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+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%Tutorial slides on Python.
+%
+% Author: FOSSEE
+% Copyright (c) 2017, FOSSEE, IIT Bombay
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+\documentclass[14pt,compress]{beamer}
+\input{macros.tex}
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+% Title page
+\title[Functions]{Python language: reading/writing files}
+
+\author[FOSSEE Team] {The FOSSEE Group}
+
+\institute[IIT Bombay] {Department of Aerospace Engineering\\IIT Bombay}
+\date[] {Mumbai, India}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+% DOCUMENT STARTS
+\begin{document}
+
+\begin{frame}
+  \titlepage
+\end{frame}
+
+\section{Reading files}
+
+\begin{frame}[fragile]
+  \frametitle{Opening files}
+  Recall:
+  \begin{lstlisting}
+% pwd # present working directory
+% cd /home/fossee # go to directory
+  \end{lstlisting}
+  {\small The file is in our present working directory}
+  \begin{lstlisting}
+   In []: f = open('pendulum.txt')
+   In []: f
+  \end{lstlisting}
+  \begin{itemize}
+  \item \texttt{f} is a file object
+  \item Shows the mode in which the file is open (read mode)
+  \end{itemize}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Reading the whole file}
+  \begin{lstlisting}
+   In []: pend = f.read()
+   In []: print(pend)
+  \end{lstlisting}
+  \begin{itemize}
+  \item We have read the whole file into the variable \texttt{pend}
+  \end{itemize}
+  \begin{lstlisting}
+   In []: type(pend)
+   In []: pend_list = pend.splitlines()
+   In []: pend_list
+  \end{lstlisting}
+  \begin{itemize}
+  \item  \texttt{pend} is a string variable
+  \item We can split it at the newline characters into a list of
+    strings
+  \item Close the file, when done; Also, if you want to read again
+  \end{itemize}
+  \begin{lstlisting}
+   In []: f.close()
+   In []: f
+ \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Reading line-by-line}
+  \begin{lstlisting}
+In []: for line in open('pendulum.txt'):
+.....:     print(line)
+  \end{lstlisting}
+  \begin{itemize}
+  \item The file object is an ``iterable''
+  \item We iterate over it and print each line
+  \item Instead of printing, collect lines in a list
+  \end{itemize}
+  \begin{lstlisting}
+In []: line_list = [ ]
+In []: for line in open('pendulum.txt'):
+.....:     line_list.append(line)
+  \end{lstlisting}
+\end{frame}
+
+\section{Writing files}
+
+\begin{frame}[fragile]
+  \frametitle{Writing files}
+  \begin{lstlisting}
+In []: f = open('new_file.txt', 'w')
+In []: f.write('Hello world!\n')
+In []: f.close()
+\end{lstlisting}
+\begin{itemize}
+\item Note the mode \typ{'w'}
+\item Will clobber existing file!
