20     Welcome friends. 

    20 	Welcome friends. 

    21     

    21 

    22     In the previous tutorial we have looked at data types for dealing

    22 	In the previous tutorial we have looked at data types for dealing

    23     with numbers. In this tutorial we shall look at strings. We shall

    23 	with numbers. In this tutorial we shall look at strings. We shall

    24     look at how to do elementary string manipulation, and simple input

    24 	look at how to do elementary string manipulation, and simple input

    25     and output operations. 

    25 	and output operations. 

    26     

    26 

    27     In Python anything enclosed within quotes is a string. Lets get 

    27     In this tuotrial we shall use concepts of writing python scripts and basics of lists that have been covered in previous session

    28     started by starting ipython interpreter. We shall create some 

    28 

    29     string variables by:

    29 	Lets get started by opening ipython interpreter.

    30 

    30 	We shall create some 

    31     a = 'This is a string'

    31 	a string by typing 

    32     print a

    32 

    33     type(a) shows it is 'str'

    33 	a = open single quote 'This is a string' close single quote

    34     b = "This too!"

    34     print a 

    35     print b

    35     a contains the string

    36 

    36 	we can check for datatype of a by using type(a) and shows it is 'str'

    37     They could either be enclosed in single or double quotes. There is

    37 

    38     also a special type of string enclosed in triple single or double

    38 	consider the case when string contains single quote.

    39     quotes. 

    39 	for example I'll be back

    40 

    40 	to store these kind of strings, we use double quotes

    41     c = '''This one too!'''

    41 	type 

    42     print c

    42 	b = open double quote "I'll be back" close double quote

    43     d = """And one more."""

    43 	print b ptints the value

    44     print d

    44 

    45 

    45 	IN python, anything enlosed in quotes is a string. Does not matter 

    46     These are special type of strings, called docstrings, which shall

    46 	if they are single quotes or double quotes.

    47     be discussed along with functions. 

    47 

    48     

    48 	There is

    49     Like lists and arrays, which we have already seen, string elements 

    49 	also a special type of string enclosed in triple single quotes or triple double

    50     can also be accessed with their indexes. The indexing here, also, 

    50 	quotes. 

    51     begins from 0. 

    51 

    52 

    52 	so when you do 

    53     print a[0] gives us 'T'

    53 	c = '''Iam also a string'''

    54     print a[5] gives us 'i' which is 6th character.

    54 	print c

    55 

    55     and c is also string variable

    56     To access the last element, we can use -1 as the index!

    56 	and even 

    57     print a[-1]

    57 	d = """And one more."""

    58     Similarly, we could access other elements with corresponding -ve

    58 	print d 

    59     indexes. This is a very handy feature of python. 

    59     d is also a string

    60 

    60 

    61     The len function, which we used with lists and arrays, works with

    61 	These strings enclosed in triple quotes are special type of strings, called docstrings, and they shall 

    62     strings too. 

    62 	be discussed in detail along with functions

    63     len(a)

    63 

    64 

    64 	We know elements in lists and arrays can be accessed with indices. 

    65     Python's strings support the operations + and *

    65 	similarly string elements 

    66     + concatenates two strings.

    66 	can also be accessed with their indexes. and here also, indexing starts from 0

    67     a + b

    67 

    68     and * is used for replicating a string for given number of times.

    68 	so

    69     a * 4

    69 	print a[0] gives us 'T' which is the first character

    70     What do you think would happen when you do a * a?

    70 	print a[5] gives us 'i' which is 6th character.

    71     It's obviously an error since, it doesn't make any logical sense. 

    71 

    72     

    72 	The len function, which we used with lists and arrays, works with

    73     One thing to note about strings, is that they are immutable, that

    73 	strings too. 

    74     is 

    74 	len(a) gives us the length of string a

    75     a[0] = 't'

    75 

    76     throws an error

    76 	Python's strings support the + and * operations 

    77     

    77 	+ concatenates two strings.

    78     Then how does one go about changing strings? Python provides

    78 	so a + b gives us the two srtings concatenated

    79     'methods' for doing various manipulations on strings. For example - 

    79 	and * is used for replicating a string for given number of times.

