Content
- Wrapping up 1d lists
- Swapping Elements
- Copying lists
- List comprehension
- Converting Between Lists and Strings
- 1d list CT
- Worked example 1: Anagrams
- Worked example 2: The Sieve of Eratosthenes
- 2d lists
- 2d lists
- 2d lists CT
- Example 1: isMagicSquare
Wrapping up 1d lists
1. 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)
Swap with parallel (tuple) assignment
a = [ 2, 3, 5, 7 ]
print("a =", a)
a[0],a[1] = a[1],a[0]
print("After swapping a[0] and a[1]:")
print(" a=",a)
2. Copying lists
Copy vs Alias
import copy
# Create a list
a = [ 2, 3 ]
# Try to copy it
b = a # Error! Not a copy, but an alias
c = copy.copy(a) # Ok
# At first, things seem ok
print("At first...")
print(" a =", a)
print(" b =", b)
print(" c =", c)
# Now modify a[0]
a[0] = 42
print("But after a[0] = 42")
print(" a =", a)
print(" b =", b)
print(" c =", c)
Other ways to copy
import copy
a = [2, 3]
b = copy.copy(a)
c = a[:]
d = a + [ ]
e = list(a)
f = copy.deepcopy(a)
g = sorted(a)
a[0] = 42
print(a, b, c, d, e, f, g)
3. List comprehension
a = [i for i in range(10)]
print(a)
a = [(i*100) for i in range(20) if i%5 == 0]
print(a)
4. Converting between Lists and Strings
# use list(s) to convert a string to a list of characters
a = list("wahoo!")
print(a) # prints: ['w', 'a', 'h', 'o', 'o', '!']
# use s1.split(s2) to convert a string to a list of strings delimited by s2
a = "How are you doing today?".split(" ")
print(a) # prints ['How', 'are', 'you', 'doing', 'today?']
# use "".join(a) to convert a list of characters to a single string
s = "".join(a)
print(s) # prints: wahoo!
# "".join(a) also works on a list of strings (not just single characters)
a = ["parsley", " ", "is", " ", "gharsley"] # by Ogden Nash!
s = "".join(a)
print(s) # prints: parsley is gharsley
5. 1d List CT
import copy
def ct1(a):
(b, c) = (a, copy.copy(a))
a[0] = 3
b[0] = 4
c[0] = 5
print(a[0], b[0], c[0], end = " ")
a = c
a[0] = 6
b[0] = 7
c[0] = 8
print (a[0], b[0], c[0], end = " ")
a = [1, 2]
ct1(a)
print(a[0])
6. Worked example 1: Anagrams
def letterCounts(s):
counts = [0] * 26
for ch in s.upper():
if ((ch >= "A") and (ch <= "Z")):
counts[ord(ch) - ord("A")] += 1
return counts
def isAnagram(s1, s2):
# First approach: same #'s of each letter
return (letterCounts(s1) == letterCounts(s2))
def isAnagram(s1, s2):
# Second approach: sorted strings must match!
return sorted(list(s1.upper())) == sorted(list(s2.upper()))
def testIsAnagram():
print("Testing isAnagram()...", end="")
assert(isAnagram("", "") == True)
assert(isAnagram("abCdabCd", "abcdabcd") == True)
assert(isAnagram("abcdaBcD", "AAbbcddc") == True)
assert(isAnagram("abcdaabcd", "aabbcddcb") == False)
print("Passed!")
testIsAnagram()
7. Worked example 2: The Sieve of Eratosthenes
# Sieve of Eratosthenes
# This function returns a list of prime numbers
# up to n (inclusive), using the Sieve of Eratosthenes.
# See http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
def sieve(n):
isPrime = [ True ] * (n+1) # assume all are prime to start
isPrime[0] = isPrime[1] = False # except 0 and 1, of course
primes = [ ]
for prime in range(n+1):
if (isPrime[prime] == True):
# we found a prime, so add it to our result
primes.append(prime)
# and mark all its multiples as not prime
for multiple in range(2*prime, n+1, prime):
isPrime[multiple] = False
return primes
print(sieve(100))
2d Lists
1. 2d lists
See here.
2. 2d lists CT
import copy
def ct1(L):
a = L
b = copy.copy(L)
c = copy.deepcopy(L)
b[0] = a[1] * a[1][0]
a[0][0] += a.pop()[0]
b[1] = c[0]
return b
L = [[1],[2],[3]]
print(ct1(L))
print(L)
3. Example 1: isMagicSquare(a)
See here.
