2. Getting started with Python

The Python programming language was created by a Dutch Computer Scientist called Guido von Rossum. Unlike languages like C/C++/Java, Python is extremely friendly to beginners - famous Universities like MIT (Massachusetts Institute of Technology) use it to teach introductory programming courses. It is also a modern, powerful general purpose programming language used by companies and research institutions all over the world to solve complex computing problems.

2.1. First steps - use Python as a calculator!

In the previous chapter, you had seen how to interact with the GNU/Linux Operating System by typing commands. You can run Python by simply typing the command python. Here is how your screen would look like:

[ram@knuth ~]$ python
Python 2.6 (r26:66714, Mar 17 2009, 11:44:21)
[GCC 4.4.0 20090313 (Red Hat 4.4.0-0.26)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

Don’t worry if the details shown above do not match exactly with what you see on your computer. It is enough that the last line of the output matches what you see above exactly. The three greater than signs constitute what is called the python prompt. When Python displays the prompt, you know that it is ready to accept commands! Type 1 + 2, hit the Enter key and see what happens:

>>> 1 + 2
3
>>>

Note

In examples like the one above, you should not type the three greater than signs - it is automatically displayed by Python.

Note

If Python shows you some error (instead of printing 3), read the section titled “use of space character in Python” which comes at the end of this chapter.

After you hit Enter, Python displays the result 3 on the next line and displays the prompt once again. This is Python’s way of telling you that it is ready to accept more commands! Now you know that using Python is as easy as using a calculator.

Try a few more experiments:

>>> 3 * 2
6
>>> 3 - 2
1
>>> 3 / 2
1
>>> 3.0 / 2
1.5
>>> 2 ** 3
8
>>> 9 % 2
1
>>> 9 % 3
0
>>>

The star symbol performs multiplication and the double star performs exponentiation. The % symbol is the remainder operator. The only confusing result is the output from the division operation 3/2. This is explained in detail in the next section.

2.2. A few things about the way Python treats numbers

A number written like this:

456

is treated as an integer by Python. A number written like this:

456.01

is called a floating point number(or simply, a float). When division is performed, if both numerator and denominator are integers, the result will be truncated to an integer. So, when you divide 3 by 2, you get 1 and not 1.5. If you wish to get the correct answer, you have to make one (or both) of the numbers float.

Note

This behaviour (of division) is confusing and it has been corrected in a more recent version of Python.

You will encounter some other problems when dealing with floating point numbers; problems mostly concerning accuracy of the result. Here is a simple experiment you can try at the Python prompt:

>>> 1.5 + 1.1
2.6000000000000001
>>>

You may be surprised at not getting exactly 2.6 as the result. The reason for this behaviour is somewhat complex and its explanation is beyond the scope of this book. At this point, it is sufficient to understand that floating point arithmetic is tricky (in all programming languages, not just Python).

Note

Floating point numbers can be written in a different way - as an example, the number 0.00000001 may be written as 1e-8 (1 * 10 to the power of -8). Another example: 1234.0 can be written as 1.234e3 or 12.34e2 or 123.4e1.

2.3. Arithmetic using very large integers

Try the following experiment:

>>> 2 ** 320
21359870359209100823950217061695521146027045223566
52769947041607822219725780640550022962086936576L
>>>

Python can handle very large integer values. Note that the number shown above has an L at the end - this is Python’s way of saying that you are dealing with a long integer! (Python does not do any kind of approximation when representing such large numbers - the number you have seen above is the exact value of 2 to the power of 320).

2.4. Using variables

Just like math, Python too has variables. The following example shows a simple use of variables:

>>> x = 1
>>> y = 2
>>> x + y
3
>>>

Here, x and y are variables having values 1 and 2. A variable should always have a value assigned to it before it is used. In the above example, Python will generate an error if you try to do x + y + z:

>>> x + y + z
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'z' is not defined
>>>

Look at the last line - it says NameError: name ‘z’ is not defined - the problem is, you have not given a value to variable z (you need not bother with the other two lines in the error message - they can be safely ignored).

Mathematicians usually use single letter variable names like x, y, z, a, b etc. But Python has no problems with longer names:

>>> mark = 90
>>> age = 19

Here, mark and age are two variables just like x and y in the previous example. There are restrictions on the kind of words you can use as variables. For example, it’s OK to use the name twenty20 as a variable name, but you can’t use the name 20twenty. The rule is that variable names have to start with an uppercase or lowercase alphabet (an underscore symbol is also permitted, but digits like 0, 1, 2 etc are not).

Variable names are case-sensitive - for example, the names Abc and abc represent two different variables.

Once you assign a value to a variable, it remains unchanged as long as you do not assign a new value:

>>> x + y
3
>>> x = 10
>>> y = 20
>>> x + y
30
>>>

Initially, x and y have values 1 and 2 which are then changed to 10 and 20.

2.5. Mistakes made by beginners while using variables

• Not realizing that variable names are case-sensitive.

• If variable x has value 1, writing x + 1 does not change the value of x to 2:

  >>> x = 1
  >>> x + 1
  2
  >>> x
  1
  >>>
  

• If variable x has value 1, then writing x.1 does not give you 1.1. Variable names simply do not work that way!

The last mistake is not common, but I have seen students making it in the class!

2.6. Use of the space character in Python

Another common problem is regarding the use of space. If you wish Python to evaluate the expression 1+2, just type 1, followed by the ‘+’ symbol, followed by 2 and then hit Enter. If you hit the spacebar key before typing 1, you will get an error which looks like this:

>>>  1 + 2
File "<stdin>", line 1
 1 + 2
 ^
IndentationError: unexpected indent
>>>

It’s OK to type as many spaces as you wish between 1 and the ‘+’ symbol and between the ‘+’ symbol and 2, but if you type a space at the beginning of the line, that will create trouble. In a later chapter, you will see situations where you have to type one or more spaces at the beginning of a line.

2.7. Exiting Python

If you wish to stop using Python, simply type Ctrl-d (hold down the key labeled Ctrl on the keyboard and then type the letter d). All the variable assignments which you have made are lost when you exit Python.

Note

It is assumed that you are running Python on a GNU/Linux system. On a Windows system, you have to type Ctrl-z and then hit Enter.

2.8. Exercises

1. In mathematics, a Mersenne number is a positive integer that is one less than a power of 2.

   

   A Mersenne prime is a Mersenne number that is prime. As of October 2009, only 47 Mersenne primes have been discovered; the largest known prime number is a Mersenne prime.The 27th Mersenne prime (with 13395 digits in it) is obtained if you use 44497 as the value of p in the above equation. Use Python to find out the value of this number. (Interested students might wish to check out the Great Internet Mersenne Prime search on the net).

2. Find factorial of 20 using Python.

3. Wilson’s theorem states that the number (p - 1)! + 1 is divisible by p for all primes p. Also, if p is not a prime number, then (p - 1)! + 1 is not divisible by p. Try a few experiments in Python to demonstrate this theorem.

4. Fermat’s little theorem states that if p is a prime number, then for any integer a:

   

   is divisible by p. Use Python and test the theorem with some large prime numbers.

5. Find out what happens if you divide a number by zero in Python.