Lecture 9 - DSA - Python Data Structures

Data Structures in Python
Crash Course

Dr.Haitham A. El-Ghareeb

Information Systems Department
Faculty of Computers and Information Sciences
Mansoura University
helghareeb@gmail.com

December 9, 2012

Dr.Haitham A. El-Ghareeb (CIS) Data Structures and Algorithms - 2012 December 9, 2012 1 / 61

Agenda

•  Whe*ng
Your
Appe0te

•  Using
Python
Interpreter

•  Informal
Introduc0on
to
Python

•  More
Control
Flows

•  Data
Structures

•  Modules


Python

•  Easy
to
learn,
powerful
programming

language.

•  Has
eﬃcient
high-‐level
data
structures
and
a

simple
but
eﬀec0ve
approach
to
object-‐
oriented
programming.

–  elegant
syntax

–  dynamic
typing

–  interpreted
nature


Python

•  Simpler
to
use

•  Available
on
Windows,
Mac
OS
X,
and
Unix

•  Help
you
get
the
job
done
more
quickly

•  Split
your
program
into
modules
that
can
be
reused

•  Python
is
an
interpreted
language:

–  Save
you
considerable
0me
during
program

development
because
no
compila0on
and
linking

is
necessary.

–  Interpreter
can
be
used
interac0vely


Python

•  Python
is
extensible:
if
you
know
how
to

program
in
C
it
is
easy
to
add
a
new
built-‐in

func0on
or
module
to
the
interpreter,

•  Named
aSer
the
BBC
show
“Monty
Python’s

Flying
Circus”
and
has
nothing
to
do
with

rep0les.


Using
the
Python
Interpreter

•  Python
interpreter
is
usually
installed
as
/usr/
local/bin/python

•  Invoking
the
Interpreter

–  A
second
way
of
star0ng
the
interpreter
is
python

-‐c
command
[arg]
...,
which
executes
the

statement(s)
in
command

–  Some
Python
modules
are
also
useful
as
scripts.

These
can
be
invoked
using
python
-‐m
module

[arg]
...,
which
executes
the
source
ﬁle
for
module


Using
Python
Interpreter

•  Argument
Passing

•  Interac0ve
Mode

•  Error
Handling


Executable
Python
Scripts


Source
Code
Encoding

•  It
is
possible
to
use
encodings
different
than

ASCII
in
Python
source
files.

•  The
best
way
to
do
it
is
to
put
one
more

special
comment
line
right
aSer
the
#!
line
to

define
the
source
file
encoding:


Informal
Introduc0on


Using
Python
as
a
Calculator


Numbers


Strings


String
Concatena0on


String
Indexing


String
Slicing


Strings
are
Immutable!


Indices
May
be
Nega0ve


How
Come?!


Lists

•  compound
data
type

•  used
to
group
together
other
values

•  The
most
versa0le

•  can
be
wricen
as
a
list
of
comma-‐separated

values
(items)
between
square
brackets.

•  List
items
need
not
all
have
the
same
type.


Shallow
Copy


First
Steps
toward
Programming


More
Control
Flow
Tools


If
Statement


For
Statement


For
Statement
with
Shallow
Copy


Range
Func0on


Break,
Con0nue,
Else


Pass

•  Does
Nothing!


Func0ons


Return
Statement


Default
Argument
Value


Documenta0on
String


Coding
Style
-‐
PEP8

•  Use
4-‐space
indenta0on,
and
no
tabs.

•  4
spaces
are
a
good
compromise
between
small

indenta0on
(allows
greater
nes0ng
depth)
and
large

indenta0on
(easier
to
read).
Tabs
introduce
confusion,

and
are
best
leS
out.

•  Wrap
lines
so
that
they
don’t
exceed
79
characters.

•  This
helps
users
with
small
displays
and
makes
it

possible
to
have
several
code
ﬁles
side-‐by-‐side
on

larger
displays.

•  Use
blank
lines
to
separate
func0ons
and
classes,
and

larger
blocks
of
code
inside
func0ons.


Coding
Style
-‐
PEP8

•  When
possible,
put
comments
on
a
line
of
their
own.

•  Use
docstrings.

•  Use
spaces
around
operators
and
aSer
commas,
but
not

directly
inside
bracke0ng
constructs:
a
=
f(1,
2)
+
g(3,
4).

•  Name
your
classes
and
func0ons
consistently;
the

conven0on
is
to
use
CamelCase
for
classes
and

lower_case_with_underscores
for
func0ons
and
methods.

Always
use
self
as
the
name
for
the
ﬁrst
method
argument.

•  Don’t
use
fancy
encodings
if
your
code
is
meant
to
be
used

in
interna0onal
environments.
Plain
ASCII
works
best
in
any

case.


Data
Structures
-‐
Lists

•  list.append(x)
Add
an
item
to
the
end
of
the
list.

•  list.extend(L)
Extend
the
list
by
appending
all
the

items
in
the
given
list.

•  list.insert(i,
x)
Insert
an
item
at
a
given
posi0on.

The
ﬁrst
argument
is
the
index
of
the
element

before
which
to
insert,
so
a.insert(0,
x)
inserts
at

the
front
of
the
list,
and
a.insert(len(a),
x)
is

equivalent
to
a.append(x).

