path: root/day2/cheatsheet2.tex

\documentclass[12pt]{article}


\title{Python: Data Structures}
\author{FOSSEE}
\usepackage{listings}
\lstset{language=Python,
    basicstyle=\ttfamily,
commentstyle=\itshape\bfseries, 
showstringspaces=false
}
\newcommand{\typ}[1]{\lstinline{#1}}
\usepackage[english]{babel}
\usepackage[latin1]{inputenc}
\usepackage{times}
\usepackage[T1]{fontenc}
\usepackage{ae,aecompl}
\usepackage{mathpazo,courier,euler}
\usepackage[scaled=.95]{helvet}

\begin{document}
\date{}
\vspace{-1in}
\begin{center}
\LARGE{Python: Data Structures}\\
\large{FOSSEE}
\end{center}
\section{Basic Looping}
\subsection{\typ{while}}
  \begin{lstlisting}
In []: a, b = 0, 1
In []: while b < 10:  
  ...:     print b,
  ...:     a, b = b, a + b # Fibonacci Sequence
  ...:
\end{lstlisting}
Basic syntax of \typ{while} loop is:
\begin{lstlisting}
while condition:
    statement1
    statement2
\end{lstlisting}
All the statements are executed, till the condition statement evaluates to True.
\subsection{\typ{for} and \typ{range}}
\typ{range(start, stop, step)}\\
returns a list containing an arithmetic progression of integers.\\
Of the arguments mentioned above, both start and step are optional.\\
For example, if we skip third argument, i.e \typ{step}, then default value is 1. So:
\begin{lstlisting}
In []: range(1,10)
Out[]: [1, 2, 3, 4, 5, 6, 7, 8, 9]
\end{lstlisting}
\textbf{Note:} stop value is not included in the list.\\
Similarly if we don't pass \typ{start} argument, default is taken to be 0.
\begin{lstlisting}
In []: range(10)
Out[]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
\end{lstlisting}
In case \typ{step} is mentioned, the jump between consecutive members of the list would be equal to that.
\begin{lstlisting}
In []: range(1,10,2)
Out[]: [1, 3, 5, 7, 9]
\end{lstlisting}
%Notice the jump between two consecutive elements is 2, i.e step.\\
\typ{for} and \typ{range}\\
As mentioned previously \typ{for} in Python is used to iterate through the list members. So \typ{for} and \typ{range} can be used together to iterate through required series. For example to get square of all numbers less then 5 and greater then equal to 0, code can be written as:
\begin{lstlisting}
In []: for i in range(5):
 ....:     print i, i * i
 ....:
 ....:
0 0
1 1
2 4
3 9
4 16
\end{lstlisting}

\section{list}
\begin{lstlisting}
In []: num = [1, 2, 3, 4] # Initializing a list
In []: num
Out[]: [1, 2, 3, 4]
\end{lstlisting}
\subsection{Accessing individual elements}
\begin{lstlisting}
In []: num[1]
Out[]: 2  
\end{lstlisting}
\textbf{Note:} Index of list starts from 0.
\begin{lstlisting}
In []: num[5]  # ERROR: throws a index error
IndexError: list index out of range
In []: num[-1]
Out[]: 4
\end{lstlisting}
\textbf{Note: }\typ{-1} points the last element in a list. Similarly to access third last element of a list one can use: 
\begin{lstlisting}
In []: num[-3]
Out[]: 2  
\end{lstlisting}
\subsection{Adding lists and elements}
\begin{lstlisting}
In []: num += [9, 10, 11] # Adding two lists
In []: num
Out[]: [1, 2, 3, 4, 9, 10, 11]
\end{lstlisting}
\typ{list} provides \typ{append} function to append objects at the end. 
\begin{lstlisting}
In []: num = [1, 2, 3, 4] 
In []: num.append(-2) 
In []: num
Out[]: [1, 2, 3, 4, -2]
\end{lstlisting}
%% In []: num += [-5] 
%% In []: num
%% Out[]: [1, 2, 3, 4, -2, -5]
Working of \typ{append} is different from \typ{+} operator on list. Till here both will behave as same. But in following case:
\begin{lstlisting}
In []: num = [1, 2, 3, 4]

In []: num + [9, 10, 11]
Out[]: [1, 2, 3, 4, 9, 10, 11]

