\long\def\antefi#1#2\fi{#2\fi#1}
\def\fornum#1=#2to#3(#4){%
	\edef#1{\number\numexpr#2}\edef\fornumtemp{\noexpand\fornumi\expandafter\noexpand\csname fornum\string#1\endcsname
		{\number\numexpr#3}{\ifnum\numexpr#4<0 <\else>\fi}{\number\numexpr#4}\noexpand#1}\fornumtemp
}
\long\def\fornumi#1#2#3#4#5#6{\def#1{\unless\ifnum#5#3#2\relax\antefi{#6\edef#5{\number\numexpr#5+(#4)\relax}#1}\fi}#1}
\def\elem(#1,#2){\numexpr(#1+#2)*(#1+#2-1)/2-(\ifodd\numexpr#1+#2\relax#1\else#2\fi)\relax}
\def\zzmat#1{%
	\noindent% quit vertical mode
	\fornum\yy=1to#1(+1){%
		\fornum\xx=1to#1(+1){%
			\ifnum\numexpr\xx+\yy\relax<\numexpr#1+2\relax
				\hbox to 2em{\hfil\number\elem(\xx,\yy)}%
			\else
				\hbox to 2em{\hfil\number\numexpr#1*#1-1-\elem(#1+1-\xx,#1+1-\yy)\relax}%
			\fi
		}%
		\par\noindent% next line + quit vertical mode
	}\par
}
\zzmat{5}
\bye