (no need for the \getRow here)
\getNbNonEmpty\nbNonEmpty{Second Parameter}{mydata}
% Using the result:
\num{\nbNonEmpty}
% Ditto for the row identified by 'Third Parameter':
\getNbNonEmpty\nbNonEmpty{Third Parameter}{mydata}
% \show\nbNonEmpty % \nbNonEmpty=macro:->2.
\end{document}