How to check if a variable contains a word in plain TeX?

The following example uses package kvsetkeys to get a parser for lists with comma separated entries:

\input kvsetkeys.sty % parser for comma separated lists
\input ltxcmds.sty % helper macros

\catcode`\@=11 % \makeatletter
\newif\ifkey@found

% \testkeywordinlist{keyword}{keyword list}{true}{false}
\def\testkeywordinlist#1#2{%
  \edef\key@word{#1}%
  \edef\key@list{#2}%
  \key@foundfalse
  \expandafter\comma@parse\expandafter{\key@list}{%
    \ifx\comma@entry\key@word
      \key@foundtrue
      \comma@break
    \fi
    \ltx@gobble
  }%
  \ifkey@found
    \expandafter\ltx@firstoftwo
  \else
    \expandafter\ltx@secondoftwo
  \fi
}
\catcode`\@=12 % \makeatother

\def\keywords{computers, business, biology, fine art, literature, zoology}

\testkeywordinlist{zoology}{\keywords}{%
  \immediate\write16{* "zoology" found.}%
}{%
  \immediate\write16{* "zoology" not found.}%
}

\end

Result:

• "zoology" found.

You can use functional-style programming to build a fully-expandable version of map and string comparison. This however requires e-TeX support. Here I am using \pdfstrcmp to compare strings, so this solution is specific to pdfTeX.

\def\map#1#2{%
  \domap{#1}#2,\end}

\def\domap#1#2,#3\end{%
  #1{\strip#2\endstrip}%
  \ifx\end#3\end\else\domap{#1}#3\end\fi}

% skips leading spaces
\def\strip#1#2\endstrip{#1#2}

% predicate (pdfTeX specific)
\def\ifstrequal#1#2#3#4{\ifnum\pdfstrcmp{#1}{#4}=0 #2\else #3\fi}

% Check if the list contains something
\def\ifinlist#1#2#3#4{%
  \ifstrequal{found}{#3}{#4}{%
    \map{\ifstrequal{#1}{found}{}}{#2}%
  }%
}

% fully expand \map
\edef\found{%
  \map
    {\ifstrequal{zoology}{``zoology'' found}{}}% predicate with empty false branch
    {computers, business, biology, fine art, literature, zoology}%
}

\tt\meaning\found

% fully expand \ifinlist
\edef\found{%
  \ifinlist
    {zoology}%
    {computers, business, biology, fine art, literature, zoology}%
    {TRUE}%
    {FALSE}%
}

\tt\meaning\found

\bye