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  practice  
  assignment  
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Strings in Python are identified as a contiguous set of characters represented in the quotation marks. Python allows for either pairs of single or double quotes. Subsets of strings can be taken using the slice operator ([ ] and [:] ) with indexes starting at 0 in the beginning of the string and working their way from -1 at the end.


The plus (+) sign is the string concatenation operator and the asterisk (*) is the repetition operator. For example −
str = 'Hello World!'
 
print str # Prints complete string
print str[0] # Prints first character of the string
print str[2:5] # Prints characters starting from 3rd to 5th
print str[2:] # Prints string starting from 3rd character
print str * 2 # Prints string two times
print str + "TEST" # Prints concatenated string
 
This will produce the following result −
Hello World!
H
llo
llo World!
Hello World!Hello World!
Hello World!TEST

 

 

escape character

Following table is a list of escape or non-printable characters that can be represented with backslash notation.
An escape character gets interpreted; in a single quoted as well as double quoted strings.

Backslash notation

Hexadecimal character

Description

\a

0x07

Bell or alert

\b

0x08

Backspace

\cx

 

Control-x

\C-x

 

Control-x

\e

0x1b

Escape

\f

0x0c

Formfeed

\M-\C-x

 

Meta-Control-x

\n

0x0a

Newline

\nnn

 

Octal notation, where n is in the range 0.7

\r

0x0d

Carriage return

\s

0x20

Space

\t

0x09

Tab

\v

0x0b

Vertical tab

\x

 

Character x

\xnn

 

Hexadecimal notation, where n is in the range 0.9, a.f, or A.F

 

 

String Special Operators

Assume string variable a holds 'Hello' and variable b holds 'Python', then −

Operator

Description

Example

+

Concatenation - Adds values on either side of the operator

a + b will give HelloPython

*

Repetition - Creates new strings, concatenating multiple copies of the same string

a*2 will give -HelloHello

[]

Slice - Gives the character from the given index

a[1] will give e

[ : ]

Range Slice - Gives the characters from the given range

a[1:4] will give ell

in

Membership - Returns true if a character exists in the given string

H in a will give 1

not in

Membership - Returns true if a character does not exist in the given string

M not in a will give 1

r/R

Raw String - Suppresses actual meaning of Escape characters. The syntax for raw strings is exactly the same as for normal strings with the exception of the raw string operator, the letter "r," which precedes the quotation marks. The "r" can be lowercase (r) or uppercase (R) and must be placed immediately preceding the first quote mark.

print r'\n' prints \n and print R'\n'prints \n

%

Format - Performs String formatting

See at next section

 

 

String Formatting Operator

One of Python's coolest features is the string format operator %. This operator is unique to strings and makes up for the pack of having functions from C's printf() family. Following is a simple example −
print "My name is %s and weight is %d kg!" % ('Zara', 21)
 
When the above code is executed, it produces the following result −
My name is Zara and weight is 21 kg!
 
Here is the list of complete set of symbols which can be used along with % −

Format Symbol

Conversion

%c

character

%s

string conversion via str() prior to formatting

%i

signed decimal integer

%d

signed decimal integer

%u

unsigned decimal integer

%o

octal integer

%x

hexadecimal integer (lowercase letters)

%X

hexadecimal integer (UPPERcase letters)

%e

exponential notation (with lowercase 'e')

%E

exponential notation (with UPPERcase 'E')

%f

floating point real number

%g

the shorter of %f and %e

%G

the shorter of %f and %E

 

Other supported symbols and functionality are listed in the following table −

Symbol

Functionality

*

argument specifies width or precision

-

left justification

+

display the sign

<sp>

leave a blank space before a positive number

#

add the octal leading zero ( '0' ) or hexadecimal leading '0x' or '0X', depending on whether 'x' or 'X' were used.

0

pad from left with zeros (instead of spaces)

%

%%' leaves you with a single literal '%'

(var)

mapping variable (dictionary arguments)

m.n.

m is the minimum total width and n is the number of digits to display after the decimal point (if appl.)

 

 

Quotation in Python

Python accepts single ('), double (") and triple (''' or """) quotes to denote string literals, as long as the same type of quote starts and ends the string.
 
Unicode String
Normal strings in Python are stored internally as 8-bit ASCII, while Unicode strings are stored as 16-bit Unicode. This allows for a more varied set of characters, including special characters from most languages in the world. I'll restrict my treatment of Unicode strings to the following −
print u'Hello, world!'
 
When the above code is executed, it produces the following result −
Hello, world!