    80 

    80 	so a * 4 gives us a replicated 4 times

    81     a.upper() returns a string with all letters capitalized.

    81 

    82 

    82 	What do you think would happen when you do a * a?

    83     and a.lower() returns a string with all smaller case letters.

    83 	It's obviously an error since, it doesn't make any logical sense. 

    84 

    84 

    85     a.startswith('Thi')

    85 	One thing to note about strings, is that they are immutable, which means when yo do 

    86     returns True if the string starts with the argument passed. 

    86 	a[0] = 't'it throws an error

    87 

    87 

    88     similarly there's endswith

    88 	Then how does one go about doing strings manipulations. Python provides

    89     a.endswith('ING')

    89 	'methods' for doing various manipulations on strings. For example - 

    90 

    90 

    91     We've seen the use of split function in the previous

    91 	a.upper() returns a string with all letters capitalized.

    92     tutorials. split returns a list after splitting the string on the

    92 

    93     given argument. 

    93 	and a.lower() returns a string with all smaller case letters.

    94     alist = a.split()

    94 

    95     will give list with four elements.

    95 	there are many other methods available and we shall use Ipython auto suggestion feature to find out

    96     print alist

    96 

    97 

    97 	type a. and hit tab

    98     Python also has a 'join' function, which does the opposite of what

    98 	we can see there are many methods available in python for string manipulation

    99     split does. 

    99 

   100     ' '.join(alist) will return the original string a. 

   100 	lets us try startswith

   101     This function takes list of elements(in our case alist) to be joined.

   101 	a.startswith('Thi')

   102     '-'.join(alist) will return a string with the spaces in the string

   102 	returns True if the string starts with the argument passed. 

   103     'a' replaced with hyphens. 

   103 

   104     

   104 	similarly there's endswith

   105     At times we want our output or message in a particular

   105 	a.endswith('ING')

   106     format with variables embedded, something like printf in C. For 

   106 

   107     those situations python provides a provision. First lets create some 

   107 	We've seen the use of split function in the previous

   108     variables say

   108 	tutorials. split returns a list after splitting the string on the

   109     * formatting - printf style *

   109 	given argument. 

   110       In []: x, y = 1, 1.234

   110 	alist = a.split()

   111       

   111 	will give list with four elements.

   112       In []: print 'x is %s, y is %s' %(x, y)

   112 	print alist

   113       Out[]: 'x is 1, y is 1.234'

   113 

   114       Here %s means string, you can also try %d or %f for integer and 

   114 	Python also has a 'join' function, which does the opposite of what

   115       float values respectively.

   115 	split does. 

   116     * formatting - printf style *

   116 	' '.join(alist) will return the original string a. 

   117 

   117 	This function takes list of elements(in our case alist) to be joined.

   118 

   118 	'-'.join(alist) will return a string with the spaces in the string

   119     Now we shall look at simple input from and output to the

   119 	'a' replaced with hyphens. 

   120     console. 

   120 

   121     The raw_input function allows us to give input from the console. 

   121 	please note that after all these operations, the original string is not changed.

   122     a = raw_input()

   122 	and print a prints the original string

   123     it is now waiting for the user input. 

   123 

   124     5

   124 	At times we want our output or message in a particular

   125     a

   125 	format with variables embedded, something like printf in C. For 

   126     raw_input also allows us to give a prompt string, as shown 

   126 	those situations python provides a provision. First lets create some 

   127     a = raw_input("Enter a value: ")

   127 	variables say

   128     Enter a value: 5

   128 

   129     Note that a, is now a string variable and not an integer. 

   129 	In []: x, y = 1, 1.234

   130     type(a)

   130 

   131     we could use type conversion similar to that shown in the tutorial

   131 	In []: print 'x is %s, y is %s' %(x, y)

   132     on numeric datatypes. 

   132 	Out[]: 'x is 1, y is 1.234'

   133     a = int(a)

   133 	Here %s means string, you can also try %d or %f for integer and 

   134     a has now been converted to an integer. 

   134 	float values respectively.

