Lecture 13, Tue 02/20

More on Lists

Recorded Lecture: 2_20_24

More on Lists

• list() function can convert a container into a list type
• Basically, anything that can be iterated through (used in a for loop) can be converted to a list
• Examples:

print(list({1, 2, 3})) # set to a list of ints
print(list((1, 2, 3))) # tuple to a list of ints
print(list({"1":10, "2":20, "3":30})) # dict to a list of its keys (strings)
print(list("123")) # string to a list of characters (strings)

List Operations / Methods

• Recall, Lists are mutable
• Certain list operations / methods directly modify the list, while other operations / methods provide an updated copy without changing the list
  • We’ve seen that del and assignment (=) to an element ([index]) modifies the original list
  • Changing the actual list is called an in-place modification

Some examples of operations that won’t do an in-place modification, but provide a copy of the updated list:

• + (concatenation)

list1 = [1,2]
list2 = ['a', 'b']
print(list1 + list2) # [1, 2, 'a', 'b']
print(list2 + list1) # ['a', 'b', 1, 2]
print(list1) # [1, 2]
print(list2) # ['a', 'b']

• : (list slicing)
  • Creates a new sublist based on elements between index values
  • [i:j] creates a list containing elements at index i up to (not including) index j
  • Note that negative indices can be used in Python (they count backwards from the end of the list)
  • Also note that not including i will start at the beginning of the list, and not having j will go to the end of the list
  • [i:j:z] can also be used where z is a step / stride when obtaining elements
  • Note: this slicing mechanic also applies to tuples and strings

list1 = [10, 20, 30, 40]
print(list1[1:3]) # [20, 30]
print(list1[-3:-1]) # [20, 30]
print(list1[1:]) # [20, 30, 40]
print(list1[:3]) # [10, 20, 30]
print(list1[:]) # [10, 20, 30, 40]
print(list1[0:4:2]) # [10, 30]
print(list1) # [10, 20, 30, 40]

• sorted() function
  • Python function that can take an iterable collection (including lists), and returns a copy with elements sorted from least-to-greatest (by default)
  • We can specify we want greatest-to-least by setting reverse=True
  • Note that all elements in the list must be comparable (support < operation) with each other or else an error occurs

list1 = [30, 20, 40, 10] 
print(sorted(list1)) # [10, 20, 30, 40]
print(list1) # [30, 20, 40, 10]
print(sorted(list1, reverse=True)) # [40, 30, 20, 10]
print(list1) # [30, 20, 40, 10]
list1 = [30, 20, 40, '10']
print(sorted(list1)) # ERROR (can’t compare int and str)

Some examples of list methods that do an in-place modification of the list

• list.append(x) - adds an element to the end of the list

list1 = [10, 20, 30, 40]
print(list1.append(50)) # None
print(list1) # [10, 20, 30, 40, 50]

• list.extend([x]) - appends all items in [x] to the list

list1 = [10, 20, 30, 40]
list2 = ['a', 'b']
print(list1.extend(list2)) # None
print(list1) # [10, 20, 30, 40, 'a', 'b']

• list.insert(i, x) - insert element x into list before index i (everything currently at index i or greater gets shifted)

list1 = [10, 20, 30, 40]
print(list1.insert(2, 25)) # None
print(list1) # [10, 20, 25, 30, 40]

• list.remove(x) / list.pop(i)
  • remove method will remove first item that has the value x
  • pop method will remove element at index i
    • Note that if i is not used, then pop() will remove the last element in the list

list1 = [10, 20, 30, 40, 30]
print(list1.remove(30)) # None
print(list1) # [10, 20, 40, 30]
print(list1.pop()) # 30
print(list1) # [10, 20, 40]
print(list1.pop(1)) # 20
print(list1) # [10, 40]

• list.sort()
  • Similar to the sorted function, but will change the list to have elements sorted from least-to-greatest (by default)
  • We can also use reverse=True to get elements from greatest-to-least
  • Note that all elements in the list must be comparable (support < operation) with each other or else an error occurs

list1 = [30, 20, 40, 10]
print(list1.sort()) # None
print(list1) # [10, 20, 30, 40]
print(list1.sort(reverse=True)) # None
print(list1) # [40, 30, 20, 10]

Common calculations on lists

• Common calculations on lists could be finding the min element, max element, sum of all elements, etc.
  • We could do this ourselves and write our own functions, but some of these operations are so common that Python provides functions that do this for us!
  • For example, we could write our own function for getting the max value in a list

def ourMax(collection):
	if collection == []:
		return None

	maxElement = collection[0]
	for item in collection:
		if item > maxElement:
			maxElement = item

	return maxElement

list1 = [30, 20, 40, 10]
print(ourMax(list1)) # 40

• Python’s max function will return the maximum element in our list
  • Note that elements in the list must be comparable or else an error will occur

list1 = [30, 20, 40, 10] # change 10 to "10" to see an error
print(max(list1)) # 40

• Examples of min, sum

list1 = [30, 20, 40, 10]
print(min(list1)) # 10
print(sum(list1)) # 100

A note on how Python interprets True and False

• We’ve seen the True and False boolean values, and use them in boolean expressions
• Technically, Python interprets values as False with False, 0, None, or an empty collection
  • Anything else is considered True
• Rapid prototype example:

# change to 0, None, [], (), {} to see x is False
# change to any other value to see x is True
x = False 

if x:
	print("x is True")
else:
	print("x is False")

• Python provides an all() function that returns True if every element in a list is considered True (or if the list is empty)
• The any() function will return True if any element in the list is considered True