Fundamental Computing Concepts

Recursion

The formal definition of recursion is an algorithmic technique where a function, calls itself to perform part of a task. A good example of recursion is the factorial function n!. Recursive solutions have two main parts:

  • Base case
  • Recursive case

A recursion function pauses, and calls itself just like calling any other function. In this way the functions stack on each other, until reaching the base case. Then it returns a value from the base case and teh functions pop back to the original function call.

python example of recursive factorial function:

                def factorial( n ):
                    if n <1:   # base case
                        return 1
                    else:
                        return n * factorial( n - 1 )  # recursive call

Exception handling

A lot of things can go wrong when you write a program. For example, you can have the following exceptions errors in Python

  • IO Error - if a file cannot be opened
  • ImportError - If python cannot find a module
  • ValueError - Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value
  • KeyboardInterrupt - Raised when the user hits the interrupt key (normally Control-C or Delete)
  • EOFError - Raised when one of the built-in functions (input() or raw_input()) hits an end-of-file condition (EOF) without reading any data

To avoid these errors, you could put check in for different possibilities like os.path.isfile or os.path.exists. However trying to cover all possibilities could take quite a lot of time. Alternatively you could go ahead and try and deal with problems as they happen. This is exactly what a try statement does. The Syntax is similar to an if statement.

python example of an if statement

                try:
                    fin = open('bad_file')
                    for line in fin:
                        print line
                    fin.close()
                except:
                    print 'Something went wrong.'

Python begins by executing the try clause. If all goes well, it will skip the except clause and continue. However if something goes wrong, it will pause, jump out of the try clause and run the except clause. Handling an exception with a try statement is called catching an exception. Catching an exception gives us an opportunity to fix the problem, or try again, or at least end the program gracefully.

There are a couple of ways the try clause can be used. You may not have a except clause at all and use it as a brute force method to continue your program. You can use except Exception: to avoid catching system exceptions like SystemExit or KeyboardInterrupt Some code may use except: pass, however this may not be good practice. Generally when you find an error, you should do something to handle it i.e. write it in a log file . Further two principles of the Zen of Python are: Explicit is better than implicit and Errors should never pass silently.

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Tags: programming