UK TeX FAQ -- question 224

### Defining a macro from an argument

It's common to want a command to create another command: often one wants the new command's name to derive from an argument. LaTeX does this all the time: for example, `\newenvironment` creates start- and end-environment commands whose names are derived from the name of the environment command.

The (seemingly) obvious approach:

    \def\relay#1#2{\def\#1{#2}}

doesn't work (the TeX engine interprets it as a rather strange redefinition of `#`). The trick is to use `\csname`, which is a TeX primitive for generating command names from random text, together with `\expandafter`. The definition above should read:

    \def\relay#1#2{%
      \expandafter\def\csname #1\endcsname{#2}%
    }

With this definition, `\relay{blah}{bleah}` is equivalent to `\def``\blah{bleah}`.

Note that the definition of `\relay` omits the braces round the 'command name' in the `\newcommand` it executes. This is because they're not necessary (in fact they seldom are), and in this circumstance they make the macro code slightly more tedious.

The name created need not (of course) be just the argument:

    \def\newrace#1#2#3{%
      \expandafter\def\csname start#1\endcsname{%
        #2%
      }%
      \expandafter\def\csname finish#1\endcsname{%
        #3%
      }%
    }

With commands

    \def\start#1{\csname start#1\endcsname}
    \def\finish#1{\csname finish#1\endcsname}

these 'races' could behave a bit like LaTeX environments.