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-* Strings
-*** Outline
-***** Strings
-******* basic manipulation
-******* operations
-******* immutability
-******* string methods
-******* split and join
-******* formatting - printf style
-***** Simple IO
-******* raw_input
-******* console output
-***** Odds and Ends
-******* dynamic typing
-******* comments
-***** Arsenal Required
-******* 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 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 = 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'
-
- 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.
-
- 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
-
- 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
-
- 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
-
- 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.
-
- 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
-
- 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
-
- 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.
-
- please note that after all these operations, the original string is not changed.
- and print a prints the original string
-
- 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
-
- 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.
-
- 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
-
- 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
-
-
- 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.
-
- 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 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!"
-
- 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
-
- 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
- In this tutorial we have learnt what are supported operations on strings
- and how to perform simple Input and Output operations in Python.
-
-*** Notes