The String Data Type
                                                                 Python Programming:                                                                                        • The most common use of personal
                                                                      An Introduction to                                                                                        computers is word processing.
                                                                     Computer Science                                                                                       • Text is represented in programs by the
                                                                                                                                                                                string data type.
                                                                                  Chapter 4                                                                                 • A string is a sequence of characters
                                                                    Computing with Strings                                                                                      enclosed within quotation marks (") or
                                                                                                                                                                                apostrophes (') or (“””).
                                                                             Coming up: The String Data Type                        1
                                                                                                                                                                    CCoommiinngg  uupp::  EExxaammpplleess                                                                  22
                                                                          Examples                                                                                                       Newline string examples
                                          >>> str1="Hello"                                                                                                                  To create a string with a newline (“return”) in it,
                                          >>> str2='spam'                                                                                                                       you can add a newline character \n .You can
                                          >>> print str1, str2                                                                                                                  think of newline as the character produced
                                          Hello spam                                                                                                                            when you press the  key
                                                                                                                                                                            >>>print “The newspaper is \n at the door”
                                          >>> type(str1)                                                                                                                    The newspaper is
                                                                                                                                                                 at the door
                                          >>> type(str2)                                                                                                                    >>> print “Mary: John? \nJohn: Yes \nMary: Your car is on fire”
                                                                                                                                                                Mary: John?
                                                                                                                                                                            John: Yes
                                                                                                                                                                            Mary: Your car is on fire
                                  CCoommiinngg  uupp::  NNeewwlliinnee  ssttrriinngg  eexxaammpplleess                                    33                        CCoommiinngg  uupp::  LLiinnee  ccoonnttiinnuuaattiioonn  eexxaammpplleess                              44
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                                                                                                             Line continuation examples                                                                                                                                                                                                                                                            “”” triple-quotes examples
                                                                                         To create a very long string you may want to define it on                                                                                                                                                                                                                           Triple-quotes “”” tells Python to include newline
                                                                                                  multiple lines, for this you can use the line                                                                                                                                                                                                                                      characters and you do not need the line continuation
                                                                                                  continuation character “\”                                                                                                                                                                                                                                                         characters. This is useful if you have a large block of
                                                                                                                                                                                                                                                                                                                                                                                     formatted text, just type it as you want it to look.
                                                                                         >>>hello = "This is a rather long string containing\n\                                                                                                                                                                                                                              >>> str1=“”"
                                                                                         several lines of text just as you would do in C.\n\                                                                                                                                                                                                                                         Usage: thingy [OPTIONS]
                                                                                             Note that whitespace at the beginning of the line is\                                                                                                                                                                                                                                        -h                         Display this usage message
                                                                                         significant."                                                                                                                                                                                                                                                                                    -H hostname         Hostname to connect to
                                                                                         >>>print hello                                                                                                                                                                                                                                                                      """
                                                                                         This is a rather long string containing                                                                                                                                                                                                                                             >>> print str1
                                                                                         several lines of text just as you would do in C.                                                                                                                                                                                                                                       Usage: thingy [OPTIONS]
                                                                                             Note that whitespace at the beginning of the line is significant.                                                                                                                                                                                                                       -h                        Display this usage message
                                                                                                                                                                                                                                                                                                                                                                                     -H hostname       Hostname to connect to
                                                                        CCoommiinngg  uupp::  ““””””  ttrriippllee--qquuootteess  eexxaammpplleess                                                                                                                                                  55                                                      CCoommiinngg  uupp::  WWhhyy  iinnppuutt  ddooeessnn’’tt  wwoorrkk                                                                                                                                                         66
                                                                                                                       Why input doesn’t work                                                                                                                                                                                                                                           Why Input Doesn’t Work (cont)
                                                                                         >>> firstName = input("Please enter your name: ")                                                                                                                                                                                                                                   • The input statement is a delayed expression.
                                                                                         Please enter your name: John                                                                                                                                                                                                                                                        • When you enter a name, it’s doing the same
                                                                                         Traceback (most recent call last):                                                                                                                                                                                                                                                          thing as:
                                                                                           File "", line 1, in -toplevel-                                                                                                                                                                                                                                                firstName = John
                                                                                             firstName = input("Please enter your name: ")                                                                                                                                                                                                                                   • The way Python evaluates expressions is to
                                                                                           File "", line 0, in -toplevel-                                                                                                                                                                                                                                                    look up the value of the variable John and
                                                                                         NameError: name 'John' is not defined                                                                                                                                                                                                                                                       store it in firstName.
                                                                                         • What happened?                                                                                                                                                                                                                                                                    • Since John didn’t have a value, we get a
                                                                                                                                                                                                                                                                                                                                                                                     NameError.
                                                                        CCoommiinngg  uupp::  WWhhyy  IInnppuutt  DDooeessnn’’tt  WWoorrkk  ((ccoonntt))                                                                                                                                            77                                                      CCoommiinngg  uupp::  UUssiinngg  qquuootteess  ttoo  aavvooiidd  tthhee  pprroobblleemm                                                                                                                                   88
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                                                                                       Using quotes to avoid the problem                                                                                                                                                                                                                                             Using raw_input to avoid the problem
                                                                                      • One way to fix this is to enter your string                                                                                                                                                                                                                              • There is a better way to handle text – the
                                                                                               input with quotes around it:                                                                                                                                                                                                                                              raw_input function.
