Define 2 variables and print them

Question

I'm just getting started with LaTeX, and I'm already struggling with the very basic stuff.

I create 2 commands, each assigning a value to a variable:

\newcommand{\firstname}[1]{\def\@firstname{#1}}
\newcommand{\lastname}[1]{\def\@lastname{#1}}

These variables are set using the commands somewhere at the beginning of the document:

\firstname{Kevin}
\lastname{De Coninck}

When I try to print the lastname, everything works fine, when I try to print the firstname, the following error is returned Use of \@ doesn't match its definition. \@f

I'm priting inside my document using the following code:

\begin{document}
    \@firstname
\end{document}

What am I missing here?

Update: Entire document

% ----------------------------------------------------------------------
% -- CONFIGURATION
% ----------------------------------------------------------------------
\documentclass[11pt, a4paper]{article}

% ----------------------------------------------------------------------
% -- PACKAGES
% ----------------------------------------------------------------------
\RequirePackage{geometry}

% ----------------------------------------------------------------------
% -- COMMAND DEFINITIONS
% ----------------------------------------------------------------------

% Summary: Configure the person's name.
% Usage:   \name{<firstname>}{<lastname>}
%          firstname{<firstname>}
%          lastname{<lastname>}
\newcommand{\firstname}[1]{\def\@firstname{#1}}
\newcommand{\lastname}[1]{\def\@lastname{#1}}

% ----------------------------------------------------------------------
% -- LAYOUT CONFIGURATION
% ----------------------------------------------------------------------
\geometry{left=2.0cm, top=5.5cm, right=2.0cm, bottom=2.0cm, footskip=.5cm}

% ----------------------------------------------------------------------
% -- PERSONAL DATA
% ----------------------------------------------------------------------
\firstname{Kevin}
\lastname{De Coninck}

% ----------------------------------------------------------------------
% -- ACTUAL DOCUMENT
% ----------------------------------------------------------------------
\begin{document}
    \@firstname
    \@lastname
\end{document}

\makeatletter\@firstname\makeatother, but you should define a wrapper command as well — , Feb 01 '17 at 14:29
@ is a special character, in the sense that, in packages, it is considered a "letter", whereas in a document, it is considered an "other", and precluded from use in variable names. As Christian indicates, you can make @ function as a letter in documents with the invocation of \makeatletter. This then allows its use in variable names such as \@firstname, etc. — Steven B. Segletes, Feb 01 '17 at 14:30
\newcommand{\printnames}{\@lastname, \@firstname} in a stylefile or between \makeatletter ... \makeatother as Christian pointed out and command \printnames in your document. — Jan, Feb 01 '17 at 14:32
I don't get it. Using the code Christian supplies renders irstname. — Complexity, Feb 01 '17 at 14:32
And it doesn't explain my why I can print the lastname property, but not the firstname. — Complexity, Feb 01 '17 at 14:34
@Complexity: You're not showing the real code, but you should \makeatletter...\makeatother around your \newcommand usages as well — , Feb 01 '17 at 14:38
Instead of writing @ in the holder macro, why not just write CMPLX, then the macros should the prefixed enough and \CMPLXfirstname works out of the box. Macros with @ in their name is genrally for intenal use and for experienced users. — daleif, Feb 01 '17 at 14:38
A suggestion for \printname: \newcommand{\printname}{% \@ifundefined{@firstname}{}{\@firstname} \@ifundefined{@lastname}{}{\@lastname}% } — , Feb 01 '17 at 14:41
@ChristianHupfer I'm showing all the code that I have right now :-) You should now that I'm just getting started. — Complexity, Feb 01 '17 at 14:41
@Complexity: Yes, but it's no document. That fragments are not really useful. Doing it my way, I get no error. — , Feb 01 '17 at 14:42
@ChristianHupfer I've updated the post with the entire document. It's small, and I just want to print my 2 variables, but it isn't working atm. — Complexity, Feb 01 '17 at 14:44
@Complexity: I've used your document right now and adding \makeatletter...\makeatother around your \@... usages works, as well for the \newcommand definitions of \firstname etc. The output is KevinDe Coninck (you're missing a \ after \@firstname this way! — , Feb 01 '17 at 14:46
http://tex.stackexchange.com/questions/8351/what-do-makeatletter-and-makeatother-do — Joseph Wright, Feb 01 '17 at 14:49
@ChristianHupfer Can you make that an answer on the question so that I can accept it? — Complexity, Feb 01 '17 at 14:50
If you say \def\@lastname{#1}, you define the macro \@ in a very specific way. It is comparable to using \def\foo#1bar#2{#1: #2}, here the bar is a type of separator for the two arguments and is gobbled when TeX processes the code. In your case, lastname is seen as a delimiter for the (not existing) argument. Thus, it is gobbled and \@ is succesfully expande. If you use \@firstname instead, \@ expects lastname to follow and complains about incorrect usage. The call of \lastname overrides the definition from \firstname, thus only one works. — Patrick Happel, Feb 01 '17 at 14:57

score 3 · Accepted Answer · 2017-02-01T14:58:55.523

I've added the usual \makeatletter...\makeatother pair around the definitions and usage.

