Here is one solution using expl3:
\documentclass{article}
\usepackage{chessboard}
\usepackage{xskak}
\usepackage[margin=2cm]{geometry}
\usepackage{tikz}
\usepackage{xparse}
\ExplSyntaxOn
\msg_new:nnn { mychess } { groups-have-different-number-of-series }
{ all~groups~must~have~the~same~number~of~series~ (found~\exp_not:n {#1}~
and~\exp_not:n {#2}). }
\seq_new:N \g_mychess_groups_seq
\cs_new_protected:Npn \mychess_define_groups:n #1
{
\seq_gset_from_clist:Nn \g_mychess_groups_seq {#1}
}
% #1: group name (A, B, C, ...)
% #2: diagram data for this group (a comma list)
\cs_new_protected:Npn \mychess_set_group_diagrams:nn #1#2
{
\seq_gclear_new:c { g_mychess_group_#1_seq }
\seq_gset_from_clist:cn { g_mychess_group_#1_seq } {#2}
}
% Expand to the number of series declared for group #1
\cs_new:Npn \mychess_nb_series_for_group:n #1
{
\seq_count:c { g_mychess_group_#1_seq }
}
\cs_generate_variant:Nn \mychess_nb_series_for_group:n { V }
\cs_generate_variant:Nn \msg_error:nnnn { nnVx }
\tl_new:N \l__mychess_a_group_tl
\int_new:N \l__mychess_nb_series_int
\cs_new_protected:Npn \mychess_print_all_diagrams:
{
% Get the name of the first group and the number of series declared for
% this group (any other group would do, as they must have the same number
% of series---which we are going to check right now as a safety measure
% against invalid user input).
\seq_get_left:NN \g_mychess_groups_seq \l__mychess_a_group_tl
\int_set:Nn \l__mychess_nb_series_int
{ \mychess_nb_series_for_group:V \l__mychess_a_group_tl }
% Check that the same number of series was defined for each group, and
% print an error message if this is not the case.
\seq_map_inline:Nn \g_mychess_groups_seq
{
\int_compare:nNnF
{ \mychess_nb_series_for_group:n {##1} }
=
{ \l__mychess_nb_series_int }
{
\msg_error:nnVx { mychess }
{ groups-have-different-number-of-series }
\l__mychess_nb_series_int
{ \mychess_nb_series_for_group:n {##1} }
}
}
% Print all diagrams in series order, then in group order inside each
% series.
\int_step_function:nN { \l__mychess_nb_series_int }
\mychess_print_diagrams_for_series:n
}
\seq_new:N \l__mychess_print_one_diagram_set_tmp_seq
% #1: group name
% #2: data (a comma list: of rounds? Not sure about what the data represents)
\cs_new_protected:Npn \mychess_print_one_diagram_set:nn #1#2
{
\par \noindent
\tikz { \node[rotate=90,font=\bfseries,align=center,text~width=4cm] {#1}; }
\seq_set_from_clist:Nn \l__mychess_print_one_diagram_set_tmp_seq {#2}
\seq_map_inline:Nn \l__mychess_print_one_diagram_set_tmp_seq
{
\chessboard[smallboard,setfen=##1]
\skip_horizontal:n { \fill }
}
\par
}
\cs_generate_variant:Nn \mychess_print_one_diagram_set:nn { nx }
% #1: 1 for the first series, 2 for second, etc.
