Nested \newcommand for creating maps

Question

I want to create a simple map using \newcommand. My \NewMapItem command defines another command which returns item's value. \MapItem simply replaces to that command.

\documentclass{article}
\newcommand{\NewMapItem}[2]{\newcommand{\Map@#1}{#2}}
\newcommand{\MapItem}[1]{\Map@#1}
\begin{document}
    \NewMapItem{Canberk}{Qwerty}
    \MapItem{Canberk}
\end{document}

But it doesn't works. Output is:

line 5: Missing number, treated as zero. \NewMapItem{Canberk}{Qwerty}
line 5: You already have nine parameters. \NewMapItem{Canberk}{Qwerty}

Could you explain?

Answer 1

See Latex dynamic macro definition for the \NewMapItem problem and also How to know whether or not we need \expandafter? for a discussion about the matter.

\newcommand{\NewMapItem}[2]{%
  \expandafter\newcommand\csname Map@#1\endcsname{#2}}
\newcommand{\MapItem}[1]{\csname Map@#1\endcsname}

You can't form a token in that way unless you use \csname.

---

A different solution with expl3:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
% allocate a global property list
\prop_new:N \g_jnbrq_maps_prop

% the user level command for adding properties
\NewDocumentCommand{\NewMapItem}{mm}
 {
  \prop_gput:Nnn \g_jnbrq_maps_prop { #1 } { #2 }
 }
% the user level command for retrieving a property
\DeclareExpandableDocumentCommand{\MapItem}{m}
 {
  \prop_item:Nn \g_jnbrq_maps_prop { #1 }
 }
\ExplSyntaxOff

\begin{document}

\NewMapItem{Canberk}{Qwerty}

\MapItem{Canberk}

\end{document}

It's possible to check whether the property already exists:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
% allocate a global property list
\prop_new:N \g_jnbrq_maps_prop
% an error message for duplicate property
\msg_new:nnnn { jnbrq/maps } { duplicate-property }
 {
  Already~existing~property~#1
 }
 {
  Property~#1~is~already~defined,~I'm~ignoring~
  this~redefinition
 }

% the user level command for adding properties
\NewDocumentCommand{\NewMapItem}{mm}
 {
  \prop_if_in:NnTF \g_jnbrq_maps_prop { #1 }
   {
    \msg_error:nnn { jnbrq/maps } { duplicate-property } { #1 }
   }
   {
    \prop_gput:Nnn \g_jnbrq_maps_prop { #1 } { #2 }
   }
 }
% the user level command for retrieving a property
\DeclareExpandableDocumentCommand{\MapItem}{m}
 {
  \prop_item:Nn \g_jnbrq_maps_prop { #1 }
 }
\ExplSyntaxOff

\begin{document}
\NewMapItem{Canberk}{Qwerty}
\NewMapItem{Canberk}{QQQ}

\MapItem{Canberk}
\end{document}

This will raise an error

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! jnbrq/maps error: "duplicate-property"
! 
! Already existing property Canberk
! 
! See the jnbrq/maps documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  

l.37 \NewMapItem{Canberk}{QQQ}

? h
|'''''''''''''''''''''''''''''''''''''''''''''''
| Property Canberk is already defined, I'm ignoring this redefinition
|...............................................

which would be more easily understood instead of the standard error issued by \newcommand.