Python - Data type, String, Operators

8/14/2019 Python - Data type, String, Operators

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Strings in Python


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Data types andoperators


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Variables and Types

Variables need no declaration >>> a=1

>>> As a variable assignment is a statement, there is no

printed result >>> a

1 Variable name alone is an expression, so the result

is printed


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Variables and Types

Variables must be created before they can be used >>> b

Traceback (innermost last): File "", line 1,in ?

NameError: b>>>


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Assignment versus EqualityTesting

Assignment performed with single = Equality testing done with double = (==

! "ensible type promotions are defined! #dentity tested with is operator$

>>> 1==11>>> 1 !==11

>>> "1"==1!


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Simple Data Types

"trings! %ay hold any data, including embedded &' s! )eclared using either single, double, or triple quotes >>> s = " i t#ere"

>>> s$ i t#ere$>>> s = "Embe%%e% $&uote$">>> s"Embe%%e% $&uote$"


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Simple Data Types

! *riple quotes useful for multi+line strings >>> s = """ a lon'

strin' it# "&uotes" or an t#in' else""">>> s$ a lon'*!1+strin' it# "&uotes" or an t#in'else$>>> len(s)

-


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Simple Data Types

#nteger ob ects implemented using - longs! i.e -, integer division returns the floor >>> -.+

+

/loat types implemented using - doubles! &o point in having single precision since execution

overhead is large anyway


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Simple Data Types

ong #ntegers have unlimited si0e! imited only by available memory >>> lon' = 1/ >> lon' -+12-345!2-3+!31!!4+23-!+15!0103--+11 0!+5! -++2-00-+5033 5! 10!54+++135+-54!0 !--!!++30+!40320-50/


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Variables and Types

1b ects always have a type>>> t pe(1)

>>> t pe(" ello")


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Strings vs. Integers

2oth strings and integer variables can storenumbers

my_speed_str="300" my_speed_integer=300

34 5hat is the difference between strings andintegers6


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Strings in use

34 5here are strings displayed and entered6


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String composition

"trings are a composite data type! 2uilt from another data type, characters! #n a string, characters are stored in sequence$! "trings can be any length (including empty $! "tring are enclosed in quotation mar.s$

str_var = "300 m/s" empty_str=""


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Length o a string

'se len(str) to return string length (7 ofcharacters sample="SERIES" len(sample)=6

empty_str="" len(empty_str) = 0


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String representation

#n strings, characters are accessed by inde! ! 8li.e mailboxes in an apartment building$

! /irst index is 9, not :$ s="LE EL" start! ar=s# $ %&st_v=ss# $

! ;ython strings are immutable"can


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#perations$%oncatenation

-ombine strings using & (concatenation operator&ll_name = " enry" * " " * "+ames"print "," * &ll_name * ","

-oncatenation is not addition visi-n_str=".0"*".0" visi-n_val=.0*.0

Try it out$ 2uild a string &ild="" ile len( &ild)12, &ild = &ild *"a" print &ild

= 0=4.0.05


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#perations$ Traversing through astring

'se a loop to examine each character in a string-r in

strng=" -&nt 7 - &s"inde8=0

-&nt=0ile inde8 1 len(strng),

i strng#inde8$ == "&4 -r strng#inde8$ == 49",-&nt*=:

inde8*=: *ry it out! ow would we traverse bac.wards6


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String operations$ ind

ind() searches for a string within a string *o use it, insert this at the top of your code4

imp-rt string

ind() returns the first index of a substring

&ll_name = " enry +ames" string; ind( &ll_nameou can specify the starting point of the search4

string; ind( &ll_name


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'igh Level Data Types

List ists hold a sequence of items

! %ay hold any ob ect! )eclared using square brac.ets

>>> l = 789 n empt list>>> l appen%(1)>>> l appen%(" i t#ere")>>> len(l)

+


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>>> l71, $ i t#ere$8>>> >>> l = 7" i t#ere", 1, +8>>> l7$ i t#ere$, 1, +8>>> l sort()>>> l71, +, $ i t#ere$8


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Tuple *uples are similar to lists

! "equence of items! ?ey difference is they are immutable

! 1ften used in place of simple structures Automatic unpac.ing >>> point = +,2

>>> ;, = point

>>> ;+


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*uples are particularly useful to return multiplevalues from a function >>> ;, = et oint() As ;ython has no concept of byref parameters, this

technique is used widely


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Dictionary )ictionaries hold .ey+value pairs

! 1ften called maps or hashes$ #mplemented usinghash+tables

! ?eys may be any immutable ob ect, values may be anyob ect

! )eclared using braces >>> %=

>>> %7!8 = " i t#ere">>> %7"6oo"8 = 1


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)ictionaries (cont$

>>> % = ! : " i t#ere", 1 : " ello">>> len(%)+>>> %7!8$ i t#ere$


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#perators


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#perators

;ython has many operators$ "ome examples are4*< < >< /< ?< @< 1< ==print

1perators perform an action on one or more

operands$ "ome operators accept operandsbefore and after themselves4 -perand: * -perand. , or 3 * 2

1thers are followed by one or more operandsuntil the end of the line, such as4 print 4 iA5 Radi&s > Radi&sprint area

will print I:H$:@ to the screen$


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eywords, Name-spaces &Scope

&ames that are defined in a function are localD tothat function$

&ames that are defined outside of a function areglobalD to the module$

ocal names overshadow global names wheninside the function that defined them$

#f you want to access a global variable from insideof a function, you should declare it globalD$


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1lobal vs Local e!amplemy aria le = CmyFaram = .0

de &n :(myFaram), my aria le = .0 print myFaram

&n :(2)print my aria le

5hat gets printed6


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1lobal vs Local e!ample my aria le = C myFaram = .0

de &n :(myFaram), gl- al my aria le my aria le = .0

print myFaram

&n :(2) print my aria le

5hat gets printed6 @ and 9 *he localD myVariable inside func: is separate from theglobalD myVariable outside of func:

*he function assigns 9 to the globalD myVariable,overwriting the G before it gets printed$

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Python - Data type, String, Operators