In []: num.append([9, 10, 11]) # appending a list to a list

In []: num
Out[]: [1, 2, 3, 4, [9, 10, 11]] # last element is a list
\end{lstlisting}
when one attempts to append a list(in above case [9, 10, 11]) to a list(num) it adds list as a single element. So the resulting list will have a element which itself is a list. But \typ{+} operator would simply add the elements of second list.\\
\subsection{Miscellaneous}
\begin{lstlisting}
In []: num = [1, 2, 3, 4]
In []: num.extend([5, 6, 7])  # extend list by adding elements
In []: num
Out[]: [1, 2, 3, 4, 5, 6, 7]
In []: num.reverse()  # reverse the current list
In []: num
Out[]: [7, 6, 5, 4, 3, 2, 1]
In []: num.remove(6) # removing first occurrence of 6
In []: num
Out[]: [7, 5, 4, 3, 2, 1]
In []: len(num) # returns the length of list
Out[]: 6
In []: a = [1, 5, 3, 7, -2, 4]
In []: min(a) # returns smallest item in a list.
Out[]: -2
In []: max(a) # returns largest item in a list.
Out[]: 7
\end{lstlisting}

\subsection{Slicing} 
General syntax for getting slice out of a list is \\
\typ{list[initial:final:step]}
\begin{lstlisting}
In []: a = [1, 2, 3, 4, 5]
In []: a[1:-1:2] 
Out[]: [2, 4]
\end{lstlisting}
Start slice from second element(1), till the last element(-1) with step size of 2.
\begin{lstlisting}
In []: a[::2]
Out[]: [1, 3, 5]
\end{lstlisting}
Start from beginning(since \typ{initial} is blank), till last of list(this time last element is included, as \typ{final} is blank), with step size of 2.\\
Apart from using \typ{reverse} command on list, one can also use slicing in special way to get reverse of a list.
\begin{lstlisting}
In []: a[-1:-4:-1]
Out[]: [5, 4, 3]
\end{lstlisting}
Above syntax of slice can be expressed as, ``start from last element(\typ{-1}), go till fourth last element(\typ{-4}), with step size \typ{-1}, which implies, go in reverse direction. That is, first element would be \typ{a[-1]}, second element would be \typ{a[-2]} and so on and so forth.''\\
So to get reverse of whole list one can write following slice syntax:
\begin{lstlisting}
In []: a[-1::-1]
Out[]: [5, 4, 3, 2, 1]
\end{lstlisting}
Since \typ{final} is left blank, it will traverse through whole list in reverse manner.\\
\textbf{Note:} While \typ{reverse} reverses the original list, slicing will just result in a instance list with reverse of original, which can be used and worked upon independently.
%%Should we include  list copy concept here?
\subsection{Containership}
\typ{in} keyword is used to check for containership of any element in a given list.
\begin{lstlisting}
In []: a = [2, 5, 4, 6, 9]
In []: 4 in a
Out[]: True

In []: b = 15
In []: b in a
Out[]: False
\end{lstlisting}
\section{Tuples}
Tuples are sequences just like Lists, but they are \textbf{immutable}, or items/elements cannot be changed in any way.
\begin{lstlisting}
In []: t = (1, 2, 3, 4, 5, 6, 7, 8) 
\end{lstlisting}
\textbf{Note:} For tupels we use parantheses in place of square brackets, rest is same as lists.
\begin{lstlisting}
In []: t[0] + t[3] + t[-1] # elements are accessed via indices
Out[]: 13
In []: t[4] = 7 # ERROR: tuples are immutable
\end{lstlisting}
\textbf{Note:} elements cant be changed!
\section{Dictionaries}
statistics section? 
\section{Sets}
are an unordered collection of unique elements.\\
Creation:
\begin{lstlisting}
In []: s = set([2,4,7,8,5]) # creating a basic set
In []: s
Out[]: set([2, 4, 5, 7, 8])
In []: g = set([2, 4, 5, 7, 4, 0, 5])
In []: g
Out[]: set([0, 2, 4, 5, 7]) # No repetition allowed.
\end{lstlisting}
Some other operations which can be performed on sets are:
\begin{lstlisting}
In []: f = set([1,2,3,5,8])
In []: p = set([2,3,5,7])
In []: f | p # Union of two sets
Out[]: set([1, 2, 3, 5, 7, 8])
In []: f & p # Intersection of two sets
Out[]: set([2, 3, 5])
In []: f - p # Elements in f not is p
Out[]: set([1, 8])
In []: f ^ p # (f - p) | (p - f)
Out[]: set([1, 7, 8])) 
In []: set([2,3]) < p # Test for subset
Out[]: True
\end{lstlisting}
\end{document}