--- a/strings.org Thu Apr 22 13:08:06 2010 +0530
+++ b/strings.org Mon Apr 26 10:32:04 2010 +0530
@@ -17,162 +17,197 @@
******* lists
******* writing to files
*** Script
- Welcome friends.
-
- In the previous tutorial we have looked at data types for dealing
- with numbers. In this tutorial we shall look at strings. We shall
- look at how to do elementary string manipulation, and simple input
- and output operations.
-
- In Python anything enclosed within quotes is a string. Lets get
- started by starting ipython interpreter. We shall create some
- string variables by:
+ Welcome friends.
+
+ In the previous tutorial we have looked at data types for dealing
+ with numbers. In this tutorial we shall look at strings. We shall
+ look at how to do elementary string manipulation, and simple input
+ and output operations.
+
+ In this tuotrial we shall use concepts of writing python scripts and basics of lists that have been covered in previous session
+
+ Lets get started by opening ipython interpreter.
+ We shall create some
+ a string by typing
- a = 'This is a string'
- print a
- type(a) shows it is 'str'
- b = "This too!"
- print b
+ a = open single quote 'This is a string' close single quote
+ print a
+ a contains the string
+ we can check for datatype of a by using type(a) and shows it is 'str'
- They could either be enclosed in single or double quotes. There is
- also a special type of string enclosed in triple single or double
- quotes.
+ consider the case when string contains single quote.
+ for example I'll be back
+ to store these kind of strings, we use double quotes
+ type
+ b = open double quote "I'll be back" close double quote
+ print b ptints the value
+
+ IN python, anything enlosed in quotes is a string. Does not matter
+ if they are single quotes or double quotes.
+
+ There is
+ also a special type of string enclosed in triple single quotes or triple double
+ quotes.
- c = '''This one too!'''
- print c
- d = """And one more."""
- print d
+ so when you do
+ c = '''Iam also a string'''
+ print c
+ and c is also string variable
+ and even
+ d = """And one more."""
+ print d
+ d is also a string
+
+ These strings enclosed in triple quotes are special type of strings, called docstrings, and they shall
+ be discussed in detail along with functions
- These are special type of strings, called docstrings, which shall
- be discussed along with functions.
-
- Like lists and arrays, which we have already seen, string elements
- can also be accessed with their indexes. The indexing here, also,
- begins from 0.
+ We know elements in lists and arrays can be accessed with indices.
+ similarly string elements
+ can also be accessed with their indexes. and here also, indexing starts from 0
+
+ so
+ print a[0] gives us 'T' which is the first character
+ print a[5] gives us 'i' which is 6th character.
+
+ The len function, which we used with lists and arrays, works with
+ strings too.
+ len(a) gives us the length of string a
- print a[0] gives us 'T'
- print a[5] gives us 'i' which is 6th character.
+ Python's strings support the + and * operations
+ + concatenates two strings.
+ so a + b gives us the two srtings concatenated
+ and * is used for replicating a string for given number of times.
+ so a * 4 gives us a replicated 4 times
+
+ What do you think would happen when you do a * a?
+ It's obviously an error since, it doesn't make any logical sense.
+
+ One thing to note about strings, is that they are immutable, which means when yo do
+ a[0] = 't'it throws an error
- To access the last element, we can use -1 as the index!
- print a[-1]
- Similarly, we could access other elements with corresponding -ve
- indexes. This is a very handy feature of python.
+ Then how does one go about doing strings manipulations. Python provides
+ 'methods' for doing various manipulations on strings. For example -
+
+ a.upper() returns a string with all letters capitalized.
- The len function, which we used with lists and arrays, works with
- strings too.
- len(a)
+ and a.lower() returns a string with all smaller case letters.
+
+ there are many other methods available and we shall use Ipython auto suggestion feature to find out
+
+ type a. and hit tab
+ we can see there are many methods available in python for string manipulation
- Python's strings support the operations + and *
- + concatenates two strings.
- a + b
- and * is used for replicating a string for given number of times.
- a * 4
- What do you think would happen when you do a * a?
- It's obviously an error since, it doesn't make any logical sense.
-
- One thing to note about strings, is that they are immutable, that
- is
- a[0] = 't'
- throws an error
-
- Then how does one go about changing strings? Python provides
- 'methods' for doing various manipulations on strings. For example -
+ lets us try startswith
+ a.startswith('Thi')
+ returns True if the string starts with the argument passed.
+
+ similarly there's endswith
+ a.endswith('ING')
+
+ We've seen the use of split function in the previous
+ tutorials. split returns a list after splitting the string on the
+ given argument.
+ alist = a.split()
+ will give list with four elements.
+ print alist
- a.upper() returns a string with all letters capitalized.
+ Python also has a 'join' function, which does the opposite of what
+ split does.
+ ' '.join(alist) will return the original string a.
+ This function takes list of elements(in our case alist) to be joined.
+ '-'.join(alist) will return a string with the spaces in the string
+ 'a' replaced with hyphens.
- and a.lower() returns a string with all smaller case letters.
+ please note that after all these operations, the original string is not changed.
+ and print a prints the original string
- a.startswith('Thi')
- returns True if the string starts with the argument passed.
