Chapter 5 Strings CSC1310 Fall 2009

Strings Stringordered storesrepresents String is an ordered collection of characters that stores and represents text-based information. immutable sequencesleft- to-right order Strings in Python are immutable (e.g., cannot be changed in place) sequences (e.g., they have a left- to-right order).

Single and Double Quotes interchangeable. Single and double quotes are interchangeable. >>> ‘Python’, “Python” Empty literal‘’“” Empty literal: ‘’ or “”. It allows you to embed a quote character of the other type inside a string: “’”‘”’ >>> “knight’s”,‘knight”s’ Python automatically concatenates adjacent strings ””‘ ’“ the ” >>>”Title” ‘ of’ “ the book”

Escape Sequences Escape sequencebyte code Escape sequence is a special byte code embedded into string, that can not be easily typed on a keyboard. \ with one (or more) character(s) single character \ with one (or more) character(s) is replaced by a single character in the resulting string. \n \n - Newline \t \t - Horizontal Tab >>> s=‘a\nb\tc’ # 5 characters! >>> s ‘a\nb\tc’ >>> print s a bc

Escape Sequences \\ \\ - Backslash \’ \’ - Single quote \” \” - Double quote \a \a - Bell \b \b - Backspace \r \r - Carriage return \xhh \xhh - Hex digits value hh \0 \0 - Null (binary zero bytes) >>> print ‘a\0m\0c’ # 5 characters! a m c

Raw Strings >>>print ‘C:\temp\new.txt’ \\\\ >>>print ‘C:\\temp\\new.txt’ Raw string suppress escape Format: r“text” r‘text’(R“text” R‘text’) Format: r“text” or r‘text’(R“text” or R‘text’) r’ >>>print r‘C:\temp\new.txt’ Raw strings may be used for directory paths, text pattern matching.

Triple Quoted Strings or Block Strings Block string Block string is a convenient literal format for coding multiline text data (error msgs, HTML or XML code). Format: “”” ””” ‘’’’’’ Format: “”” text””” or ‘’’text’’’

Unicode Strings Unicode (“wide” character) strings Unicode (“wide” character) strings are used to support non-latin characters that require more than one byte in memory. Format: u“text” u‘text’(U“text” U‘text’) Format: u“text” or u‘text’(U“text” or U‘text’) Expression with Unicode and normal strings has Unicode string as a result. >>>’fall’+u’08’ u’fall08’ strunicode >>>str(u’fall08’),unicode(‘fall08’) ‘fall08’,u’fall08’

Basic operations: len(), +, *,in len(str)str. len(str) function returns the length of a string str. >>>len(‘abc’) str1+str2 (concatenation) str1str2 str1+str2 (concatenation) creates a new string by joining operands str1 and str2. >>>‘abc’ + ‘def’,len(‘abc’ + ‘def’) str*i (repeat) stri str*i (repeat) adds a string str to itself i times. >>> print ‘-’ * 80 str 1 in str2 (membership)str1 str2 str 1 in str2 (membership) returns true if str1 is a substring of str2; otherwise, returns false. >>>day=‘Monday 8th Sept 2008’ >>>’sep’ in day >>>’th Sep’ in day

Indexing index Each character in the string can be accessed by its position (offset) – index. >>>S = ‘STRINGINPYTHON’ –xx th Negative offset can be viewed as counting backward from the end(offset –x is x th character from the end). >>>S[0],S[10],S[13],S[-5],S[-14] (‘S’,’T’,’N’,’Y’,’S’) >>>S[14],S[-15]

Slicing substring Slicing allows us to extract an entire section (substring) in a single step. Str1[offset1:offset2]substringstr1 from offset1 (including)at offset2 (excluding). Str1[offset1:offset2] returns a substring of str1 starting from offset1 (including) and ending at offset2 (excluding). >>>S[1:3] #extract item at offsets1 and 2 >>>S[1:] #all items past the first >>>S[:3] # extract items at offsets 0,1,2 >>>S[:-1] #fetch all but the last item >>>S[-1:] # extract last item copy >>>S[:] # a copy of the string

In Python 2.3 Third index – stride(step) >>>S=‘ ’ >>>S[1:10:2] ‘13579’ To reverse string use step =-1 >>>”hello”[::-1]

String Conversion You cannot add a number and a string together. int(str1)str1 int(str1) converts string str1 into integer. float(str1)str1 float(str1) converts string str1 into floating-point number. string.atoi(str1) string.atof(str1). Older techniques: functions string.atoi(str1) and string.atof(str1). >>>int(“42”)+1,float(“42”)+1 str(i)i`i` str(i) converts numeric i to string(`i`) >>>”fall0”+str(8)

Changing Strings You cannot change a string in-place by assigning value to an index(S[0] = ‘X’) To modify: create new string with concatenation and slicing. >>>s=‘spam’ += >>>s=s+” again” # s+=” again!” >>>s >>>s=s[:3]+” is here”+s[-1:] >>>s Alternatively, format a string.

Formatting Strings “format string” % “object to insert” “format string” % “object to insert” >>>s=“Sales tax” >>>s=”%s is %d percent!” % (s,8) %s %s string %d %d decimal integer %i%u %i integer (%u - unsigned integer) %o %o octal integer %x%X %x hex integer (%X – uppercase hex integer) %e%E %e floating-point exponent (%E - uppercase) %f%F %f floating-point decimal (%F – uppercase)

String Methods (p.91 table 5-4) Str1.replace(str2,str3)str2 Str1str3 Str1.replace(str2,str3) replaces each substring str2 in Str1 to str3. >>>‘string in python’.replace( ‘in’, ‘XXXX’) Str1.find(str2) str2Str1, Str1.find(str2) returns the offset where substring str2 appears in Str1, or -1. >>>where =‘string in python’.find( ‘in’) >>>’string in python’[:where] >>>‘in1in2in3in4in5’.replace( ‘in’, ‘XX’,3)

String Methods Str.upper(), str.lower(), str.swapcase() Str1.count(substr,start,end) Str1.endswith(suffix,start,end) Str1.startswith(prefix,start,end) Str1.index(substr,start,end) Str1.isalnum(),str1.isalpha(), str1.isdigit(), str1.islower(),str1.isspace(),str1.issupper()

String Module Maketrans()/translate() Maketrans()/translate() >>>import string >>>convert=string.maketrans(“ _-”,”_-+”) >>>input=“It is a two_part – one_part” >>>input.translate(convert) ‘It_is_a_two-part_+_one-part’

String Module Constants Constants  digits ‘ ’  octdigits ‘ ’  hexdigits ‘ abcdefABCDEF’  lowercase ‘abcdefghijklmnopqrstuvwxyz’  uppercase ‘ABCDEFGHIJKLMNOPRQSTUVWXYZ ’  letters lowercase+uppercase  whitespace ‘\t\n\r\v’ >>>import string >>>x=raw_input() >>>x in string.digits