\cs_new_protected:Npn \mychess_print_diagrams_for_series:n #1
{
\seq_map_inline:Nn \g_mychess_groups_seq
{
\mychess_print_one_diagram_set:nx
{##1} % group name
{ \seq_item:cn { g_mychess_group_##1_seq } {#1} }
}
}
% Processing the “big structure”
\seq_new:N \g__mychess_rosfg_discovered_groups_seq
% #1: group name
% #2: data (a comma list: of rounds? Not sure about what the data represents)
\cs_new_protected:Npn \mychess_read_one_series_for_group:nn #1#2
{
\seq_if_in:NnF \g__mychess_rosfg_discovered_groups_seq {#1}
{ % First time we see group #1 in the “big structure”
\seq_gput_right:Nn \g__mychess_rosfg_discovered_groups_seq {#1}
% Clear all data for this group
\seq_gclear_new:c { g_mychess_group_#1_seq }
}
% Append the data to what we already have for this group
\seq_gput_right:cn { g_mychess_group_#1_seq } {#2}
}
\cs_generate_variant:Nn \mychess_read_one_series_for_group:nn { VV }
\cs_new_protected:Npn \mychess_read_big_structure:n #1
{
\seq_gclear:N \g__mychess_rosfg_discovered_groups_seq
\foreach \group / \data in {#1}
{ \mychess_read_one_series_for_group:VV \group \data }
\seq_gset_eq:NN \g_mychess_groups_seq
\g__mychess_rosfg_discovered_groups_seq
}
% #1: comma list of group names
\NewDocumentCommand \definegroups { m }
{
\mychess_define_groups:n {#1}
}
% #1: group name
% #2: data for this group (a comma list containing one item per series, each
% item being itself a comma list describing the diagrams for the (group,
% series) combination).
\NewDocumentCommand \setgroupdiagrams { m m }
{
\mychess_set_group_diagrams:nn {#1} {#2}
}
\NewDocumentCommand \readbigstructure { m }
{
\mychess_read_big_structure:n {#1}
}
\NewDocumentCommand \printalldiagrams { }
{
\mychess_print_all_diagrams:
}
\ExplSyntaxOff
\begin{document}
% Load the data as provided in the question
\readbigstructure{%
basic/{3k4/8/2Q5/4K3/8/8/8/8 w - - 0 1,
8/8/2k5/7R/6R1/8/5K2/8 w - - 0 1,
1k6/7R/3K4/8/8/8/8/8 w - - 0 1},
basic/{3k4/8/2Q5/4K3/8/8/8/8 w - - 0 1,
8/8/2k5/7R/6R1/8/5K2/8 w - - 0 1,
1k6/7R/3K4/8/8/8/8/8 w - - 0 1},
%******* PAWN LEVEL *****************
pawn/{8/8/8/8/8/1k6/7P/4K3 w - - 0 1,
8/8/8/8/4k3/4p3/4K3/8 w - - 0 1,
8/1k6/8/8/1PK5/8/8/8 w - - 0 1},
pawn/{8/8/8/8/8/1k6/7P/4K3 w - - 0 1,
8/8/8/8/4k3/4p3/4K3/8 w - - 0 1,
8/1k6/8/8/1PK5/8/8/8 w - - 0 1},
%************ 2 PAWN LEVEL *******************
2 pawns/{8/8/8/4p1K1/2k1P3/8/8/8 w - - 0 1,
8/8/8/p7/P7/7k/7P/6K1 w - - 0 1,
k7/P1K5/6p1/7p/7P/8/8/8 w - - 0 1},
2 pawns/{8/8/8/4p1K1/2k1P3/8/8/8 w - - 0 1,
8/8/8/p7/P7/7k/7P/6K1 w - - 0 1,
k7/P1K5/6p1/7p/7P/8/8/8 w - - 0 1},
%************ MINOR + PAWN LEVEL ********************
Minor + Pawns/{4BK2/6P1/5k2/8/8/1b6/8/8 w - - 0 1,
8/8/4k3/2p4n/2P1K3/8/7B/8 w - - 0 1,
8/p7/P5p1/1P4P1/5P2/3kb3/8/3K4 w - - 0 1},
Minor + Pawns/{4BK2/6P1/5k2/8/8/1b6/8/8 w - - 0 1,
8/8/4k3/2p4n/2P1K3/8/7B/8 w - - 0 1,
8/p7/P5p1/1P4P1/5P2/3kb3/8/3K4 w - - 0 1}}
% Print it
\printalldiagrams
\newpage
