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% Tutorial slides on Python.

%

% Author: Prabhu Ramachandran <prabhu at aero.iitb.ac.in>

% Copyright (c) 2005-2009, Prabhu Ramachandran

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% Title page

\title[Basic Python]{Python:\\Functions and basic data structures}

\author[FOSSEE Team] {Asokan Pichai\\Prabhu Ramachandran}

\institute[IIT Bombay] {Department of Aerospace Engineering\\IIT Bombay}

\date[] {10, October 2009\\Day 1, Session 2}

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%\pgfdeclareimage[height=0.75cm]{iitmlogo}{iitmlogo}

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%% Delete this, if you do not want the table of contents to pop up at

%% the beginning of each subsection:

\AtBeginSection[]

{

  \begin{frame}<beamer>

    \frametitle{Outline}

    \tableofcontents[currentsection,subsections]

  \end{frame}

}

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% the following command: 

%\beamerdefaultoverlayspecification{<+->}

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% DOCUMENT STARTS

\begin{document}

\begin{frame}

  \titlepage

\end{frame}

\section{Control Flow}

\subsection{Exercises}

\begin{frame}

  \frametitle{Problem set 1}

  \begin{itemize}

    \item All the problems can be\\

      solved using \kwrd{if} and \kwrd{while} 

  \end{itemize}

\end{frame}

\begin{frame}{Problem 1.1}

  Write a program that displays all three digit numbers that are equal to the sum of the cubes of their digits. That is, print numbers $abc$ that have the property $abc = a^3 + b^3 + c^3$\\

\vspace*{0.2in}

\emphbar{These are called $Armstrong$ numbers.}

\end{frame}

\begin{frame}{Problem 1.2 - Collatz sequence}

\begin{enumerate}

  \item Start with an arbitrary (positive) integer. 

  \item If the number is even, divide by 2; if the number is odd, multiply by 3 and add 1.

  \item Repeat the procedure with the new number.

  \item It appears that for all starting values there is a cycle of 4, 2, 1 at which the procedure loops.

\end{enumerate}

    Write a program that accepts the starting value and prints out the Collatz sequence.

\end{frame}

\begin{frame}[fragile]{Problem 1.4}

  Write a program that prints the following pyramid on the screen. 

  \begin{lstlisting}

1

2  2

3  3  3

4  4  4  4

  \end{lstlisting}

The number of lines must be obtained from the user as input.\\

\pause

\emphbar{When can your code fail?}

\only<2->{\inctime{20}}

\end{frame}

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% TIME: 20 m, running 20m 

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\section{Functions}

\subsection{Defining}

\begin{frame}[fragile]

\frametitle{Functions: examples}

  \begin{lstlisting}

def signum( r ):

    """returns 0 if r is zero

    -1 if r is negative

    +1 if r is positive"""

    if r < 0:

        return -1

    elif r > 0:

        return 1

    else:

        return 0

  \end{lstlisting}

\end{frame}

\begin{frame}[fragile]

  \frametitle{Functions: examples}

  \begin{lstlisting}

def pad( n, size ): 

    """pads integer n with spaces

    into a string of length size

    """

    SPACE = ' '

    s = str( n )

    padSize = size - len( s )

    return padSize * SPACE + s

  \end{lstlisting}

\pause

\emphbar{What about \% formatting?}

\end{frame}

\begin{frame}[fragile]

  {What does this function do?}

  \begin{lstlisting}

def what( n ):

    if n < 0: n = -n

    while n > 0:

        if n % 2 == 1:

            return False

        n /= 10

    return True

  \end{lstlisting}

\end{frame}

\begin{frame}[fragile]

  {What does this function do?}

\begin{lstlisting}

def what( n ):

    i = 1    

    while i * i < n:

        i += 1

    return i * i == n, i

  \end{lstlisting}

\end{frame}

\begin{frame}[fragile]