   135     type(a)

   135 	* formatting - printf style *

   136 

   136 

   137     For console output, we use print which is pretty straightforward. 

   137 	we have seen how to output data

   138     We shall look at a subtle feature of the print statement. 

   138 	Now we shall look at taking input from the console.

   139     We shall first put the following code snippet in the file

   139 

   140     "hello1.py"

   140 	The raw_input function allows us to take input from the console. 

   141     print "Hello"

   141 	type a = raw_input() and hit enter

   142     print "World"

   142 	now python is waiting for input

   143     We save the file and run it from the ipython interpreter. Make

   143 	type 5 and hit enter

   144     sure you navigate to the place, where you have saved it. 

   144 

   145     %run -i hello1.py

   145 	we can check for the value of a by typing print a and we see that it is 5

   146 

   146 

   147     Now we make a small change to the code snippet and save it in the

   147 	raw_input also allows us to give a prompt string.

   148     file named "hello2.py"

   148 	we type 

   149     print "Hello", 

   149 	a = raw_input("Enter a value: ")

   150     print "World"

   150 	and we see that the string given as argument is prompted at the user.

   151     We now run this file, from the ipython interpreter. 

   151 	5

   152     %run -i hello2.py

   152 	Note that a, is now a string variable and not an integer. 

   153     Note the difference in the output of the two files that we

   153 	type(a)

   154     executed. The comma adds a space at the end of the line, instead

   154 	raw_input takes input only as a string

   155     of a new line character that is normally added. 

   155 

   156 

   156 	we cannot do mathematical operations on it

   157     Before we wind up, a couple of miscellaneous things. 

   157     but we can use type conversion similar to that shown in previous tutorial

   158     As you may have already noticed, Python is a dynamically typed

   158 

   159     language, that is you don't have to specify the type of a variable

   159 	b = int(a)

   160     when using a new one. You don't have to do anything special, to 'reuse'

   160 	a has now been converted to an integer and stored in b

   161     a variable that was of int type as a float or string. 

   161 	type(b) gives int

   162     

   162     b can be used here for mathematical operations.

   163     a = 1 here a is integer

   163 

   164     a = 1.1 now a float

   164 	For console output, we have been using print which is pretty straightforward. 

   165     a = "Now I am a string!"

   165 

   166 

   166 	We shall look at a subtle feature of the print statement. 

   167     Comments in Python start with a pound or hash sign. Anything after

   167 

   168     a #, until the end of the line is considered a comment, except of

   168 	Open scite editor and type

   169     course, if the hash is in a string. 

   169 	print "Hello"

   170     a = 1 # in-line comments

   170 	print "World"

   171     # a comment line

   171 	We save the file as hello1.py run it from the ipython interpreter. Make

   172     a = "# not a comment"

   172 	sure you navigate to the place, where you have saved it. 

   173 

   173 	%run hello1.py

   174     we come to the end of this tutorial on strings introduction of Data types in

   174 

   175     Python. In this tutorial we have learnt what are supported operations and 

   175 	Now we make a small change to the code snippet and save it in the

   176     performing simple IO operations in Python.

   176 	file named "hello2.py"

   177 	print "Hello", 

   178 	print "World"

   179 	We now run this file, from the ipython interpreter. 

   180 	%run hello2.py

   181 

   182 

   183 	Note the difference in the output.

   184     The comma adds a space at the end of the line, instead

   185 	of a new line character that is normally added. 

   186 

   187 	Before we wind up, a couple of miscellaneous things. 

   188 	As you may have already noticed, Python is a dynamically typed

   189 	language, that is you don't have to specify the type of a variable

   190 	when using a new one. You don't have to do anything special, to 'reuse'

   191 	a variable that was of int type as a float or string. 

   192 

   193 	a = 1 and here a is integer

   194 	lets store a float value in a by doing 

   195     a = 1.1

   196     and print a

   197     now a is float

   198 	a = "Now I am a string!"

   199 

   200 	Comments in Python start with a pound or hash sign. Anything after

   201 	a #, until the end of the line is considered a comment, except of

   202 	course, if the hash is in a string. 

   203 	a = 1 # in-line comments

   204 

   205 	pritn a and we see that comment is not a part of variable a

   206 

   207 	a = "# not a comment"

   208 

   209 	we come to the end of this tutorial on strings 

   210         In this tutorial we have learnt what are supported operations on strings 

   211 	and how to perform simple Input and Output operations in Python.