>>>Authors = [Medhamsh, [email protected]](Mail suggestions, typos please)
Type 'python' at your shell and that should take you to the screen like this:
[email protected]:~$python Python 2.7.2 (default, Jun 12 2011, 03:11:18) [GCC 4.6.0 20110603 (prerelease)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>>
That ">>>" is called a “prompt”, which means it’s something the computer displays to tell you it’s ready for your instructions. You can type things into that window, and the computer will obey them.
What if we want to find out the sum of two and three?
Try the following at your python shell!
>>> Tell me the sum of two and three
Unfortunately the computer does not understand english! And we have to talk to it in special language. That special language here being python, fortunately is pretty easy for humans as well!!!
So, try it out like this: Type 2+3 and hit enter.
>>> 2+3 5 >>>
As python is an interpreted programming language, you will have the results as you just hit on enter. Try various mathematical calculations.
Try using python as your pocket calculator!
Now try this. 1+2*3-4
>>> 1+2*3-4 3 >>>
Is that what you expected? If you expected 5, try getting the output. You can use all sorts of paranthesis.
>>> (3+4)*(4+7) 77 >>>
Incidentally, if you’re still confused about the fact that 1+2*3-4 gives 3 and not 5, the reason is that “multiplication happens before addition”. Your maths teacher at school probably calls this BODMAS or something similar.
Now, try this. 8/6
>>> 8/6 1 >>>
Did you expect it to be 1.3333333? Then you should try it like this: 8.0/6.0
>>> 8.0/6.0 1.3333333333333333 >>>
Did you see the magical output?
This is how python treats numbers and how mathematical operations are performed.
Don’t be afraid to experiment. Whenever you learn something new that you can do with Python, try making slight changes (or bigger changes!) and play around until you’re confident that you understand just what’s going on. Don’t limit yourself to what’s actually printed here.
Till now we have played with python as your pocket calculator. Apart from that python can handle many things.
For Instance: Lets consider the hello world program.
>>> 'hello, ' + 'world' 'hello, world' >>>
Things between quotation marks are called “strings”. As you might guess from the lines above, you can apply + to strings as well as to numbers. It “concatenates” strings; that is, puts one immediately after the other.
Try this: 3 * 'hella '
>>>3*'hella ' 'hella hella hella ' >>>
You can surround strings in either single quotes or double quotes; Python doesn’t mind.
>>>'Doll' + "ar" Dollar >>>
Why would this(' , “) all matter?? Try printing: I'm Sorry!
Giving names to things!
Suppose you know that you’re going to need to do a lot of calculations involving the number 123456. (Maybe it’s your annual salary in pounds, or something.) You could just type the number in every time:
>>> 123456*3 370368 >>> 123456/6 20576 >>> 123456-1000 122456
This might get very boring after a while. And if anyone else wanted to read what you were doing, they might be confused by the mysterious number 123456 and wonder why it appeared so often. We can solve either of these problems by giving the number a name. To save typing, give it a short name, like n (short for “number”, maybe). To make it more obvious what it means, give it a longer name, like salary. Here’s how we do that.
>>> salary=123456 >>> salary*4 493824 >>> salary/12 10288 >>> salary 123456
What we’ve called “names”, most people call “variables”. You’ll find out later why they’re called that.For now, “names” is fine.
Python also has lists!
>>> [1,2,3]+[7,8] [1, 2, 3, 7, 8] >>>
Methods of Lists
The list methods make it very easy to use a list as a stack, where the last element added is the first element retrieved (“last-in, first-out”). To add an item to the top of the stack, use append(). To retrieve an item from the top of the stack, use pop() without an explicit index.
>>> stack = [3, 4, 5] >>> stack.append(6) >>> stack.append(7) >>> stack [3, 4, 5, 6, 7] >>> stack.pop() 7 >>> stack [3, 4, 5, 6] >>> stack.pop() 6 >>> stack.pop() 5 >>> stack [3, 4]
So far we have done what your pocke calculator could do in math. Here's something your pocket calculator isn't so good at:
>>> for x in 1,2,3,4,5: ... print x,x*x ...
The prompt changes to ... from >>>.It tells you that Python is expecting more. Just press Enter.
This is basically what people call as a 'for loop'.
Sometimes one list isn’t enough. Suppose you wanted to keep a list of what you had eaten for breakfast; it’s easy enough to write
>>> breakfast = [’idly’, ’puri’, ’dosa’, ’chapathi’]
and carry on. But suppose you wanted to list what you had eaten for breakfast every day, and you don’t always eat the same thing. What would we do?
Fortunately, Python is very helpful about this. Remember that we said earlier that lists were just collections of things. Well, lists are things too, so making lists of lists is just like making lists of anything else!
>>>breakfast = [ ... 'Monday', ['idly', 'coffee'], ... 'Tuesday', ['dosa', 'juice', 'chutney'], ... 'Wednesday', ['coffee'] ] >>> breakfast ['dosa', 'juice', 'chutney'] >>> breakfast ['idly', 'coffee'] >>> breakfast 'Monday' >>> breakfast 'Wednesday'
Observed what happened? Try several things like that! Numbers in the square brackets indicate the indices.
Some people call lists of lists “2-dimensional arrays” or “tables”, because you can write them out in rows and columns (two dimensions!) like a table. Unlike real tables (and many other computer languages), in Python you don’t have to have every row the same length.
This is the text. Copy the the text to a file and give the name of the file as fs-dataset.txt
Finding and matching strings.
'.' matches with any character
'*' repeats the previous expression zero or more times
a. matches with as, ab, arch etc ^ matches the begining of a line $ matches end of a line | alternation eg. H|h matches h or H () grouping eg. H|hello matches H or hello (H|h)ello matches Hello or hello \ escapes any meta character Mr. matches Mr. and Mrs. Mr\. matches only Mr.
Consider the following two strings
STRING1 Mozilla/4.0 (compatible; MSIE 5.0; Windows NT; DigExt) STRING2 Mozilla/4.75 [en](X11;U;Linux2.2.16-22 i586)
Consider the following searches and find out what pattern gets returned in both the above given strings.