1. IDLE Hints
To re-load a module after making changes:

2. Useful Pylab
savefig('filename.pdf')

3. Elements of Programming
CSMC 120: Visualizing Information
2/12/08

4. What’s in a Name?

5. Names are useful because...
Functions
Modules
Variables
Identifiers
Start with letter or underscore (“_”)
Follow with any sequence of letters, digits, or underscores
No spaces!
 give name to values

6. A rose by any other name... Case-sensitive
rose rose2 ROSE rOSE RoSe R_O_S_e Rose
Case-sensitive

7. Reserve Words Some identifiers are part of Python and del for is raise
assert
elif
from
lambda
return
break
else
global
not
try
class
except if or
while
continue
exec
import
pass
yield
def
finally in
print

8. Naming Values <VARIABLE> = <EXPRESSION> Variables
Designated using assignment statements
x = 5
speed = 53.1
myFavoriteFood = 'curry'
<VARIABLE> = <EXPRESSION>

9. Expressions >>> 7 >>> 5 + 2
Literal
>>> 7
>>> 5 + 2
>>> 3.0 / 1.7 * 25
>>> mean(x)
An expression is a Python command that returns a result when evaluated

10. Illuminating the Black Box

11. A peak inside the black box...
So, what exactly is a variable?
a space saver
a place to store an unknown (address)
can be used many times
>>> x = 4 4 5 x
>>> x = x + 1

12. Storage Space or Sticky Note?
Hold address of memory location where value is stored
Pointer
>>> x = 4 x 4
>>> x = x + 1 x 4 5

13. Parameters Parameters are a type of variable
>>> time = [10, 20, 30, 40]
>>> distance = [5, 17, 28, 35]
>>> plot(time, distance)
[10, 20, 30, 40]
[10, 20, 30, 40] x
time
[5, 17, 28, 35]
[5, 17, 28, 35]
distance y

14. Scope Region of the program in which a name has meaning
>>> def plus(x, y):
x = x + y
print x
>>> x = 5
>>> x = plus(x, 2) 7
>>> print x
>>> def plus(x, y):
x = x + y
print x
>>> x = 5
>>> plus(x, 2) 7
>>> print x 5

15. Values

16. Types Computers are “number crunchers”
A piece of information stored and manipulated by computer is a datum
Different types of data are stored and manipulated in different ways
e.g., 2, 3, 4  whole numbers (integers)
e.g., 2.7, 3.8, 4.9  real numbers (floats)
A data’s type determines:
operations that can be used
limits on size and storage of the value
use of value

17. Int: The Integers Whole Numbers: i.e., no fractional part Use when:1 50
11,234,139,514,510587,180,238,401
-75
Use when:
Representing counts
Value cannot be a fraction
Require efficiency or precision
Finite Range

18. The Limits of Int = Data stored in binary representation
24 / 2 = 8
Range = -8 to 7
1000
1111
1110
1101
1100
1011
1010
1001
0000
0001
0010
0011
0100
0101
0110
0111 -8 -7 -6 -5 -4 -3 -2 -1 1 2 3 4 5 6 7
Data stored in binary representation
1 1 = 3
0 1 1 = 3
= 3
Number of bits used depends on computers
32 bits  232 possible integer values
centered at 0, positive and negative
232/2 = 231
integers from -231 to
With 32 bits: 2,147,483,647 =

19. Long Ints >>> x = 4L Not fixed in size
Expands to accommodate value
Limit is amount of memory in computer
Ints that grow too large to be represented with 32 bits are automatically converted to long ints
Less efficient
>>> x = 4L

20. Floats Any literal that has a decimal point:
1.1
-1.0
573, 1.
Stores an approximation to real numbers
limit to precision
limit to accuracy

21. Useful Python type(<VALUE>) or type(<VARIABLE>)
>>> type(100L)
<type 'long int'>
>>> type(2.4)
<type 'float‘>

22. Playing with Numbers +, -, *, / are operators %: mod (remainder)
6 / 4 = 1
6 % 4 = 2
**: exponention
2**3 = 8
Operations performed on floats  floats
Operations performed on ints  ints
6.0 / 4.0 = 1.5

23. Type Conversion 231 + 1  converts int to long int
x = 5.0 / 2  mixed type expression
Python adds .0 to 2
Explicit type conversion
e.g., calculating an average of a set of integers:
average = sum / n
average = float(sum) / n
average = float(sum / n)
int(<VALUE>)
long(<VALUE>)

24. Booleans Used for logical expressions 2 values: hold(1) hold(True)
True or False
hold(1) hold(True)

25. Strings Text Sequence of characters Deliminated by single quotes
'my text'

26. Python Power: Creating Your Own Types

27. A Point Defined by two pieces of data: x and y
(x2,y2)
(x,y)
def color(x, y, c):
def label(x, y, l):
def move(x, y, a, b):
(x1,y1)
Defined by two pieces of data: x and y
Methods: color, label, move

28. Object Oriented Programming
A Point x y
color(c):
label(l):
move(a, b): p
Point p3
Object p
Object Oriented Programming p2
Defined by two pieces of data: x and y
Methods: color, label, move

29. Object Oriented Programming
Traditional Programming
Data are passive
Manipulated and combined via active operations
Objects are active types that combine data and operations
know stuff
do stuff

30. Classes Our point is an example of a class
a construct used to define objects
abstractions
A value whose type is an object is an instance of the class that defines the object

31. Python Demystified plot(x,y) is an expression
>>> 5 + 2 7
When an expression is evaluated and not assigned, Python outputs the result of the expression to the screen
>>> plot(Year, AnyDrug)
[<matplotlib.lines.2D instance at 0x01A0A788>]
plot(x,y) is an expression
The result it returns is a pointer to an object
Python has created the defining elements of a figure and stored it at a specific address in memory
>>> myplot = plot(Year, AnyDrug)

32. Useful Pylab >>> myplot = plot(Year, AnyDrug)
>>> setp(myplot, linestyle = '--')
>>> x = xlabel('Year')
>>> setp(x, color = 'r')

33. Lines.2D An object defining a two-dimensional line plot Data
Dimensions
Line Style
Marker Style
Values being plotted
Labels
Axes Scales
Background Format
Operations
Sizing
Zooming
Labeling
Coloring
Displaying
Scaling
Copying

34. Using Classes <OBJECT>.<METHOD>(<PARAMETERS>)
To perform an operation on an object, we send the object a message
Messages to which the object responds are its methods
A method is invoked using dot-notation:
<OBJECT>.<METHOD>(<PARAMETERS>)
>>> p.move(a,b)

35. Create an Instance of an Object
Useful Pylab
Create an Instance of an Object
>>> fig = figure(1)
>>> fig.set_facecolor('g‘)

36. Useful Pylab figure(<#>): create a figure
>>> fig2 = figure(2)
>>> fig2.savefig('fig2.pdf')
figure(<#>): create a figure
.savefig(<FILENAME>)
.set_facecolor(<COLOR>)
.set_edgecolor(<COLOR>)
.set_frameon(<BOOLEAN>)
.set_figheight(<#>)
.set_figwidth(<#>)
.hold(<BOOLEAN>)

37. Useful Pylab >>> y = ylabel('fig2.pdf')
>>> y.set(color = 'r')