--- a/basic_python/strings_dicts.rst Thu Sep 17 14:33:08 2009 +0530
+++ b/basic_python/strings_dicts.rst Fri Sep 18 03:04:02 2009 +0530
@@ -31,3 +31,277 @@
String Formatting
=================
+String formatting can be performed using the string formatting operator represented
+as the percent (%) sign. The string placed before the % sign is formatted with
+the value placed to the right of it. Let us look at a simple example.
+
+::
+
+ >>> format = 'Hello %s, from PythonFreak'
+ >>> str1 = 'world!'
+ >>> print format % str1
+ Hello world!, from PythonFreak
+
+The %s parts of the format string are called the coversion specifiers. The coversion
+specifiers mark the places where the formatting has to be performed in a string.
+In the example the %s is replaced by the value of str1. More than one value can
+also be formatted at a time by specifying the values to be formatted using tuples
+and dictionaries (explained in later sections). Let us look at an example.
+
+::
+
+ >>> format = 'Hello %s, from %s'
+ >>> values = ('world!', 'PythonFreak')
+ >>> print format % values
+ Hello world!, from PythonFreak
+
+In this example it can be observed that the format string contains two conversion
+specifiers and they are formatted using the tuple of values as shown.
+
+The s in %s specifies that the value to be replaced is of type string. Values of
+other types can be specified as well such as integers and floats. Integers are
+specified as %d and floats as %f. The precision with which the integer or the
+float values are to be represented can also be specified using a **.** (**dot**)
+followed by the precision value.
+
+String Methods
+==============
+
+Similar to list methods, strings also have a rich set of methods to perform various
+operations on strings. Some of the most important and popular ones are presented
+in this section.
+
+**find**
+~~~~~~~~
+
+The **find** method is used to search for a substring within a given string. It
+returns the left most index of the first occurence of the substring. If the
+substring is not found in the string then it returns -1. Let us look at a few
+examples.
+
+::
+
+ >>> longstring = 'Hello world!, from PythonFreak'
+ >>> longstring.find('Python')
+ 19
+ >>> longstring.find('Perl')
+ -1
+
+**join**
+~~~~~~~~
+
+The **join** method is used to join the elements of a sequence. The sequence
+elements that are to be join ed should all be strings. Let us look at a few
+examples.
+
+::
+
+ >>> seq = ['With', 'great', 'power', 'comes', 'great', 'responsibility']
+ >>> sep = ' '
+ >>> sep.join(seq)
+ 'With great power comes great responsibility'
+ >>> sep = ',!'
+ >>> sep.join(seq)
+ 'With,!great,!power,!comes,!great,!responsibility'
+
+*Try this yourself*
+
+::
+
+ >>> seq = [12,34,56,78]
+ >>> sep.join(seq)
+
+**lower**
+~~~~~~~~~
+
+The **lower** method, as the name indicates, converts the entire text of a string
+to lower case. It is specially useful in cases where the programmers deal with case
+insensitive data. Let us look at a few examples.
+
+::
+
+ >>> sometext = 'Hello world!, from PythonFreak'
+ >>> sometext.lower()
+ 'hello world!, from pythonfreak'
+
+**replace**
+~~~~~~~~~~~
+
+The **replace** method replaces a substring with another substring within
+a given string and returns the new string. Let us look at an example.
+
+::
+
+ >>> sometext = 'Concise, precise and criticise is some of the words that end with ise'
+ >>> sometext.replace('is', 'are')
+ 'Concaree, precaree and criticaree are some of the words that end with aree'
+
+Observe here that all the occurences of the substring *is* have been replaced,
+even the *is* in *concise*, *precise* and *criticise* have been replaced.
+
+**split**
+~~~~~~~~~
+
+The **split** is one of the very important string methods. split is the opposite of the
+**join** method. It is used to split a string based on the argument passed as the
+delimiter. It returns a list of strings. By default when no argument is passed it
+splits with *space* (' ') as the delimiter. Let us look at an example.
+
+::
+
+ >>> grocerylist = 'butter, cucumber, beer(a grocery item??), wheatbread'
+ >>> grocerylist.split(',')
+ ['butter', ' cucumber', ' beer(a grocery item??)', ' wheatbread']
+ >>> grocerylist.split()
+ ['butter,', 'cucumber,', 'beer(a', 'grocery', 'item??),', 'wheatbread']
+
+Observe here that in the second case when the delimiter argument was not set
+**split** was done with *space* as the delimiter.
+
+**strip**
+~~~~~~~~~
+
+The **strip** method is used to remove or **strip** off any whitespaces that exist
+to the left and right of a string, but not the whitespaces within a string. Let
+us look at an example.