+ At times we want our output or message in a particular
+ format with variables embedded, something like printf in C. For
+ those situations python provides a provision. First lets create some
+ variables say
+
+ In []: x, y = 1, 1.234
- similarly there's endswith
- a.endswith('ING')
+ In []: print 'x is %s, y is %s' %(x, y)
+ Out[]: 'x is 1, y is 1.234'
+ Here %s means string, you can also try %d or %f for integer and
+ float values respectively.
+ * formatting - printf style *
+
+ we have seen how to output data
+ Now we shall look at taking input from the console.
- We've seen the use of split function in the previous
- tutorials. split returns a list after splitting the string on the
- given argument.
- alist = a.split()
- will give list with four elements.
- print alist
+ The raw_input function allows us to take input from the console.
+ type a = raw_input() and hit enter
+ now python is waiting for input
+ type 5 and hit enter
+
+ we can check for the value of a by typing print a and we see that it is 5
+
+ raw_input also allows us to give a prompt string.
+ we type
+ a = raw_input("Enter a value: ")
+ and we see that the string given as argument is prompted at the user.
+ 5
+ Note that a, is now a string variable and not an integer.
+ type(a)
+ raw_input takes input only as a string
+
+ we cannot do mathematical operations on it
+ but we can use type conversion similar to that shown in previous tutorial
- Python also has a 'join' function, which does the opposite of what
- split does.
- ' '.join(alist) will return the original string a.
- This function takes list of elements(in our case alist) to be joined.
- '-'.join(alist) will return a string with the spaces in the string
- 'a' replaced with hyphens.
-
- At times we want our output or message in a particular
- format with variables embedded, something like printf in C. For
- those situations python provides a provision. First lets create some
- variables say
- * formatting - printf style *
- In []: x, y = 1, 1.234
-
- In []: print 'x is %s, y is %s' %(x, y)
- Out[]: 'x is 1, y is 1.234'
- Here %s means string, you can also try %d or %f for integer and
- float values respectively.
- * formatting - printf style *
+ b = int(a)
+ a has now been converted to an integer and stored in b
+ type(b) gives int
+ b can be used here for mathematical operations.
+
+ For console output, we have been using print which is pretty straightforward.
+
+ We shall look at a subtle feature of the print statement.
+
+ Open scite editor and type
+ print "Hello"
+ print "World"
+ We save the file as hello1.py run it from the ipython interpreter. Make
+ sure you navigate to the place, where you have saved it.
+ %run hello1.py
+
+ Now we make a small change to the code snippet and save it in the
+ file named "hello2.py"
+ print "Hello",
+ print "World"
+ We now run this file, from the ipython interpreter.
+ %run hello2.py
- Now we shall look at simple input from and output to the
- console.
- The raw_input function allows us to give input from the console.
- a = raw_input()
- it is now waiting for the user input.
- 5
- a
- raw_input also allows us to give a prompt string, as shown
- a = raw_input("Enter a value: ")
- Enter a value: 5
- Note that a, is now a string variable and not an integer.
- type(a)
- we could use type conversion similar to that shown in the tutorial
- on numeric datatypes.
- a = int(a)
- a has now been converted to an integer.
- type(a)
+ Note the difference in the output.
+ The comma adds a space at the end of the line, instead
+ of a new line character that is normally added.
- For console output, we use print which is pretty straightforward.
- We shall look at a subtle feature of the print statement.
- We shall first put the following code snippet in the file
- "hello1.py"
- print "Hello"
- print "World"
- We save the file and run it from the ipython interpreter. Make
- sure you navigate to the place, where you have saved it.
- %run -i hello1.py
+ Before we wind up, a couple of miscellaneous things.
+ As you may have already noticed, Python is a dynamically typed
+ language, that is you don't have to specify the type of a variable
+ when using a new one. You don't have to do anything special, to 'reuse'
+ a variable that was of int type as a float or string.
- Now we make a small change to the code snippet and save it in the
- file named "hello2.py"
- print "Hello",
- print "World"
- We now run this file, from the ipython interpreter.
- %run -i hello2.py
- Note the difference in the output of the two files that we
- executed. The comma adds a space at the end of the line, instead
- of a new line character that is normally added.
+ a = 1 and here a is integer
+ lets store a float value in a by doing
+ a = 1.1
+ and print a
+ now a is float
+ a = "Now I am a string!"
- Before we wind up, a couple of miscellaneous things.
- As you may have already noticed, Python is a dynamically typed
- language, that is you don't have to specify the type of a variable
- when using a new one. You don't have to do anything special, to 'reuse'
- a variable that was of int type as a float or string.
-
- a = 1 here a is integer
- a = 1.1 now a float
- a = "Now I am a string!"
+ Comments in Python start with a pound or hash sign. Anything after
+ a #, until the end of the line is considered a comment, except of
+ course, if the hash is in a string.
+ a = 1 # in-line comments
- Comments in Python start with a pound or hash sign. Anything after
- a #, until the end of the line is considered a comment, except of
- course, if the hash is in a string.
- a = 1 # in-line comments
- # a comment line
- a = "# not a comment"
+ pritn a and we see that comment is not a part of variable a
+
+ a = "# not a comment"
- we come to the end of this tutorial on strings introduction of Data types in
- Python. In this tutorial we have learnt what are supported operations and
- performing simple IO operations in Python.
+ we come to the end of this tutorial on strings
+ In this tutorial we have learnt what are supported operations on strings
+ and how to perform simple Input and Output operations in Python.
*** Notes