Lists (also called Arrays)

1
Lists (also called Arrays)

• A list is an example of a collection a data type
  that is capable of storing other data types.
• foods "spam", "eggs", "sausage", "baked
  beans"
• drinks "milk", "coffee"
• oddDigits 1, 3, 5, 7, 9
• anEmptyList
• Accessing elements of a list
• gtgtgt foods2
• 'sausage'
• gtgtgt foods0
• 'spam'
• gtgtgt foods-1
• 'baked beans'
• Adding to a list
• gtgtgt juiceName "OJ"
• gtgtgt drinks.append(juiceName)

2
Things You Can Do With Lists

• gtgt foods "spam", "eggs", "sausage", "baked
  beans"
• gtgt waysToSayYes "yes", "Yes", "yep", "you
  bet", "oh yeah"
• Picking a random item from a list
• gtgtgt import random
• gtgtgt print random.choice(foods)
• baked beans
• gtgtgt print random.choice(foods)
• spam
• How many items in a list?
• gtgtgt print len(foods)
• 4
• gtgtgt print len(anEmptyList)
• 0
• Testing if an item is in a list using the in
  keyword
• while True
• userResp raw_input("Want to play again?")

3
More List Stuff. . .

• gtgtgt my_list 3, 9, 7, 5, 22, 6
• Sorting a list
• gtgtgt my_list.sort()
• gtgtgt print my_list
• 3, 5, 6, 7, 9, 22
• Slicing a list
• these responses are for the sorted version of
  my_list.
• gtgtgt my_list3
• 7, 9, 22
• gtgtgt my_list3
• 3, 5, 6
• gtgtgt my_list24
• 6, 7
• what values would you get for the unsorted
  list at the top?
• Concatenating two lists
• gtgtgt my_list my_list 16, 25, 36, 49 you
  can do but not -

4
Lists and Strings. . .

• Splitting a string into a list of words
• gtgtgt phrase "When in the Course of human events.
  . . "
• gtgtgt print phrase.split()
• 'When', 'in', 'the', 'Course', 'of', 'human',
  'events.', '.', '.'
• Counting instances of a particular item in a list
• gtgtgt myScores 18, 22, 17, 18, 21, 22, 22
• gtgtgt print myScores.count(22)
• 3
• Strings can also be treated a lot like lists
• gtgtgt my_string "Four score and seven years ago,
  etc."
• gtgtgt print my_string0
• F
• gtgtgt print my_string-9
• ago, etc.
• gtgtgt if letter in "aeiouAEIUO"
• print "Its a vowel!"
• gtgtgt if word in my_string

5
for loops in Python
for loops work with lists. A for loop is used
when you want to repeat some operation on every
element in a list. Here is a for loop in Python
for x in 5,4,3,2,1 print x print blast
off!
What do you think will be printed? It can also
be done like this
countdown 5,4,3,2,1 for x in countdown
print x print blast off!
The values can be in any order. This also works
fine
for x in 1,3,4,5,2,77,22 print x
Do you think the following is okay?
for x in apples, bananas, cherries
print I like, x
6
How exactly does a for loop work?

• the format of a for loop is
• for someVariable in someList
• do something. . .
• the first time through the loop, someVariable
  gets the value of the first element in someList
• the second time through the loop, someVariable
  gets the value of the second element
• and so forth. . .
• This is called iterating through the list
• Questions
• What happens after it processes the last element
  in the list?
• What happens if the list is empty?
• Can you use break in a for loop?

