ktu python programming lab - Dr Binu V P

While a program is running, its data is in main memory. When the program ends, or the computer shuts down, data in memory disappears. To store data permanently, you have to put it in a file. Files are usually stored on a secondary storage device(hard disk, pen drive, DVD,CD etc).  When there are a large number of files, they are often organized into directories (also called “folders”). Each file is identified by a unique name, or a combination of a file name and a directory name.  By reading and writing files, programs can exchange information with each other and generate printable formats like PDF. Working with files is a lot like working with books. To use a book, you have to open it. When you’re done, you have to close it. While the book is open, you can either write in it or read from it. In either case, you know where you are in the book. Most of the time, you read the whole book in its natural order, but you can also skip around. All of this applies to files as well. 

Recursion is a method of solving problems that involves breaking a problem down into smaller and smaller sub problems until you get to a small enough problem that it can be solved trivially. Usually recursion involves a function calling itself. While it may not seem like much on the surface, recursion allows us to write elegant solutions to problems that may otherwise be very difficult to program. It is legal for one function to call another, and you have seen several examples of that. It is also legal for a function to call itself. It may not be obvious why that is a good thing, but it turns out to be one of the most magical and interesting things a program can do. For example, look at the following function: def countdown(n):   if n == 0:     print "Blastoff!"  else:     print n countdown(n-1) A call to this function countdown(5) will print 5 4 3 2 1 Blastoff! Advantages of recursion Recursive functions make the code look clean and elegant. A complex

So far we have only seen the functions which come with Python either in some file (module) or in interpreter itself (built in), but it is also possible for programmer to write  their own function(s) and are called User Defined Functions . These functions can then be combined to form a module which can then be used in other programs by importing them. In the context of programming, a function is a named sequence of statements that performs a desired operation. This operation is specified in a function definition. To define a function keyword def is used. After the keyword comes an identifier i.e.name of the function, followed by parenthesized list of parameters and the colon which ends up the line. Next follows the block of statement(s) that are the part of function. A block is one or more lines of code, grouped together so that they are treated as one big sequence of statements while executing. In Python, statements in a block are written  with indentation. Usually, a bloc