- Creating Lists
print('Empty lists:')
a1 = [ ]
a2 = list()
print(' ', a1, a2)
print('Non-empty lists:')
a3 = [2, 3, 5, 7]
a4 = ['mixed types', True, 42]
print(' ', a3, a4)
print('Variable-length lists:')
a5 = [42] * 10
print(' ', a5)
print('Converting non-lists to lists:')
a6 = list(range(5))
a7 = list('yes!')
print(' ', a6, a7)
- List Functions
a = [ 2, 3, 5, 2 ]
print('a = ', a)
print('len =', len(a))
print('min =', min(a))
print('max =', max(a))
print('sum =', sum(a))
- Accessing Elements
a = [2, 3, 5, 7, 11, 13]
print('Indexing works just like strings:')
print(' a[0] =', a[0])
print(' a[-1] =', a[-1])
print('Slicing also works just like strings:')
print(' a[1:3] =', a[1:3])
print(' a[3:] =', a[3:])
- Finding Elements
a = [ 2, 3, 5, 2, 6, 5, 5, 7, 5 ]
print('in, not in, count, and index all work just like strings:')
print(' ', 3 in a)
print(' ', 3 not in a)
print(' ', a.count(5))
print(' ', a.index(5))
- Adding Elements
- Using
append is destructive (modifies the list)
a = [ 2, 3 ]
a.append(7)
print(a)
- Using
+ is non-destructive (creates a new list)
a = [ 2, 3 ]
b = a + [ 7 ]
print(a)
print(b)
- Removing Elements
- Using
remove
a = [ 2, 3, 5, 4, 5 ]
print('a =', a)
a.remove(5)
print("After a.remove(5), a=", a)
a.remove(5)
print("After another a.remove(5), a=", a)
- Using
pop
a = [ 2, 3, 4, 5 ]
print('a =', a)
print('pop with no argument removes (pops) the last value:')
v = a.pop()
print(' We just popped:', v)
print(' And now a =', a)
print('pop with an argument removes the value at that index:')
v = a.pop(1)
print(' We just popped:', v)
print(' And now a =', a)
- Swapping Elements
- Failed swap
a = [ 2, 3, 5, 7 ]
print('a =', a)
a[0] = a[1]
a[1] = a[0]
print('After failed swap of a[0] and a[1]:')
print(' a=',a)
- Swap with a temp variable
a = [ 2, 3, 5, 7 ]
print('a =', a)
temp = a[0]
a[0] = a[1]
a[1] = temp
print('After swapping a[0] and a[1]:')
print(' a=',a)
- Looping Over Lists
- Looping over items
a = [ 2, 3, 5, 7 ]
print('Here are the items in a:')
for item in a:
print(item)
- Looping over indexes
a = [ 2, 3, 5, 7 ]
print('Here are the items in a with their indexes:')
for i in range(len(a)):
print('a[', i, '] =', a[i])
- Copying Lists
print('Use (a + [ ]) to copy a list:')
a = [2, 3]
b = a + [ ] # creates a copy of a
a[0] = 42
print(a)
print(b)
- List Aliases
- Aliases example:
# Create a list
a = [ 2, 3, 5, 7 ]
# Create an alias to the list
b = a
# We now have two references (aliases) to the SAME list
a[0] = 42
b[1] = 99
print(a)
print(b)
- Copies are not aliases:
# Create a list
a = [ 2, 3, 5, 7 ]
# Create a COPY of the list
b = a + [ ] # creates a copy of a
# These are not aliases -- changes to one do not affect the other
a[0] = 42
b[1] = 99
print(a)
print(b)
- Function parameters are aliases:
def f(a):
a[0] = 42
a = [2, 3, 5, 7]
f(a)
print(a)
- Sorting Lists
- Using
sort is destructive (modifies the list)
a = [ 7, 2, 5, 3, 5, 11, 7 ]
a.sort()
print(a)
- Using
sorted is non-destructive (creates a new list)
a = [ 7, 2, 5, 3, 5, 11, 7 ]
b = sorted(a)
print(a)
print(b)
- Some Worked Examples Using Lists
- The Locker Problem
def lockerProblem(lockers):
isOpen = [ False ] * (lockers+1)
students = lockers
for student in range(1,students+1):
for locker in range(student, lockers+1, student):
isOpen[locker] = not isOpen[locker]
openLockers = [ ]
for locker in range(1, lockers+1):
if isOpen[locker]:
openLockers.append(locker)
return openLockers
print(lockerProblem(2000))
- Anagrams
def letterCounts(s):
counts = [0] * 26
for c in s.upper():
if (c.isalpha() == True):
counts[ord(c) - ord('A')] += 1
return counts
# Here are two very different ways to solve isAnagram:
def isAnagram(s1, s2):
return (letterCounts(s1) == letterCounts(s2))
def isAnagram(s1, s2):
return sorted(s1.upper()) == sorted(s2.upper())
def testIsAnagram():
print('Testing isAnagram()...')
assert(isAnagram('', '') == True)
assert(isAnagram('abCdabCd', 'abcdabcd') == True)
assert(isAnagram('abcdaBcD', 'AAbbcddc') == True)
assert(isAnagram('abcdaabcd', 'aabbcddcb') == False)
print('Passed!')
testIsAnagram()