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\section{I/O}
\begin{frame}[fragile]
\frametitle{Printing}
\begin{lstlisting}
In[]: a = "This is a string"
In[]: a
In[]: print a
\end{lstlisting}
\begin{itemize}
\item Both \texttt{a}, and \texttt{print a} are showing the value
\item What is the difference?
\item Typing \texttt{a} shows the value; \texttt{print a} prints it
\item Typing \texttt{a} shows the value only in interpreter
\item In a script, it has no effect.
\end{itemize}
\begin{lstlisting}
In[]: b = "A line \n New line"
In[]: b
In[]: print b
\end{lstlisting}
\end{frame}
\begin{frame}[fragile]
\frametitle{String formatting}
\small
\begin{lstlisting}
In[]: x = 1.5
In[]: y = 2
In[]: z = "zed"
In[]: print "x is %2.1f y is %d z is %s" %(x, y, z)
\end{lstlisting}
\end{frame}
\begin{frame}[fragile]
\frametitle{\texttt{print x} \& \texttt{print x,}}
\begin{itemize}
\item Open an editor
\item Type the following code
\item Save as \texttt{print\_example.py}
\end{itemize}
\begin{lstlisting}
In[]: print "Hello"
In[]: print "World"
In[]: print "Hello",
In[]: print "World"
\end{lstlisting}
\begin{itemize}
\item Run the script using \texttt{\% run print\_example.py}
\item \texttt{print x} adds a newline whereas \texttt{print x,} adds
a space
\end{itemize}
\end{frame}
\begin{frame}[fragile]
\frametitle{\texttt{raw\_input}}
\begin{lstlisting}
In[]: ip = raw_input()
\end{lstlisting}
\begin{itemize}
\item The cursor is blinking; waiting for input
\item Type \texttt{an input} and hit <ENTER>
\end{itemize}
\begin{lstlisting}
In[]: print ip
\end{lstlisting}
\end{frame}
\begin{frame}[fragile]
\frametitle{\texttt{raw\_input} \ldots}
\begin{lstlisting}
In[]: c = raw_input()
5.6
In[]: c
In[]: type(c)
\end{lstlisting}
\begin{itemize}
\item \alert{\texttt{raw\_input} always takes a string}
\end{itemize}
\begin{lstlisting}
In[]: name = raw_input("Enter your name: ")
Enter your name: George
\end{lstlisting}
\begin{itemize}
\item \texttt{raw\_input} can display a prompt string for the user
\end{itemize}
\end{frame}
\section{Files}
\begin{frame}[fragile]
\frametitle{Opening files}
\begin{lstlisting}
pwd # present working directory
cd /home/fossee # go to location of the file
\end{lstlisting}
{\tiny 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}
\begin{frame}[fragile]
\frametitle{File parsing -- Problem}
\begin{lstlisting}
A;010002;ANAND R;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 (if no argument)
\item Returns a list of strings
\item It can be given an argument to split on that argrument
\end{itemize}
\begin{lstlisting}
In[]: record = "A;015163;JOSEPH RAJ S;083;042;47;AA;72;244;;;"
In[]: record.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]
\frametitle{File parsing -- Solution}
\begin{lstlisting}
In[]: math_B = [] # empty list to store marks
In[]: for line in open("sslc1.txt"):
....: fields = line.split(";")
....: reg_code = fields[0]
....: reg_code_clean = reg_code.strip()
....: math_mark_str = fields[5]
....: math_mark = float(math_mark_str)
....: if reg_code == "B":
....: math_B.append(math_mark)
In[]: math_B_mean = sum(math_B) / len(math_B)
In[]: math_B_mean
\end{lstlisting}
\end{frame}
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