PyClass

Today

• Lists (aka Arrays)
• Tuples
• Loops

Examples of lists:

list1 = [1,2,3,4]
list2 = [1, "a", "c", 3, "d"]
list3 = list1 + list2

• Lists have multiple items
• Lists can have different types of items

Is This Allowed?

def hello():
  print "hi"

l = [1, 2, hello]

hello != "hello"

    l = [1, 2, hello] # hello refers to a variable name
    l2 = [1, 2, "hello"] # "hello" is a string
    

Getting Elements In a List

l = [1, 2, hello]
print l[0] # prints 1
print l[1] # prints 2

# Tougher Examples:
l[2]() # prints "hi"
l[-1]() # prints "hi"
print l[-2] # prints 2

List Slicing

l = [2, 4, 6, 8, 10]
l[0:2] # [2,4]
l[2:4] # [6,8]
l[:2]  # [2,4]
l[2:]  # [6,8,10]
l[:]   # [2, 4, 6, 8, 10]

Strings Behave Like Lists!

a = "foo"
a[0] # "f"
a[1] # "o"
a[:2] # "fo"
    

What Else Can Lists Do?

 
l = [2, 4, 6, 8, 10]
l.append(12) # l is now [2, 4, 6, 8, 10, 12]
len(l) # 6
l.remove(4) # l is now [2, 6, 8, 10, 12]
a = l.pop() # l is now [6, 8, 10, 12], a is now 2
l.index(10) # 2
l += [10, 20, 30] # l is now [6, 8, 10, 12, 10, 20, 30]
l.count(10) # 2
l.sort() # l is now [6, 8, 10, 10, 12, 20, 30]
l.reverse() # l is now [30, 20, 12, 10, 10, 8, 6]
    

Join An Array Back To a String

 
array = ['A','dead','parrot']
delim = '.'
str1 = delim.join(array) #str1 is 'A.dead.parrot'
str2 = ' '.join(array) #str2 is 'A dead parrot'

Shuffle a List

 
from random import shuffle
l = [1,2,3,4]
shuffle(l)
    

List Comprehensions

l = [1,2,3,4,5]
squares = [x**2 for x in l] # [1, 4, 9, 16, 25]
even_squares = [x**2 for x in l if x % 2 == 0] # [4, 16]

Tuples

Tuples are immutable lists:

list1 = [1, 2, 3, 4]
list1.append(5)
tuple1 = (1, 2, 3, 4)
tuple1.append(5) # error

The For Loop

l = [1,2,3,4,5]
squares = []
for x in l:
  squares.append(x**2)

Range and Enumerate

squares = [1, 4, 9, 16, 25]

for i in range(len(squares)):
  print "the element at index", i, "is", squares[i]

for i, x in enumerate(squares):
  print "the element at index", i, "is", x

Zip

L1 = [1,2,3,4]
L2 = [5,6,7,8]
L3 = zip(L1,L2) # L3 is now [ (1,5) , (2,6) , (3,7) , (4,8) ]
# Processing both sequences simultaneously in a for loop:
for (x,y) in zip(L1,L2):
  print x + y,   
# prints out: 6 8 10 12

Problem?