Python Lists

• a kind of container along with tuples, and dictionaries

general format and indexes

• format is list_name = [element_0, element_1, element2]
• can mix types on a single list
• can have empty list list = []
• reference an element by list_name[element_index]
• to get length of list len(list_name)
• 0 indexed
• when using negative indexes -1 is the last item on the list
  • think length of 'list - negative_index = positive_index'; ex: if you have a list whose length is 5 and you want index -3, then 'len(list) - 3 = 2', +2 is the same index as -3
• trying to access things outside index range gives index error
• list.index(element) - returns the index of the first occurrence of the element
  • error if element isn't on list, so check first

adding elements

• list.insert(index, element) - -inserts element at that index and shifts all the following elements down
  • note: if you assign an element with list[index] = element to an index that already has an element in it the existing element is overwritten rather than inserted
• add elements to list with list_name.append(new_element) and the new element will be added to the end of the list
• list.extend(other_list) - all the elements on other_list are now at the end of list
• note: .append adds single element, .extend merges a list onto another
  • example:

    >>> list1 = [1, 2, 3]
    >>> list = [4, 5]
    >>> list1.append(list2)
    >>> list1
    [1, 2, 3, [4, 5]]

    >>> list1 = [1, 2, 3]
    >>> list = [4, 5]
    >>> list1.extend(list2)
    >>> list1
    [1, 2, 3, 4, 5]

• list1 + list2 - works like extend but creates a new list and leaves the original lists unchanged
• list1 += list2 - works just like extend

removing elements

• remove elements from list with list_name.pop() removes the last element
  • use list.pop(index) to get an element that isn't the last one on the list
  • it returns the popped element
• list.remove(element) - removes the element
  • if the element occurs multiple times in the list it removes only the first occurrence
  • get error if try to remove an element that isn't on the list, so good to check that it is in the list first
• note: .pop takes an index, .remove takes an element

checking if element in list

• to see if a list contains an element use in and it will evaluate to a Boolean, so it can be used anywhere you can use a Boolean test --- assignment, loops, conditionals
  • example:

    x = 1
    if x in list_name:
        # do something

misc.

• list.copy() - returns copy of the list
• max(list), min(list), sum(list) - returns the min value, max value, or sum of the elements in the list

iterating through list

• standard for loop:

  for i in range(len(list)):
    # do something

• for each loop:

  for elem in collection:
    # do something with element

• both for loops iterate over all elements of the list in order

loops

• for i in range(len(list)):
    list[i] += 1    # modifying list in place

  • use 'for loop with range' when modifying primitive type elements in a list
  • you can always use this one

• for elem in list  # modifying local variable elem; if elem is primitive type, not changing the list!!!
    elem += 1

  • use 'for-each loop' when not modifying elements or when the elements are not primitive types

passing as parameters

• lists are passed by reference
• in the function, value changes in list persist after the function ends
• creating a new list in a function means you're not dealing with the list passed in as parameter
  • it's like the URL you are using is pointing to a new page and you are no longer changing the parameter list
  • example:

    def create_new_list(num_list):
        num_list.append(9)
        num_list = [1, 2, 3]
    
    def main():
        values = [5, 6, 7, 8]
        create_new_list(values)
        print(values)

    Output is [5, 6, 7, 8, 9]