  {What does this function do?}

  \begin{lstlisting}

def what( n, x ):

    z = 1.0

    if n < 0:

        x = 1.0 / x

        n = -n

    while n > 0:

        if n % 2 == 1:

            z *= x

        n /= 2

        x *= x

    return z

  \end{lstlisting}

\end{frame}

\subsection{Built-in functions}

\begin{frame}

  {Before writing a function}

  \begin{itemize}

      \item Variety of builtin functions are available

      \item \typ{abs, any, all, len, max, min}

      \item \typ{pow, range, sum, type}

      \item Refer here:

          \url{http://docs.python.org/library/functions.html}

  \end{itemize}

  \inctime{10} 

\end{frame}

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% TIME: 10 m, running 30m 

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\subsection{Exercises}

\begin{frame}{Problem set 2: Problem 2.1}

  Write a function to return the gcd of two numbers.

\end{frame}

\begin{frame}{Problem 2.2}

Write a program to print all primitive pythagorean triads (a, b, c) where a, b are in the range 1---100 \\

A pythagorean triad $(a,b,c)$ has the property $a^2 + b^2 = c^2$.\\By primitive we mean triads that do not `depend' on others. For example, (4,3,5) is a variant of (3,4,5) and hence is not primitive. And (10,24,26) is easily derived from (5,12,13) and is also not primitive.

\end{frame}

\begin{frame}{Problem 2.3}

  Write a program that generates a list of all four digit numbers that have all their digits even and are perfect squares.\newline\\\emph{For example, the output should include 6400 but not 8100 (one digit is odd) or 4248 (not a perfect square).}

\end{frame}

\begin{frame}{Problem 2.4}

  The aliquot of a number is defined as: the sum of the \emph{proper} divisors of the number. For example, aliquot(12) = 1 + 2 + 3 + 4 + 6 = 16.\\

  Write a function that returns the aliquot number of a given number. 

\end{frame}

\begin{frame}{Problem 2.5}

  A pair of numbers (a, b) is said to be \alert{amicable} if the aliquot number of a is b and the aliquot number of b is a.\\

  Example: \texttt{220, 284}\\

  Write a program that prints all four digit amicable pairs.

  \inctime{25}

\end{frame}

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% TIME: 25 m, running 55m 

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\section{Lists}

\subsection{Manipulating}

\begin{frame}[fragile]

  \frametitle{List creation and indexing}

\begin{lstlisting}

>>> a = [] # An empty list.

>>> a = [1, 2, 3, 4] # More useful.

>>> len(a) 

4

>>> a[0] + a[1] + a[-1]

7

\end{lstlisting}

  \begin{itemize}

  \item Indices start with ?

  \item Negative indices indicate ?

  \end{itemize}

\end{frame}

\begin{frame}[fragile]

  \frametitle{List: slices}

  \begin{itemize}

  \item Slicing is a basic operation

  \item \typ{list[initial:final:step]}

  \item  The step is optional

  \end{itemize}

\begin{lstlisting}

>>> a[1:3] # A slice.

[2, 3]

>>> a[1:-1]

[2, 3]

>>> a[1:] == a[1:-1]

False  

\end{lstlisting}

Explain last result

\end{frame}

\begin{frame}[fragile]

  \frametitle{List: more slices}

\begin{lstlisting}

>>> a[0:-1:2] # Notice the step!

[1, 3]

>>> a[::2]

[1, 3]

>>> a[-1::-1]

\end{lstlisting}

What do you think the last one will do?

  \emphbar{Strings also use same indexing and slicing.}

\end{frame}

\begin{frame}[fragile]

  \frametitle{List: examples}

\begin{lstlisting}

>>> a = [1, 2, 3, 4]

>>> a[:2]

[1, 2]

>>> a[0:-1:2]

[1, 3]

\end{lstlisting}

\pause

\alert{Lists are mutable (unlike strings)}

\begin{lstlisting}

>>> a[1] = 20

>>> a

[1, 20, 3, 4]

\end{lstlisting}

\end{frame}

\begin{frame}[fragile]

  \frametitle{Lists are mutable and heterogenous}

\begin{lstlisting}

>>> a = ['spam', 'eggs', 100, 1234]