+
+::
+
+ >>> spacedtext = " Where's the text?? "
+ >>> spacedtext.strip()
+ "Where's the text??"
+
+Observe that the whitespaces between the words have not been removed.
+
+::
+
+ Note: Very important thing to note is that all the methods shown above do not
+ transform the source string. The source string still remains the same.
+ Remember that **strings are immutable**.
+
+
+I/O: Reading and Writing Files
+==============================
+
+Files are very important aspects when it comes to computing and programming.
+Up until now the focus has been on small programs that interacted with users
+through **input()** and **raw_input()**. Generally, for computational purposes
+it becomes necessary to handle files, which are usually large in size as well.
+This section focuses on basics of file handling.
+
+Opening Files
+~~~~~~~~~~~~~
+
+Files can be opened using the **open()** method. **open()** accepts 3 arguments
+out of which 2 are optional. Let us look at the syntax of **open()**:
+
+*f = open( filename, mode, buffering)*
+
+The *filename* is a compulsory argument while the *mode* and *buffering* are
+optional. The *filename* should be a string and it should be the complete path
+to the file to be opened (The path can be absolute or relative). Let us look at
+an example.
+
+::
+
+ >>> f = open ('basic_python/interim_assessment.rst')
+
+The *mode* argument specifies the mode in which the file has to be opened.
+The following are the valid mode arguments:
+
+**r** - Read mode
+**w** - Write mode
+**a** - Append mode
+**b** - Binary mode
+**+** - Read/Write mode
+
+The read mode opens the file as a read-only document. The write mode opens the
+file in the Write only mode. In the write mode, if the file existed prior to the
+opening, the previous contents of the file are erased. The append mode opens the
+file in the write mode but the previous contents of the file are not erased and
+the current data is appended onto the file.
+The binary and the read/write modes are special in the sense that they are added
+onto other modes. The read/write mode opens the file in the reading and writing
+mode combined. The binary mode can be used to open a files that do not contain
+text. Binary files such as images should be opened in the binary mode. Let us look
+at a few examples.
+
+::
+
+ >>> f = open ('basic_python/interim_assessment.rst', 'r')
+ >>> f = open ('armstrong.py', 'r+')
+
+The third argument to the **open()** method is the *buffering* argument. This takes
+a boolean value, *True* or *1* indicates that buffering has to be enabled on the file,
+that is the file is loaded on to the main memory and the changes made to the file are
+not immediately written to the disk. If the *buffering* argument is *0* or *False* the
+changes are directly written on to the disk immediately.
+
+Reading and Writing files
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+**write()**
+-----------
+
+**write()**, evidently, is used to write data onto a file. It takes the data to
+be written as the argument. The data can be a string, an integer, a float or any
+other datatype. In order to be able to write data onto a file, the file has to
+be opened in one of **w**, **a** or **+** modes.
+
+**read()**
+----------
+
+**read()** is used to read data from a file. It takes the number of bytes of data
+to be read as the argument. If nothing is specified by default it reads the entire
+contents from the current position to the end of file.
+
+Let us look at a few examples:
+
+::
+
+ >>> f = open ('randomtextfile', 'w')
+ >>> f.write('Hello all, this is PythonFreak. This is a random text file.')
+ >>> f = open ('../randomtextfile', 'r')
+ >>> f = open ('../randomtextfile', 'r')
+ >>> f.read(5)
+ 'Hello'
+ >>> f.read()
+ ' all, this is PythonFreak. This is a random text file.'
+ >>> f.close()
+
+**readline()**
+--------------
+
+**readline()** is used to read a file line by line. **readline()** reads a line
+of a file at a time. When an argument is passed to **readline()** it reads that
+many bytes from the current line.
+
+One other method to read a file line by line is using the **read()** and the
+**for** construct. Let us look at this block of code as an example.
+
+::
+
+ >>> f = open('../randomtextfile', 'r')
+ >>> for line in f:
+ ... print line
+ ...
+ Hello all!
+
+ This is PythonFreak on the second line.
+
+ This is a random text file on line 3
+
+**close()**
+-----------
+
+One must always close all the files that have been opened. Although, files opened
+will be closed automatically when the program ends. When files opened in read mode
+are not closed it might lead to uselessly locked sometimes. In case of files
+opened in the write mode it is more important to close the files. This is because,
+Python maybe using the file in the buffering mode and when the file is not closed
+the buffer maybe lost completely and the changes made to the file are lost forever.
+
+
+Dictionaries
+============
+