% Alternate input syntax written for the duplicate question:
% <https://tex.stackexchange.com/questions/497019/foreach-looping-error>
\definegroups{A, B, C, D}
\setgroupdiagrams{A}{
{3k4/8/2Q5/4K3/8/8/8/8 w - - 0 1,
8/8/2k5/7R/6R1/8/5K2/8 w - - 0 1,
1k6/7R/3K4/8/8/8/8/8 w - - 0 1},
{3k4/8/2Q5/4K3/8/8/8/8 w - - 0 1,
8/8/2k5/7R/6R1/8/5K2/8 w - - 0 1,
1k6/7R/3K4/8/8/8/8/8 w - - 0 1},
}
\setgroupdiagrams{B}{
{8/8/8/8/8/1k6/7P/4K3 w - - 0 1,
8/8/8/8/4k3/4p3/4K3/8 w - - 0 1,
8/1k6/8/8/1PK5/8/8/8 w - - 0 1},
{8/8/8/8/8/1k6/7P/4K3 w - - 0 1,
8/8/8/8/4k3/4p3/4K3/8 w - - 0 1,
8/1k6/8/8/1PK5/8/8/8 w - - 0 1},
}
\setgroupdiagrams{C}{
{8/8/8/4p1K1/2k1P3/8/8/8 w - - 0 1,
8/8/8/p7/P7/7k/7P/6K1 w - - 0 1,
k7/P1K5/6p1/7p/7P/8/8/8 w - - 0 1},
{8/8/8/4p1K1/2k1P3/8/8/8 w - - 0 1,
8/8/8/p7/P7/7k/7P/6K1 w - - 0 1,
k7/P1K5/6p1/7p/7P/8/8/8 w - - 0 1},
}
\setgroupdiagrams{D}{
{4BK2/6P1/5k2/8/8/1b6/8/8 w - - 0 1,
8/8/4k3/2p4n/2P1K3/8/7B/8 w - - 0 1,
8/p7/P5p1/1P4P1/5P2/3kb3/8/3K4 w - - 0 1},
{4BK2/6P1/5k2/8/8/1b6/8/8 w - - 0 1,
8/8/4k3/2p4n/2P1K3/8/7B/8 w - - 0 1,
8/p7/P5p1/1P4P1/5P2/3kb3/8/3K4 w - - 0 1},
}
\printalldiagrams
\end{document}
Page 1
(page 2 has the same diagrams as page 1 because the input data is the same)
Page 3
(page 4 has the same diagrams as page 3 because the input data is the same)
Notes:
The alternate input syntax used the second time (see end of the document) declares group names A, B, C and D as in this duplicate question.
You can use \readbigstructure, \definegroups and \setgroupdiagrams before or after \begin{document}, it doesn't matter (these commands don't produce any output, contrary to \printalldiagrams).
If you want to learn expl3, the best starting point I know is expl3.pdf found here, then l3styleguide.pdf1 and finally the main reference, interface3.pdf. In any case, reading the TeXbook before will be of great help, because many essential TeX concepts2 are underlying everywhere in the LaTeX3 language. For instance, once you understand \edef, you'll know pretty well what x argument specifiers do and why they can't be used in such and such situations.
Footnotes:
Very short; you may skip the Documentation style section at this point.
In particular, character and category codes, tokens, how characters from the input are turned into tokens and how the resulting stream of tokens is processed (part of which is the concept of expansion), \edef, \csname and \string.
pdflatex. Any chance you mean by error "undesired output"? If so, what would you like to have changed? – Jun 22 '19 at 20:31basic,basic,pawns,pawns, ...? If so, this is the order that you have in the LaTeX code, so of course this is what you get. Please clearly explain what is wrong with the current output and what you want it to look like. I am sure that it is clear to you what you want but from what you have written, and perhaps I am being thick, it is not clear to me. – Jun 23 '19 at 00:41