Please keep daleif's proposition in mind too: \CMPLXfirstname seems to be better and easier.

I've also changed \RequirePackage into \usepackage (although the first one is allowed too, but rather meant for packages only, not for normal documents)

Now, @ is special character that is reserved as part of names of internal commands which should not be accessible easily for a normal document user. Outside of packages or class files @ is not considered to be an allowed letter for command names, so \makeatletter changes this (temporarily), permitting @ to be part of such names and \makeatother reverts this.

% ----------------------------------------------------------------------
% -- CONFIGURATION
% ----------------------------------------------------------------------
\documentclass[11pt, a4paper]{article}

% ----------------------------------------------------------------------
% -- PACKAGES
% ----------------------------------------------------------------------
\usepackage{geometry}

% ----------------------------------------------------------------------
% -- COMMAND DEFINITIONS
% ----------------------------------------------------------------------

% Summary: Configure the person's name.
% Usage:   \name{<firstname>}{<lastname>}
%          firstname{<firstname>}
%          lastname{<lastname>}
\makeatletter
\newcommand{\firstname}[1]{\def\@firstname{#1}}
\newcommand{\lastname}[1]{\def\@lastname{#1}}

 \newcommand{\printname}{%
  \@ifundefined{@firstname}{}{\@firstname} \@ifundefined{@lastname}{}{\@lastname}%
}

\makeatother

% ----------------------------------------------------------------------
% -- LAYOUT CONFIGURATION
% ----------------------------------------------------------------------
\geometry{left=2.0cm, top=5.5cm, right=2.0cm, bottom=2.0cm, footskip=.5cm}

% ----------------------------------------------------------------------
% -- PERSONAL DATA
% ----------------------------------------------------------------------
\firstname{Kevin}
\lastname{De Coninck}

% ----------------------------------------------------------------------
% -- ACTUAL DOCUMENT
% ----------------------------------------------------------------------
\begin{document}
\makeatletter
    \@firstname\ \@lastname
\makeatother

Or \printname%
\end{document}

Do \firstname and \lastname really need to process arguments if all they do with the arguments is fetching them and putting them at the end of their replacement text? — Ulrich Diez, Feb 01 '17 at 23:37

Ulrich Diez · Answer 2 · 2017-02-01T23:53:04.693

In case you don't wish to use \makeatletter..\makeatother, you can use
\csname..\endcsname:

\csname @firstname\endcsname → \@firstname.
\csname @lastname\endcsname → \@lastname.

I have written a small wrapper for \csname..\endcsname:

The macro \CreateCsFromName works as follows:

\CreateCsFromName<preceding tokens with no curly braces>{ControlSequence}
→ <preceding tokens with no curly braces>\ControlSequence.

If you omit <preceding tokens with no curly braces>, i.e., if you leave <preceding tokens with no curly braces> empty, then \CreateCsFromName will just construct the control sequence token from the name of the control sequence:

\CreateCsFromName{macro} → \macro

Instead of leaving <preceding tokens with no curly braces> empty, you can supply whatsoever commands for (re)defining:

\CreateCsFromName\long\def{macro} → \long\def\macro

\CreateCsFromName\long\def{macro}#1#2{First argument: #1, second argument: #2}
→ \long\def\macro#1#2{First argument: #1, second argument: #2}

\CreateCsFromName\newcommand*{macro} → \newcommand*\macro

\CreateCsFromName\newcommand*{macro}[2]{First argument: #1, second argument: #2}
→ \newcommand*\macro[2]{First argument: #1, second argument: #2}

Example:

\documentclass{article}

\newcommand\Exchange[2]{#2#1}
\newcommand\CreateCsFromName{}
\long\def\CreateCsFromName#1#{\romannumeral0\InnerCreateCsFromName{#1}}
\newcommand\InnerCreateCsFromName[2]{%
  \expandafter\Exchange\expandafter{\csname#2\endcsname}{ #1}%
}

% raise an error in case \@firstname and \@lastname are already defined:
\CreateCsFromName\newcommand*{@firstname}{}
\CreateCsFromName\newcommand*{@lastname}{}

% commands for silently redefining \@firstname and \@lastname:
\newcommand{\firstname}{\CreateCsFromName\def{@firstname}}
\newcommand{\lastname}{\CreateCsFromName\def{@lastname}}

\firstname{Kevin}
\lastname{De Coninck}

\begin{document}

\CreateCsFromName{@firstname} \CreateCsFromName{@lastname}

\end{document}

Define 2 variables and print them

2 Answers2