0_Variables_Types.md

Python 0: Variable Types (with focus on String and String methods)

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Based on lecture materials by Mike Jackson and Stephen McGough

Modified by Karin Lagesen

There are many different kinds of programming languages. The language we are teaching you today is python, which is an interpreted language. This means that we can write in code and it will be executed right away.

You can deal with python in two ways, either directly in an interpreter, or you can write bits of code in a file and have that run by the interpreter for you.

NOTE: a line anywere in python that is preceeded with a # is a comment and is ignored by python!

The python interpreter

Let's bring up the Python interpreter. This is what it looks like on cygwin - linux is pretty similar:

Python 2.7.3 (default, Dec 18 2012, 13:50:09)
[GCC 4.5.3] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

See the three >? You type in your commands after that.

The interpreter can be used as a calculator:

print 2 + 2
print 2*8

We can also work with strings in python:

print "Hello world!"
print 'Ada ' + ' Lovelace'

You can see that these are actually strings and part of the python language in that they have quotation marks around them.

As you can see, strings can actually be added together.

Please note that there is a space before Lovelace in the last instruction. Without that one, python would have returned 'AdaLovelace' instead.

Assigning values to variables

In a programming situation, we usually have data that we want to do something to. We want to be able to work on the data without having to specify it directly, like we did with the numbers and "Hello World" above. Instead of specifying the data directly, we can use variables instead. Variables contains data that we can do things to. Many languages require you to specify or declare what kind the data a variable should contain should be, like strings or numbers. Python does not require this, we can use them directly.

Python creates variables when we assign a value to them:

a = 1
print a
a = 'pigeon'
print a
a = 2 + 7 
print a

Variables in Python are not returned to you by the interpreter, as you can see. They can be thought of as being just names. However, beware that Python does not assume default variables - you have to assign them before use:

print b

What happens when we try to "combine" two Python variables: string and integer:

string = "text"
number = 4
print string * number 

Here you see that the word is repeated four times, as many times as number

However, what happens if we add string and number?

print string + number

Python has what we call dynamic and strong typing - dynamic in that you do not have to declare variables, and strong in that it objects if you try to do something to a variable that the data type in that variable does not permit.

Type conversion

Sometimes a number can be interpreted as a text string when you need it to work as a number. This can be fixed by converting it to an integer (a whole number). We can also convert to decimal numbers - floats. These have a decimal point in them.

print int('2') + 3
print '2' + str('3')
print string + str(number)

Python supports typical (and popular) arithmetic operations: Adding:

print 10 + 3

Integer division:

print 10 / 3

Remainder:

print 10 % 3

Note: python 2x does integer division, i.e. it floors the results.

String operations

Python provides a number of very useful methods for string variables. We can check the string length:

a = 'Krakow'
print len(a)

We can also split the string - splitting at space:

a = 'Ada Lovelace'
b = a.split()
print b

Or by any other character:

a = 'Ada-Lovelace'
b = a.split('-')
print b

In order to list all Python built-in methods for string:

dir(str)

Next: Data Types