Sequences CMSC 120: Visualizing Information 2/26/08

Step by Step Algorithm: a set of sequential instructions that are followed to solve a problem Computer programs are algorithms that provide a set of sequential instructions to the CPU CPU processes instructions one at a time, in the order that they are received; i.e., sequentially

Graphics Window Step by Step def drawStuff(P1, P2, P3, win): # create a line L = Line(P2, P3) # create a circle C = Circle(P3, 10) C.setFill('red') # create a rectangle R = Rectangle(P1, P3) R.setFill('blue') # draw the objects L.draw(win) R.draw(win) C.draw(win)

Graphics Window Step by Step def drawStuff(P1, P2, P3, win): # create a line L = Line(P2, P3) # create a circle C = Circle(P3, 10) C.setFill('red') # create a rectangle R = Rectangle(P1, P3) R.setFill('blue') # draw the objects C.draw(win) R.draw(win) L.draw(win)

The Alternative Sequential Processing Perception Programming Parallel Processing Perception Programming Multiple CPUs Control Flow Statements

Control Flow Alter the direction of the flow of the code No longer sequential Loops: repetition Conditionals: make decisions Gotos: jump to another section

Repetition Print out the numbers from 0 to 9 for-statement loop statement Repeat something a fixed number of times >>> for i in range(10): print i

for-Statements for in :... Sets up number of times the loop is repeated or iterated Body: stuff to be repeated

for-Statements is the index for the loop Assign successive values of to is a list of values Statements in the body are executed for each value for in :

Repetition Print out the numbers from 0 to 9 is range(10) generates a sequence of numbers up to but not including the specified parameter value >>> range(5) [0, 1, 2, 3, 4] >>> for i in range(10): print i

Repetition Print out the numbers from 0 to 9 The index variable, i will take on the values of 0 through 9 and for each value, it will execute the print statement >>> for i in range(10): print i

Lists Collections of information Any sequence of “things” Numbers Letters Strings Points figures

Types of Lists Empty List >>> [] [] or >>> L = [] >>> print L [] An empty list does not contain anything

Types of Lists N = [2, 43, 6, 27] FamousNumbers = [3.1415, 2.718, 42] Cities = ['Philadelphia', 'Hicksville', 'Troy', 'Austin'] MyPoints = [Point(100, 42), Point(75, 64), Point(83, 310)] MyCar = ['Mazda Protege', 1999, 'red'] Any collection of things, even those with different data types

Lists Operations >>> N = [2, 43, 6, 27] >>> len(N) 4 len( ) : takes a list and returns the length, or the number of objects in the list

Lists Operations >>> N = [2, 43, 6, 27] >>> FamousNumbers = [3.1415, 2.718, 42] >>> N + FamousNumbers [2, 43, 6, 27, , 2.718, 42] Concatenate (‘+’): joins two lists together

List Operations >>> Cities = ['Philadelphia', 'Hicksville', 'Troy', 'Austin'] >>> Cities[0] 'Philadelphia' Indexing: allows access individual elements in a list by their index First element has an index of 0 Last element has an index of n-1 where n is the length of the list

Lists Operations >>> Cities = ['Philadelphia', 'Hicksville', 'Troy', 'Austin'] >>> Cities[1:3] ['Hicksville', 'Troy'] Slice: allows access to a sublist by a set of indices List[a:b] returns the elements from a to b-1

Lists Operations >>> Cities = ['Philadelphia', 'Hicksville', 'Troy', 'Austin'] >>> 'Troy' in Cities True >>> 'Cairo' in Cities False in: checks whether an element is in a list

Lists are Objects >>> N = [2, 43, 6, 27] >>> N.sort() >>> print N [2, 6, 27, 43] sort: arrange elements in ascending order

Lists are Objects >>> N = [2, 43, 6, 27] >>> N.reverse() >>> print N [27, 6, 43, 2] reverse: reverse order of elements

Lists are Objects >>> N = [2, 43, 6, 27] >>> N.append(3) >>> print N [2, 43, 6, 27, 3] append: add an element to the end of the list

All Lists are Sequences All lists can be used to perform repetitions from graphics import * MyPoints = [Point(100, 42), Point(75, 64), Point(83, 310)] win = GraphWin('My Win', 400, 400) for point in MyPoints: c = Circle(point, 5) c.setFill('red') c.draw(win)

Strings Strings are also sequences >>> ABC = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' >>> len(ABC) 26 >>> for letter in ABC: print letter A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Lists in Pylab All the data we have plotted so far has been stored as a set of lists: Year = [1979, 1980, 1982,…] Marijuana = [14.1, 13.3, 13.3, 12.5, …]

Lists in Pylab Thus, the actual syntax of the plot command is: where the x- and y-data are actually lists of either int or float values The syntax of the mean command is: And the function calculates the mean of a list of int or float values plot( ) Plot(, ) plot( ) Plot(, ) mean( )

Lists in Pylab

Ticks: Labels = ['Marijuana', 'Hallucinogens', 'Cocaine', 'Heroin', 'Cigarettes', 'Alcohol'] xticks(arange(6), Labels) xticks(arange( ), )

Lists in Pylab