+\item \typ{write} will not add new lines
+\item Always remember to call \typ{close}
+\end{itemize}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Using print to write files}
+  \begin{small}
+    On Python 2.x
+  \begin{lstlisting}
+In []: from __future__ import print_function
+\end{lstlisting}
+\hrule
+\end{small}
+
+  \begin{lstlisting}
+In []: f = open('new_file.txt', 'w')
+In []: print('Hello world!', file=f)
+In []: f.close()
+\end{lstlisting}
+\begin{itemize}
+\item Just pass the \typ{file} keyword arg
+\item \typ{print} works normally, so adds new lines
+\end{itemize}
+\end{frame}
+
+
+\section{Exercise: parsing data from file}
+
+\begin{frame}[fragile,plain]
+  \frametitle{File parsing -- Problem}
+  \begin{lstlisting}
+A;010002;AMY A;058;037;42;35;40;212;P;;
+  \end{lstlisting}
+  \begin{itemize}
+  \item File with records like the one above is given
+  \item Each record has fields separated by \verb+;+
+  \item region code; roll number; name;
+  \item marks --- $1^{st}$ L; $2^{nd}$ L; math; science; social; total
+  \item pass/fail indicated by P/F; W if withheld and else empty
+    \end{itemize}
+
+  \begin{itemize}
+  \item We wish to calculate mean of math marks in region B
+  \end{itemize}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Tokenization}
+  \begin{lstlisting}
+In []: line = "parse this        string"
+In []: line.split()
+  \end{lstlisting}
+  \begin{itemize}
+  \item Original string is split on white-space
+  \item Returns a list of strings
+  \item It can be given an argument to split on that argrument
+  \end{itemize}
+  \begin{lstlisting}
+r = "A;01;JOSE R;083;042;47;AA;72;244;;;"
+r.split(';')
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Tokenization \ldots}
+  \begin{itemize}
+  \item Since we split on commas, fields may have extra spaces at ends
+  \item We can strip out the spaces at the ends
+  \end{itemize}
+  \begin{lstlisting}
+   In []: word = "     B    "
+   In []: word.strip()
+  \end{lstlisting}
+  \begin{itemize}
+  \item \texttt{strip} is returning a new string
+  \end{itemize}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{\texttt{str} to \texttt{float}}
+  \begin{itemize}
+  \item After tokenizing, the marks we have are strings
+  \item We need numbers to perform math operations
+  \end{itemize}
+  \begin{lstlisting}
+  In []: mark_str = "1.25"
+  In []: mark = int(mark_str)
+  In []: type(mark_str)
+  In []: type(mark)
+  \end{lstlisting}
+  \begin{itemize}
+  \item \texttt{strip} is returning a new string
+  \end{itemize}
+\end{frame}
+
+\begin{frame}[fragile, plain]
+  \frametitle{File parsing -- Solution}
+  \begin{lstlisting}
+math_B = [] # empty list to store marks
+for line in open("sslc1.txt"):
+    fields = line.split(";")
+    reg_code = fields[0]
+    reg_code = reg_code.strip()
+
+    math_mark_str = fields[5]
+    math_mark = float(math_mark_str)
+
+    if reg_code == "B":
+        math_B.append(math_mark)
+
+math_B_mean = sum(math_B) / len(math_B)
+print(math_B_mean)
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile, plain]
+  \frametitle{File parsing -- Solution}
+  An Error!
+  \small
+  \begin{lstlisting}
+ValueError: could not convert string to float: AA
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile, plain]
+  \frametitle{File parsing -- debugging}
+  \begin{lstlisting}
+math_B = [] # empty list to store marks
+for line in open("sslc1.txt"):
+    fields = line.split(";")
+    reg_code = fields[0]
+    reg_code = reg_code.strip()
+    print(fields) # <-- Added
+    math_mark_str = fields[5]
+    math_mark = float(math_mark_str)
+    # ...
+  \end{lstlisting}
+\end{frame}
+
+
+\begin{frame}[fragile, plain]
+  \frametitle{File parsing -- debugging}
+  \begin{lstlisting}
+math_B = [] # empty list to store marks
+for i, line in enumerate(open("sslc1.txt")):
+    fields = line.split(";")
+    reg_code = fields[0]
+    reg_code = reg_code.strip()
+    print(i, fields) # <-- Added
+    math_mark_str = fields[5]
+    math_mark = float(math_mark_str)
+    # ...
+  \end{lstlisting}
+\end{frame}
+
+
+\begin{frame}[fragile, plain]
+  \frametitle{File parsing -- Solution}
+  \begin{lstlisting}
+math_B = [] # empty list to store marks
+for line in open("sslc1.txt"):
+    fields = line.split(";")
+    reg_code = fields[0].strip()
+    m = fields[5]
+    mark = float(m) if m != 'AA' else 0
+    if reg_code == "B":
+        math_B.append(mark)
+
+math_B_mean = sum(math_B) / len(math_B)
+print(math_B_mean)
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}
+  \frametitle{Summary}
+  \begin{itemize}
+  \item Reading files
+  \item Writing files
+  \item Simple file parsing
+  \end{itemize}
+\end{frame}
+
+\end{document}