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\author{Piyush P Kurur\\
  Office no: 224\\
  Dept. of Comp. Sci. and Engg.\\
  IIT Kanpur}


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\usepackage{multicol}

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\title{Fundamentals of Computing: Lecture 17}
\date{September 7, 2009}
\begin{document}
\begin{frame}
  \maketitle
\end{frame}

\begin{frame}
  \frametitle{Summary of the previous lecture}

  \begin{itemize}
    \item Arrays are passed to functions as pointers.
    \item A function that expects an array of type |T| should
      have as argument type |T *|. 
    \item There is no way a function can know the lenght of the array
      passed unless one of the argument is itself the length.  This is
      unlike arrays in Java.
  \end{itemize} 
    
\end{frame}

\begin{frame}
  \frametitle{Pointers to arrays and array of pointers}
  \begin{spec}
    int a[2][3], *b[3], (*c)[3];
  \end{spec}

  \pause
  \begin{itemize}
    \item |a| is a 2 dimensional array integer.
    \item |b| is an array 3 of pointers to integers.
    \item |c| is a pointer to an array of 3 integers.
  \end{itemize}

\end{frame}

\begin{frame}
  \frametitle{Two dimensional arrays}
  \begin{itemize}
  \item A two dimensional array is declared as |T a[2][3]|.
  \uncover<2->{
  \item The array is row major, i.e. the second index moves faster.
  }
  \uncover<4->{
  \item The expression |a| is of type pointer to array of 3 ints and
    therefore |a| points to |a[0][0]| and |a+1| points to a[1][0].
  }
  \end{itemize}

 
  \begin{tikzpicture}
    \begin{scope}
      [every node/.style={box,draw=black}, node distance=0pt];
      \node (a00) {$a_{0,0}$};
      \node [right=of a00] (a01) {$a_{0,1}$};
      \node [right=of a01] (a02) {$a_{0,2}$};
      \node [below=of a00] (a10) {$a_{1,0}$};  
      \node[right=of a10] (a11) {$a_{1,1}$};
      \node[right=of a11] (a12) {$a_{1,2}$};
    \end{scope}
 
    \node [above] (decl) at (a01.north) {$\texttt{int a[2][3]}$};
  
    \uncover<3->{
      \node [right=of a02, draw = black,box] (b00){$a_{0,0}$};
      \begin{scope}
        [every node/.style={box,draw=black}, node distance=0pt];
        \node [right=of b00] (b01) {$a_{0,1}$};
        \node [right=of b01] (b02) {$a_{0,2}$};
        \node [right=of b02] (b10) {$a_{1,0}$};  
        \node[right=of b10] (b11) {$a_{1,1}$};
        \node[right=of b11] (b12) {$a_{1,2}$};
      \end{scope}
      \node [above] (actual) at (b02.north east) {Actual storage in memory};
    }

    \uncover<5->{
      \node [below=of b00] (a) {$\texttt{a}$}; \draw [->,thick, out=180, in = 270] (a) to (b00);
      \node [right=of a] (ap1) {$\texttt{a + 1}$}; \draw [->,thick, out=0, in = 270] (ap1) to (b10);
    }
  \end{tikzpicture}

\end{frame}

  \begin{frame}
    \frametitle{Two dimensional array and pointer to array}
    \begin{code}
      int a[2][3] = { 
                      00,01,02,
                      10,11,12
                    } 
      int (*ptr)[3], qtr[][3];
      ptr = a;
      qtr = a + 1;
    \end{code}

 \begin{tikzpicture}
   
   \begin{scope}
     [every node/.style={box,draw=black}, node distance=0pt];
     \node (a00) {$0$};
     \node [right=of a00] (a01) {$1$};
     \node [right=of a01] (a02) {$2$};
     \node [below=of a00] (a10) {$10$};  
     \node[right=of a10] (a11) {$11$};
     \node[right=of a11] (a12) {$12$};
   \end{scope}
    
   \node [right=of a02, draw = black,box] (b00){$0$};
   
   \begin{scope}
     [every node/.style={box,draw=black}, node distance=0pt];
     \node [right=of b00] (b01) {$1$};
     \node [right=of b01] (b02) {$2$};
     \node [right=of b02] (b10) {$10$};  
     \node[right=of b10] (b11) {$11$};
     \node[right=of b11] (b12) {$12$};
   \end{scope}

    
   % \node [above] (decl) at (a01.north) {$\texttt{int a[2][3]}$};
   % \node [above] (actual) at (b02.north east) {Actual storage in memory};
    
   \node [below=of b00, box, label=left:$\mathtt{ptr}$] (ptr) {$\cdot$}; 
   \draw [->,thick, out=180, in = 270] (ptr.mid) to (b00);
   \node [below=of b11, box, label=right:$\mathtt{qtr}$] (qtr) {$\cdot$}; 
   \draw [->,thick, out=0, in = 270] (qtr.mid) to (b10);
 \end{tikzpicture}


\end{frame}

\begin{frame}
  \frametitle{Passing multi-dimensional array to function}
  \begin{itemize}
  \item For a 2-d array |T a[m][n]| remember the
    value of a is pointer to an array of |n| T's.
  \item Therefore to pass such an array declare the
    argument as |T (*ptr)[n]|
  \end{itemize}
\end{frame}

\begin{frame}
  \frametitle{Example code adding matrices}
  
  \begin{code}
    
/* adding 5x5 matrix */
int addMatrix(double (*a)[5], double (*b)[5], double (*c)[5])
{
  for(int i = 0; i < 5; i++)
  {
    for(int j = 0; j < 5; j++)
    {
      c[i][j] = a[i][j] + b[i][j];
    }
  }
}
  
  \end{code}
      
\end{frame}

\begin{frame}

\end{frame}

\end{document}