7
range()
range() returns a list of integers. With one
parameter it generates a list from 0 to one less
than the number you give gtgtgt print range(10) 0,
1, 2, 3, 4, 5, 6, 7, 8, 9 With two parameters
it generates a list from the first number to one
less than the second number gtgtgt print
range(10,20) 10, 11, 12, 13, 14, 15, 16, 17, 18,
19 With three parameters it generates a list
from the first number to one less than the second
number, but stepping by the third parameter. Use
this to count by twos gtgtgt print
range(10,20,2) 10, 12, 14, 16, 18 or to count
backwards gtgtgt print range(20,10,-1) 20, 19, 18,
17, 16, 15, 14, 13, 12, 11 or to count backwards
by twos gtgtgt print range(20,10,-2) 20, 18, 16,
14, 12
8
Using for with range
It's extremely common in Python to combine for
and range to create a loop that will run a
specified number of times. Remember this? for
count in range(3) turn_left() Above, we
aren't using the value of the variable called
count we're just using the fact that the list
that range(3) creates has 3 elements. But
sometimes the value of the loop counter variable
is very useful. . .
Remember this code from the IDLE worksheet? for
radius in range(10, 150, 10) circle(150,
150, radius) If the graphics API circle(x,y,r)
draws a circle of radius r at the point x,y, why
does the code above draw the picture at the
right? (Bonus question can you tell from the
code above how many rings there are? No fair
counting from the picture. . . )
9
for/range and while equivalence
You can generally rewrite a for/range using
while, and vice versa
for loop while loop
for val in range(10) print val val 0 while val lt 10 print val val 1
for val in range(2,10,2) print val val 2 while val lt 10 print val val 2
for val in range(10,5,-2) print val val 10 while val gt 5 print val val - 2
10
What can you put in a loop?

• You can put any valid Python code in a loop,
  including
• conditional jumps
• calls to subroutines
• even other loops! yes, loops can be nested
• What does the following code print?

for day in "Mon", "Tue", "Wed", "Thu", "Fri"
for myclass in "PCT","AP","Span","Math"
print "On", day, "I go to", myclass print
"then I go home" print "I do homework and sleep
on weekends"
11
Loops can be nested

• The following will print out the multiplication
  table
• for i in range(1,10)
• for j in range(1,10)
• print i, "times", j, "", ij
• Can you figure out what the following will do?
• for i in range(1,10)
• for j in range(i,10)
• print i, "times", j, "", ij

12
Livewires Graphics APIs
Livewires is a set of Python routines that
simplifies some computer graphics operations. To
use it, your code must start with from
livewires import To open up a graphics window
that you can draw on, you must call
begin_graphics() If you call it as above, you
will get a white window 640 pixels wide x 480
high You can also call it with arguments to
specify the height, width, background, and title.
This is done with keyword arguments or keyword
parameters. For example, the following creates a
tall, skinny, red window begin_graphics(heigh
t500,width100, backgroundColour.red) You do
not need to specify all keyword arguments, just
the ones for which you dont want the default
values To close the window, call
end_graphics()
13
More Livewires Graphics APIs
box(x1, y1, x2, y2) draw a box given two
opposite corners. circle(x, y, r) draw a
circle with center at (x, y), radius r. box()
and circle() also have two keyword parameters of
interest filled and colour. For filled, 1 means
fill in the box or circle, 0 (the default) means
dont. Colour allows you to set the color of the
circle. More about specifying colors on the next
slide clear_screen() empty the graphics
window Examples circle(100,75,20) will draw the
outline of a circle of radius 20 at x100,
y75 circle(300,320,75, filled1,
colourColour.blue) will draw a filled-in blue
circle of radius 75 at x300, y320 box(20,20,50,
50,colourColour.red) will draw the outline of a
red box with corners at 20,20 and 50,50
14
Livewires Colours

• Just like there is a current position or point,
  there is a current color, er, colour. You change
  it by calling
• set_colour(Colour.foo) where foo is either red,
  green, blue, black, white, dark_grey, grey,
  light_grey, dark_red, dark_green, dark_blue,
  yellow, or brown.
• So you call it like this
• set_colour(Colour.dark_green)
• You can also make custom colors from RGB values,
  using
• make_colour() and set_colour() together, like
  this
• newShade make_colour(red, green, blue)
• set_colour(newShade)
• In the call to make_colour above, red, green, and
  blue are variables their values must be
  floating-point numbers between 0.0 and 1.0. You
  can also use a custom color like this
• circle(90,90,50, colourmake_colour(0.3, 0.3,
  0.3))
• will give you a nice grey circle.

15
Using Circles and a for Loop to Make a
Surprisingly Cool Design

• from livewires import
• begin_graphics(300,300)
• for radius in range(10,150,5)
• circle(radius, radius, radius)
• circle(300 - radius, radius, radius)
• circle(radius, 300 - radius, radius)
• circle(300 - radius, 300 - radius, radius)