Latex - 关于newcommand和Tabular的问题

Question

以下代码产生错误：Incomplete \if; all text was ignored after line.

我尝试在每个\\的末尾添加if (...\eval{#1+#2}}}} \\%)。但这只在我的桌子上增加了一条线。

\begin{table}[H]
    \tiny
    \centering
    \def\arraystretch{2}
    \setlength{\tabcolsep}{5.2pt}
    \caption{xxxxx}
    \ExplSyntaxOn
    \DeclareExpandableDocumentCommand{\eval}{m}{\int_eval:n {#1}}
    \ExplSyntaxOff
    \newcommand*{\drawtable}[5]{%
        \if a#5%
            \multicolumn{1}{c}{\multirow{1}{*}{#1}} & \multicolumn{1}{c}{\multirow{1}{*}{#2}} & \multicolumn{1}{c}{\multirow{1}{*}{\textbf{\eval{#1+#2}}}}%
        \else\if b#5%
            \multicolumn{1}{c}{\multirow{1}{*}{#3}} & \multicolumn{1}{c}{\multirow{1}{*}{#4}} & \multicolumn{1}{c}{\multirow{1}{*}{\textbf{\eval{#3+#4}}}}%
        \fi\fi%
    }%  
    \begin{tabular}{ccccccccccccccccccccccccc}
        \multicolumn{1}{c}{\multirow{1}{*}{}} &
        \multicolumn{1}{c}{\multirow{1}{*}{}} &
        \multicolumn{3}{c}{\multirow{1}{*}{\textbf{xxxxx}}} & \\ \hline

        \multicolumn{1}{c}{\multirow{1}{*}{}} &
        \multicolumn{1}{c}{\multirow{1}{*}{}} &
        \multicolumn{2}{c}{\multirow{1}{*}{\textbf{yyy}}} & \\ \cline{3-4}

        \multicolumn{1}{c}{\multirow{1}{*}{}} &
        \multicolumn{1}{c}{\multirow{1}{*}{}} &
        \multicolumn{1}{c}{\multirow{1}{*}{\textbf{aa}}} & 
        \multicolumn{1}{c}{\multirow{1}{*}{\textbf{bb}}} & 
        \multicolumn{1}{c}{\multirow{1}{*}{\textbf{ccc}}} & \\ \hline

        \multicolumn{1}{c}{\multirow{2}{*}{\textbf{rr}}} &
        \multicolumn{1}{c}{\multirow{1}{*}{\textbf{N.}}} &
        \drawtable{18}{8}{4}{7}{a} 
         & \\

        \multicolumn{1}{c}{\multirow{1}{*}{}} &
        \multicolumn{1}{c}{\multirow{1}{*}{\textbf{N.}}} &
        \drawtable{18}{8}{4}{7}{b} 
        & \\ \hline
    \end{tabular}
\end{table}