                                                                                               >>> firstName = input("Please enter your name: ")                                                                                                                                                                                                                 • raw_input is like input, but it doesn’t evaluate
                                                                                               Please enter your name: "John"                                                                                                                                                                                                                                            the expression that the user enters.
                                                                                               >>> print "Hello", firstName
                                                                                               Hello John                                                                                                                                                                                                                                                        >>> firstName = raw_input("Please enter your name: ")
                                                                                      • Even though this works, this is                                                                                                                                                                                                                                          Please enter your name: John
                                                                                               cumbersome!                                                                                                                                                                                                                                                       >>> print "Hello", firstName
                                                                                                                                                                                                                                                                                                                                                                 Hello John
                                                                      CCoommiinngg  uupp::  UUssiinngg  rraaww__iinnppuutt  ttoo  aavvooiidd  tthhee  pprroobblleemm                                                                                                                       99                                                    CCoommiinngg  uupp::  AAcccceessssiinngg  iinnddiivviidduuaall  cchhaarraacctteerrss                                                                                                                              1010
                                                                                              Accessing individual characters                                                                                                                                                                                                                                                                                 Indexing example
                                                                                      • We can access the individual
                                                                                               characters in a string through indexing.                                                                                                                                                                                                                                                    H e                            l            l              o                            B o                          b
                                                                                      • The positions in a string are numbered                                                                                                                                                                                                                                                           0    1     2    3     4    5     6     7     8
                                                                                               from the left, starting with 0.                                                                                                                                                                                                                                   >>> greet = "Hello Bob"
                                                                                      • The general form is [],                                                                                                                                                                                                                                    >>> greet[0]
                                                                                               where the value of expr determines                                                                                                                                                                                                                                'H'
                                                                                                                                                                                                                                                                                                                                                                 >>> print greet[0], greet[2], greet[4]
                                                                                               which character is selected from the                                                                                                                                                                                                                              H l o
                                                                                               string.                                                                                                                                                                                                                                                           >>> x = 8
                                                                                                                                                                                                                                                                                                                                                                 >>> print greet[x - 2]
                                                                                                                                                                                                                                                                                                                                                                 B
                                                                      CCoommiinngg  uupp::  IInnddeexxiinngg  eexxaammppllee                                                                                                                                                             1111                                                    CCoommiinngg  uupp::  IInnddeexxiinngg  eexxaammppllee  --  ffrroomm  tthhee  rriigghhtt                                                                                                                          1212
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                                                                                         Indexing example - from the right                                                                                                                                                                                                                                                                       What about a substring?
                                                                                                                                                                                                                                                                                                                                                                                                                    Slicing a string
                                                                                                                                                                                                                                                                                                                                                         • Slicing:
                                                                                                              H e                           l           l              o                            B o                         b                                                                                                                                [:]
                                                                                                            0    1     2    3     4    5     6     7     8                                                                                                                                                                                               • start and end should both be ints
                                                                                    • In a string of n characters, the last character                                                                                                                                                                                                                    • The slice contains the substring
                                                                                             is at position n-1 since we start counting with
                                                                                             0.                                                                                                                                                                                                                                                                  beginning at position start and runs up
                                                                                    • We can index from the right side using                                                                                                                                                                                                                                     to but doesn’t include the position
                                                                                             negative indexes.                                                                                                                                                                                                                                                   end.
                                                                                    >>> greet[-1]
                                                                                    'b'
                                                                                    >>> greet[-3]
                                                                                    'B'
                                                                    CCoommiinngg  uupp::  WWhhaatt  aabboouutt  aa  ssuubbssttrriinngg??                                                                                                                                          1313                                                   CCoommiinngg  uupp::  SSlliicciinngg  EExxaammppllee                                                                                                                                                          1414
                                                                    SSlliicciinngg  aa  ssttrriinngg
                                                                                                                                    Slicing Example                                                                                                                                                                                                                                                     String Operators
                                                                                                              H e                           l           l              o                            B o                         b
                                                                                                     0    1     2    3     4    5     6     7     8    9                                                                                                                                                                                                                   Operator                                                                Meaning
                                                                                    >>> greet[0:3]                                                                                                                                                                                                                                                                         +                                                                       Concatenation
                                                                                    'Hel'
                                                                                    >>> greet[5:9]                                                                                                                                                                                                                                                                         *                                                                       Repetition
                                                                                    ' Bob'
                                                                                    >>> greet[:5]                                                                                                                                                                                                                                                                          []                                                              Indexing
                                                                                    'Hello'                                                                                      Hint: When slicing it helps 
                                                                                    >>> greet[5:]                                                                              to think of the slide indexes                                                                                                                                                               [:]                                                             Slicing
                                                                                    ' Bob'                                                                                          between the characters, 
                                                                                    >>> greet[:]                                                                                                                                                                                                                                                                           len()                                                           Length
                                                                                    'Hello Bob'                                                                                           then 0:3 is very clear
                                                                                                                                                                                                                                                                                                                                                                           for  in                                                    Iteration through
                                                                                                                                                                                                                                                                                                                                                                                                                                                   characters
                                                                    CCoommiinngg  uupp::  SSttrriinngg  OOppeerraattoorrss                                                                                                                                                        1515                                                   CCoommiinngg  uupp::  CCoonnccaatteennaattiioonn  ((ssttrr  ++  ssttrr))                                                                                                                                      1616
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