Triple Quotes
Python's triple quotes comes to the rescue by allowing strings to span multiple lines, including verbatim NEWLINEs, TABs, and any other special characters.
 
The syntax for triple quotes consists of three consecutive single or double quotes.
para_str = """this is a long string that is made up of
several lines and non-printable characters such as TAB ( \t ).
NEWLINEs within the string, whether explicitly given like this within the
brackets [ \n ], or just a NEWLINE within the variable assignment will also show up.
"""
print para_str

The triple quotes are used to span the string across multiple lines. For example, all the following are legal −
word = 'word'
sentence = "This is a sentence."
paragraph = """This is a paragraph. It is
made up of multiple lines and sentences."""


 

Built-in String Methods

Python includes the following built-in methods to manipulate strings −

capitalize()

Capitalizes first letter of string

center(width, fillchar)

Returns a space-padded string with the original string centered to a total of width columns.

count(str, beg= 0,end=len(string))

Counts how many times str occurs in string or in a substring of string if starting index beg and ending index end are given.

decode(encoding='UTF-8',errors='strict')

Decodes the string using the codec registered for encoding. encoding defaults to the default string encoding.

encode(encoding='UTF-8',errors='strict')

Returns encoded string version of string; on error, default is to raise a ValueError unless errors is given with 'ignore' or 'replace'.

endswith(suffix, beg=0, end=len(string))

Determines if string or a substring of string (if starting index beg and ending index end are given) ends with suffix; returns true if so and false otherwise.

expandtabs(tabsize=8)

Expands tabs in string to multiple spaces; defaults to 8 spaces per tab if tabsize not provided.

find(str, beg=0 end=len(string))

Determine if str occurs in string or in a substring of string if starting index beg and ending index end are given returns index if found and -1 otherwise.

index(str, beg=0, end=len(string))

Same as find(), but raises an exception if str not found.

isalnum()

Returns true if string has at least 1 character and all characters are alphanumeric and false otherwise.

isalpha()

Returns true if string has at least 1 character and all characters are alphabetic and false otherwise.

isdigit()

Returns true if string contains only digits and false otherwise.

islower()

Returns true if string has at least 1 cased character and all cased characters are in lowercase and false otherwise.

isnumeric()

Returns true if a unicode string contains only numeric characters and false otherwise.

isspace()

Returns true if string contains only whitespace characters and false otherwise.

istitle()

Returns true if string is properly "titlecased" and false otherwise.

isupper()

Returns true if string has at least one cased character and all cased characters are in uppercase and false otherwise.

join(seq)

Merges (concatenates) the string representations of elements in sequence seq into a string, with separator string.

len(string)

Returns the length of the string

ljust(width[, fillchar])

Returns a space-padded string with the original string left-justified to a total of width columns.

lower()

Converts all uppercase letters in string to lowercase.

lstrip()

Removes all leading whitespace in string.

maketrans()

Returns a translation table to be used in translate function.

max(str)

Returns the max alphabetical character from the string str.

min(str)

Returns the min alphabetical character from the string str.

replace(old, new [, max])

Replaces all occurrences of old in string with new or at most max occurrences if max given.

rfind(str, beg=0,end=len(string))

Same as find(), but search backwards in string.

rindex( str, beg=0, end=len(string))

Same as index(), but search backwards in string.

rjust(width,[, fillchar])

Returns a space-padded string with the original string right-justified to a total of width columns.

rstrip()

Removes all trailing whitespace of string.

split(str="", num=string.count(str))

Splits string according to delimiter str (space if not provided) and returns list of substrings; split into at most num substrings if given.

splitlines( num=string.count('\n'))

Splits string at all (or num) NEWLINEs and returns a list of each line with NEWLINEs removed.

startswith(str, beg=0,end=len(string))

Determines if string or a substring of string (if starting index beg and ending index end are given) starts with substring str; returns true if so and false otherwise.

strip([chars])

Performs both lstrip() and rstrip() on string

swapcase()

Inverts case for all letters in string.

title()

Returns "titlecased" version of string, that is, all words begin with uppercase and the rest are lowercase.

translate(table, deletechars="")

Translates string according to translation table str(256 chars), removing those in the del string.

upper()

Converts lowercase letters in string to uppercase.

zfill (width)

Returns original string leftpadded with zeros to a total of width characters; intended for numbers, zfill() retains any sign given (less one zero).

isdecimal()

Returns true if a unicode string contains only decimal characters and false otherwise.

 

 

 
         
 
 
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