>>> a[2] = a[2] + 23

>>> a

['spam', 'eggs', 123, 1234]

>>> a[0:2] = [1, 12] # Replace items

>>> a

[1, 12, 123, 1234]

>>> a[0:2] = [] # Remove items

>>> a.append( 12345 )

>>> a

[123, 1234, 12345]

\end{lstlisting}

\inctime{10}

\end{frame}

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% TIME: 10 m, running 65m 

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\subsection{Methods}

\begin{frame}[fragile]

  \frametitle{List methods}

\begin{lstlisting}

>>> a = ['spam', 'eggs', 1, 12]

>>> a.reverse() # in situ

>>> a

[12, 1, 'eggs', 'spam']

>>> a.append(['x', 1])

>>> a

[12, 1, 'eggs', 'spam', ['x', 1]]

>>> a.extend([1,2]) # Extend the list.

>>> a.remove( 'spam' )

>>> a

[12, 1, 'eggs', ['x', 1], 1, 2]

\end{lstlisting}

\end{frame}

\begin{frame}[fragile]

  \frametitle{List containership}

  \begin{lstlisting}

>>> a = ['cat', 'dog', 'rat', 'croc']

>>> 'dog' in a

True

>>> 'snake' in a

False

>>> 'snake' not in a

True

>>> 'ell' in 'hello world'

True

  \end{lstlisting}

  \inctime{5}

\end{frame}

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\section{Tuples}

\begin{frame}[fragile]

  \frametitle{Tuples: immutable}

\begin{lstlisting}

>>> t = (0, 1, 2)

>>> print t[0], t[1], t[2], t[-1] 

0 1 2 2

>>> t[0] = 1

Traceback (most recent call last):

  File "<stdin>", line 1, in ?

TypeError: object does not support

item assignment

\end{lstlisting}  

\begin{itemize}

    \item Multiple return values are actually a tuple.

    \item Exchange is tuple (un)packing

\end{itemize}

\inctime{5}

\end{frame}

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% TIME: 5 m, running 75m 

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\section{for and range()}

\begin{frame}[fragile]

  \frametitle{\typ{range()} function}

  \begin{lstlisting}

>>> range(7)

[0, 1, 2, 3, 4, 5, 6]

>>> range( 3, 9)

[3, 4, 5, 6, 7, 8]

>>> range( 4, 17, 3)

[4, 7, 10, 13, 16]

>>> range( 5, 1, -1)

[5, 4, 3, 2]

>>> range( 8, 12, -1)

[]

  \end{lstlisting}

\end{frame}

\begin{frame}[fragile]

  \frametitle{\typ{for\ldots range(\ldots)} idiom}

  \begin{lstlisting}

In [83]: for i in range(5):

   ....:     print i, i * i

   ....:     

   ....:     

0 0

1 1

2 4

3 9

4 16

\end{lstlisting}

\end{frame}

\begin{frame}[fragile]

  \frametitle{\typ{for}: the list companion}

  \begin{lstlisting}

In [84]: a = ['a', 'b', 'c']

In [85]: for x in a:

   ....:    print x, chr( ord(x) + 10 )

   ....:

a  k

b  l

c  m

  \end{lstlisting}

  Iterating over the list and not the index + reference\\

  what if you want the index?

\end{frame}

\begin{frame}[fragile]

  \frametitle{\typ{for}: the list companion}

  \begin{lstlisting}

In [89]: for p, ch in enumerate( a ):

   ....:     print p, ch

   ....:     

   ....:     

0 a

1 b

2 c

  \end{lstlisting}

Try: \typ{print enumerate(a)}

\inctime{10}

\end{frame}

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% TIME: 10 m, running 85m 

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\begin{frame}

  \frametitle{What did we learn?}

  \begin{itemize}

    \item Control flow in action

    \item Functions

    \item Manipulating Lists 

    \item Tuples

    \item range() function

    \item for loops

    \item for...range() idiom

  \end{itemize}

\end{frame}

\end{document}