•  list.remove(x)
Remove
the
ﬁrst
item
from
the
list

whose
value
is
x.
It
is
an
error
if
there
is
no
such

item.


Data
Structures
-‐
Lists

•  list.pop([i])
Remove
the
item
at
the
given
posi0on
in

the
list,
and
return
it.
If
no
index
is
speciﬁed,
a.pop()

removes
and
returns
the
last
item
in
the
list.

•  list.index(x)
Return
the
index
in
the
list
of
the
ﬁrst
item

whose
value
is
x.
It
is
an
error
if
there
is
no
such
item.

•  list.count(x)
Return
the
number
of
0mes
x
appears
in

the
list.

•  list.sort()
Sort
the
items
of
the
list,
in
place.

•  list.reverse()
Reverse
the
elements
of
the
list,
in
place.


Lists
Example


Using
Lists
as
Stack


Using
List
as
a
Queue


Func0onal
Programming
Tools

•  There
are
three
built-‐in
func0ons
that
are
very

useful
when
used
with
lists:
ﬁlter(),
map(),
and

reduce().


Filter()

•  ﬁlter(func0on,
sequence)
returns
a
sequence

consis0ng
of
those
items
from
the
sequence

for
which
func0on(item)
is
true.

•  If
sequence
is
a
string
or
tuple,
the
result
will

be
of
the
same
type;
otherwise,
it
is
always
a

list.

•  For
example,
to
compute
a
sequence
of

numbers
not
divisible
by
2
and
3:


Filter()


Map()

•  map(func0on,
sequence)
calls
func0on(item)

for
each
of
the
sequence’s
items
and
returns
a

list
of
the
return
values.

•  For
example,
to
compute
some
cubes:


Map()

•  More
than
one
sequence
may
be
passed;
the

func0on
must
then
have
as
many
arguments

as
there
are
sequences
and
is
called
with
the

corresponding
item
from
each
sequence
(or

None
if
some
sequence
is
shorter
than

another).
For
example:


Reduce()

•  reduce(func0on,
sequence)
returns
a
single

value
constructed
by
calling
the
binary

func0on
func2on
on
the
ﬁrst
two
items
of
the

sequence,
then
on
the
result
and
the
next

item,
and
so
on.

•  For
example,
to
compute
the
sum
of
the

numbers
1
through
10:


Tuples
and
Sequences


Comma
at
the
end!


Sets

•  A
set
is
an
unordered
collec0on
with
no

duplicate
elements.

•  Basic
uses
include
membership
tes0ng
and

elimina0ng
duplicate
entries.

•  Set
objects
also
support
mathema0cal

opera0ons
like
union,
intersec0on,
diﬀerence,

and
symmetric
diﬀerence.


Dic0onaries

•  Dic0onaries
are
some0mes
found
in
other

languages
as
“associa0ve
memories”
or

“associa0ve
arrays”.

•  Unlike
sequences,
which
are
indexed
by
a
range

of
numbers,
dic0onaries
are
indexed
by
keys,

which
can
be
any
immutable
type;
strings
and

numbers
can
always
be
keys.

•  Tuples
can
be
used
as
keys
if
they
contain
only

strings,
numbers,
or
tuples;
if
a
tuple
contains

any
mutable
object
either
directly
or
indirectly,
it

cannot
be
used
as
a
key.


Dic0onaries

•  You
can’t
use
lists
as
keys,
since
lists
can
be
modiﬁed
in

place
using
index
assignments,
slice
assignments,
or

methods
like
append()
and
extend().

•  It
is
best
to
think
of
a
dic0onary
as
an
unordered
set
of

key:
value
pairs,
with
the
requirement
that
the
keys

are
unique
(within
one
dic0onary).

•  A
pair
of
braces
creates
an
empty
dic0onary:
{}.
Placing

a
comma-‐separated
list
of
key:value
pairs
within
the

braces
adds
ini0al
key:value
pairs
to
the
dic0onary;

this
is
also
the
way
dic0onaries
are
wricen
on
output.


Dic0onaries

•  The
main
opera0ons
on
a
dic0onary
are
storing
a
value

with
some
key
and
extrac0ng
the
value
given
the
key.

•  It
is
also
possible
to
delete
a
key:value
pair
with
del.
If

you
store
using
a
key
that
is
already
in
use,
the
old

value
associated
with
that
key
is
forgocen.
It
is
an

error
to
extract
a
value
using
a
non-‐existent
key.

•  The
keys()
method
of
a
dic0onary
object
returns
a
list

of
all
the
keys
used
in
the
dic0onary,
in
arbitrary
order

(if
you
want
it
sorted,
just
apply
the
sorted()
func0on

to
it).

•  To
check
whether
a
single
key
is
in
the
dic0onary,
use

the
in
keyword.


Dict()
Constructor


Looping
Techniques


Looping
over
Two
Sequences


Loop
in
Reverse


More
on
Condi0ons

•  Include
and,
or


Comparing
Objects
of
Diﬀerent
Types


Lecture 9 - DSA - Python Data Structures

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