# CMU 15-112: Fundamentals of Programming and Computer Science Class Notes: 1d Lists

1. Creating Lists
• Empty List
print("Two standard ways to create an empty list:") a = [ ] b = list() print(type(a), len(a), a) print(type(b), len(b), b) print(a == b)

• List with One Element (Singleton)
a = [ "hello" ] b = [ 42 ] print(type(a), len(a), a) print(type(b), len(b), b) print(a == b)

• List with Multiple Elements
a = [2, 3, 5, 7] b = list(range(5)) c = ["mixed types", True, 42] print(type(a), len(a), a) print(type(b), len(b), b) print(type(c), len(c), c)

• Variable-Length List
n = 10 a = [0] * n b = list(range(n)) print(type(a), len(a), a) print(type(b), len(b), b)

2. List Properties (len, min, max, sum)
a = [ 2, 3, 5, 2 ] print("a = ", a) print("len =", len(a)) print("min =", min(a)) print("max =", max(a)) print("sum =", sum(a))

3. Accessing Elements (Indexing and Slicing)
a = [2, 3, 5, 7, 11, 13] print("a =", a) # Access non-negative indexes print("a[0] =", a[0]) print("a[2] =", a[2]) # Access negative indexes print("a[-1] =", a[-1]) print("a[-3] =", a[-3]) # Access slices a[start:end:step] print("a[0:2] =", a[0:2]) print("a[1:4] =", a[1:4]) print("a[1:6:2] =", a[1:6:2])

4. List 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)

• Function Parameters are Aliases:
def f(a): a[0] = 42 a = [2, 3, 5, 7] f(a) print(a)

• Another Example:
# Create a list a = [ 2, 3, 5, 7 ] # Create an alias to the list b = a # Create a different list with the same elements c = [ 2, 3, 5, 7 ] # a and b are references (aliases) to the SAME list # c is a reference to a different but EQUAL list print("initially:") print(" a==b :", a==b) print(" a==c :", a==c) print(" a is b:", a is b) print(" a is c:", a is c) # Now changes to a also change b (the SAME list) but not c (a different list) a[0] = 42 print("After changing a[0] to 42") print(" a=",a) print(" b=",b) print(" c=",c) print(" a==b :", a==b) print(" a==c :", a==c) print(" a is b:", a is b) print(" a is c:", a is c)

5. Finding Elements
• Check for list membership: in
a = [ 2, 3, 5, 2, 6, 2, 2, 7 ] print("a =", a) print("2 in a =", (2 in a)) print("4 in a =", (4 in a))

• Check for list non-membership: not in
a = [ 2, 3, 5, 2, 6, 2, 2, 7 ] print("a =", a) print("2 not in a =", (2 not in a)) print("4 not in a =", (4 not in a))

• Count occurrences in list: list.count(item)
a = [ 2, 3, 5, 2, 6, 2, 2, 7 ] print("a =", a) print("a.count(1) =", a.count(1)) print("a.count(2) =", a.count(2)) print("a.count(3) =", a.count(3))

• Find index of item: list.index(item) and list.index(item, start)
• Example
a = [ 2, 3, 5, 2, 6, 2, 2, 7 ] print("a =", a) print("a.index(6) =", a.index(6)) print("a.index(2) =", a.index(2)) print("a.index(2,1) =", a.index(2,1)) print("a.index(2,4) =", a.index(2,4))

• Problem: crashes when item is not in list
a = [ 2, 3, 5, 2 ] print("a =", a) print("a.index(9) =", a.index(9)) # crashes! print("This line will not run!")

• Solution: use (item in list)
a = [ 2, 3, 5, 2 ] print("a =", a) if (9 in a): print("a.index(9) =", a.index(9)) else: print("9 not in", a) print("This line will run now!")

• Destructively (Modifying Lists)
• Add an item with list.append(item)
a = [ 2, 3 ] a.append(7) print(a)

• Add a list of items with list += list2
a = [ 2, 3 ] a += [ 11, 13 ] print(a)

• Add a list of items with list.extend(list2)
a = [ 2, 3 ] a.extend([ 17, 19 ]) print(a)

• Insert an item at a given index
a = [ 2, 3, 5, 7, 11 ] a.insert(2, 42) # at index 2, insert 42 print(a)

• Non-Destructively (Creating New Lists)
• Add an item with list1 + list2
a = [ 2, 3 ] b = a + [ 13, 17 ] print(a) print(b)

• Insert an item at a given index (with list slices)
a = [ 2, 3 ] b = a[:2] + [5] + a[2:] print(a) print(b)

• Destructive vs Non-Destructive Example
print("Destructive:") a = [ 2, 3 ] b = a a += [ 4 ] print(a) print(b) print("Non-Destructive:") a = [ 2, 3 ] b = a a = a + [ 4 ] print(a) print(b)

7. Removing Elements
• Destructively (Modifying Lists)
• Remove an item with list.remove(item)
a = [ 2, 3, 5, 3, 7, 6, 5, 11, 13 ] print("a =", a) a.remove(5) print("After a.remove(5), a=", a) a.remove(5) print("After another a.remove(5), a=", a)

