为什么 \foreach 循环似乎会干扰 \let 代码？？？？？请参见下面的代码。在循环之外，重复相同的代码两次可以正常工作

Question

 \documentclass[ aspectratio = 1610 ]{beamer} 
 
 \usepackage{psfrag}
 
 \usepackage{sidecap}
 
 \usepackage{tikz}
 
 \usepackage{pgfplots}
 
 \usepackage{amsmath}
 
  \begin{document}
  
 \begin{frame}
 
 \begin{tikzpicture} [scale = 0.6]
 
 \def\score{10}
 
  \draw (0,3) node [right] {Score is initialized to 10 and will be augmented  by 3 twice by repeating code};

  \pgfmathparse{\score + 3 } \let\score\pgfmathresult

 \draw  (0,2)  node [right] { New score \quad    $\score$ correct};

  \pgfmathparse{\score + 3 } \let\score\pgfmathresult

 \draw  (0,1)  node [right] {New score \quad    $\score$ correct };

 \draw [dashed](0, 0.5)--(20,.5);

 \draw(0,0) node[right] {Score re-initialized   to 10  and will be augmented  by 3 in a  foreach loop , again twice};

 \def\score{10}

  \foreach \k in { 1,2 }  {

  \pgfmathparse{\score + 3 } \let\score\pgfmathresult

  \draw  (0,   -  \k)  node [right] {    New score    $\score$    \ifthenelse{\k=1} {  Correct} {should be 16  Wrong.  Why?}};
 }
 \end{tikzpicture}
 
 \end{frame}
  
 \end{document}

Answer 1

在循环内使用\global\let而不是。循环是组有限的，因此使用 只会进行局部更改，该更改会在每次迭代结束时丢失。\let\foreach\let

\documentclass[ aspectratio = 1610 ]{beamer} 
 
 \usepackage{psfrag}
 
 \usepackage{sidecap}
 
 \usepackage{tikz}
 
 \usepackage{pgfplots}
 
 \usepackage{amsmath}
 
  \begin{document}
  
 \begin{frame}
 
 \begin{tikzpicture} [scale = 0.6]
 
 \def\score{10}
 
  \draw (0,3) node [right] {Score is initialized to 10 and will be augmented  by 3 twice by repeating code};

  \pgfmathparse{\score + 3 } \let\score\pgfmathresult

 \draw  (0,2)  node [right] { New score \quad    $\score$ correct};

  \pgfmathparse{\score + 3 } \let\score\pgfmathresult

 \draw  (0,1)  node [right] {New score \quad    $\score$ correct };

 \draw [dashed](0, 0.5)--(20,.5);

 \draw(0,0) node[right] {Score re-initialized   to 10  and will be augmented  by 3 in a  foreach loop , again twice};

 \def\score{10}

  \foreach \k in { 1,2 }  {

  \pgfmathparse{\score + 3 } \global\let\score\pgfmathresult

  \draw  (0,   -  \k)  node [right] {    New score    $\score$    \ifthenelse{\k=1} {  Correct} {should be 16  Wrong.  Why?}};
 }
 \end{tikzpicture}
 
 \end{frame}
  
 \end{document}

Answer 2

循环中的赋值\foreach仅限于当前循环周期。您可以使用索引\k。

您确实要加载吗psfrag？

\documentclass[ aspectratio = 1610 ]{beamer} 
% \usepackage{psfrag} % really?
%\usepackage{sidecap}
\usepackage{tikz}
\usepackage{pgfplots}
%\usepackage{amsmath}% already loaded by beamer
 
\begin{document}
  
\begin{frame}
 
\begin{tikzpicture} [scale = 0.6]
\def\score{10}
\draw (0,3) node [right] {Score is initialized to 10 and will be 
                          augmented  by 3 twice by repeating code};
\pgfmathsetmacro\score{\score + 3 }
\draw  (0,2)  node [right] { New score \quad    $\score$ correct};
\pgfmathsetmacro\score{\score + 3 }
\draw  (0,1)  node [right] {New score \quad    $\score$ correct };
\draw [dashed](0, 0.5)--(20,.5);
\draw(0,0) node[right] {Score re-initialized to 10 and will be
                        augmented by 3 in a foreach loop, again twice};
\def\score{10}
\foreach \k in { 1,2 } {
  \pgfmathsetmacro\score{\score + 3*\k}
  \draw (0,-\k) node [right] {New score $\score$};
}
\end{tikzpicture}
 
\end{frame}
  
\end{document}