• Remove an item at a given index with list.pop(index)
a = [ 2, 3, 4, 5, 6, 7, 8 ] print("a =", a) item = a.pop(3) print("After item = a.pop(3)") print(" item =", item) print(" a =", a) item = a.pop(3) print("After another item = a.pop(3)") print(" item =", item) print(" a =", a) # Remove last item with list.pop() item = a.pop() print("After item = a.pop()") print(" item =", item) print(" a =", a)

• Remove an item with slice assignment
a = [ 2, 3, 4, 5, 6, 7, 8 ] a[2:4] = [ ] print("a =", a)

• Remove an item with the del operator
a = [ 2, 3, 4, 5, 6, 7, 8 ] del a[2:4] print("a =", a)

• Non-Destructively (Creating New Lists)
• Remove an item at a given index (with list slices)
a = [ 2, 3, 5, 3, 7, 5, 11, 13 ] print("a =", a) b = a[:2] + a[3:] print("After b = a[:2] + a[3:]") print(" a =", a) print(" b =", b)

8. 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)

9. Looping Over Lists
• Looping with: for item in list
a = [ 2, 3, 5, 7 ] print("Here are the items in a:") for item in a: print(item)

• Looping with: for index in range(len(list))
a = [ 2, 3, 5, 7 ] print("Here are the items in a with their indexes:") for index in range(len(a)): print("a[", index, "] =", a[index])

• Looping backward
a = [ 2, 3, 5, 7 ] print("And here are the items in reverse:") for index in range(len(a)): revIndex = len(a)-1-index print("a[", revIndex, "] =", a[revIndex])

• Looping backward with reversed(list)
a = [ 2, 3, 5, 7 ] print("And here are the items in reverse:") for item in reversed(a): print(item) print(a)

• Looping backward with destructive list.reverse()
a = [ 2, 3, 5, 7 ] print("And here are the items in reverse:") a.reverse() for item in a: print(item) print(a)

• Hazard: modifying while looping
a = [ 2, 3, 5, 3, 7 ] print("a =", a) # Failed attempt to remove all the 3's for index in range(len(a)): if (a[index] == 3): # this eventually crashes! a.pop(index) print("This line will not run!")

10. Comparing Lists
# Create some lists a = [ 2, 3, 5, 3, 7 ] b = [ 2, 3, 5, 3, 7 ] # same as a c = [ 2, 3, 5, 3, 8 ] # differs in last element d = [ 2, 3, 5 ] # prefix of a print("a =", a) print("b =", b) print("c =", c) print("d =", d) print("------------------") print("a == b", (a == b)) print("a == c", (a == c)) print("a != b", (a != b)) print("a != c", (a != c)) print("------------------") print("a < c", (a < c)) print("a < d", (a < d))

11. 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)

12. Sorting Lists
• Destructively with list.sort()
a = [ 7, 2, 5, 3, 5, 11, 7 ] print("At first, a =", a) a.sort() print("After a.sort(), a =",a)

• Non-Destructively with sorted(list)
a = [ 7, 2, 5, 3, 5, 11, 7 ] print("At first") print(" a =", a) b = sorted(a) print("After b = sorted(a)") print(" a =", a) print(" b =", b)

• Sorting with a key function
a = [ 10, 2, -5, 8, -3, 7, 1 ] print(sorted(a)) print(sorted(a, key=abs))

13. Usings Lists with Functions
• List Parameters Example: countOdds(list)
def countOdds(a): count = 0 for item in a: if (item % 2 == 1): count += 1 return count print(countOdds([2, 3, 7, 8, 21, 23, 24]))

• Modifying list elements is visible to caller: fill(list, value)
def fill(a, value): for i in range(len(a)): a[i] = value a = [1, 2, 3, 4, 5] print("At first, a =", a) fill(a, 42) print("After fill(a, 42), a =", a)

• Modifying list reference is not visible to caller
import copy def destructiveRemoveAll(a, value): while (value in a): a.remove(value) def nonDestructiveRemoveAll(a, value): a = copy.copy(a) while (value in a): a.remove(value) return a a = [ 1, 2, 3, 4, 3, 2, 1 ] print("At first") print(" a =", a) destructiveRemoveAll(a, 2) print("After destructiveRemoveAll(a, 2)") print(" a =", a) b = nonDestructiveRemoveAll(a, 3) print("After b = nonDestructiveRemoveAll(a, 3)") print(" a =", a) print(" b =", b)

• List Return Types
def numbersWith3s(lo, hi): result = [ ] for x in range(lo, hi): if ("3" in str(x)): result.append(x) return result print(numbersWith3s(250, 310))

14. Summary of List Operations and Methods
See this table and this list of list methods.

15. Tuples (Immutable Lists)
• Tuple syntax
t = (1, 2, 3) print(type(t), len(t), t) a = [1, 2, 3] t = tuple(a) print(type(t), len(t), t)

• Tuples are immutable
t = (1, 2, 3) print(t[0]) t[0] = 42 # crash! print(t[0])

• Parallel (tuple) assignment
(x, y) = (1, 2) print(x) print(y) (x, y) = (y, x) print(x) print(y)

• Singleton tuple syntax
t = (42) print(type(t), t*5) t = (42,) print(type(t), t*5)

16. List Comprehensions
a = [i for i in range(10)] print(a) a = [(i*100) for i in range(20) if i%5 == 0] print(a)

17. 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

18. Some Worked Examples Using Lists