CSE 231 Lab 6

Topics to cover Creating Lists & tuples Lists methods In-place vs not in-place Patterns List comprehension A Python list is a sequence of objects. It is a sequence like a string. It shares characteristics such as indexing & slicing, iteration (for) and membership (in) as well as functions such as len().

LISTS and tuples A sequence of objects. It is a sequence like a string. It shares characteristics such as indexing & slicing, iteration (for) and membership (in) as well as functions such as len(). Mutable vs Immutable Strings, ints, floats, tuples are all immutable Lists are Mutable Tuples are immutable lists

Lists my_list = ["Arthur", "King", "of", "the", "Britons"]

Lists – Creating Lists my_list = [] # start off with nothing in the list my_list = ['I', 'am', 'Full', 'of', 'stuff'] my_list = list(‘CSE231 Rocks') # using list() ['C', 'S', 'E', '2', '3', '1', ' ', 'R', 'o', 'c', 'k', 's'] mixed_list = [“string”, 1, 42.0, True] Matrix = [[1, 1], [1,1]]

List methods: In-place my_list.append('A') # takes element arg. & puts to end of list my_list.pop(3) # takes index arg. & takes it out of list If no argument is given, it pops out the last element What does this return? my_list.sort() # sorts entire list & modifies it in place. What does the function sorted(my_list) do? my_list.insert(index, value) # Insert value into position index my_list.remove(value) # removes the first instance of value # crashes if the value doesn't exist

Lists – What is in-place? some_list = [] some_list.append(1) print(some_list) >>> [1] # Why didn’t I need to do some_list = some_list.append(1) ? # Methods (like append) on lists happen IN-PLACE meaning there is no return!

Lists – What is in-place? >>> some_list = [3, 1, 7, 2, 4] >>> some_list = some_list.sort() >>> print(some_list) None # uh oh. What happened to the data??? # You reassigned some_list when sort is done in-place! # The return of a function that does not return something is what?

Lists – What is in-place? >>> some_list = [3, 1, 7, 2, 4] >>> some_list.sort() >>> print(some_list) [1, 2, 3, 4, 7] # All good again!

List methods: NOT-IN-PLACE ‘-’.join(my_list) # joins entire list with a delimiter ‘-’ >>my_list = [‘I’, ’am’, ‘Full’, ‘of’, ‘stuff’] >>my_string= ‘-’.join(my_list) >>print(my_string) I-am-Full-of-stuff

Split Method >>my_str='I am in my 231 lab‘ split and also strip methods are your best friends with data parsing By default it separates strings by whitespaces into a list Very helpful when given lines of data. >>my_str='I am in my 231 lab‘ >>my_str.split() ['I', 'am', 'in', 'my', '231', 'lab'] >>my_str.split('m') ['I a', ' in ', 'y 231 lab']

Patterns Counter: x = 0 # initialize at zero x += 1 # increment the counter Strings: s = ‘’ # initialize with empty string s += ch # concatenate characters Lists: L = [] # initialize with empty list L.append(value) # append values to the list

List Comprehensions L=[] For i in range(3) L.append(i**3) L = [i**3 for i in range(3)] Other examples >> L = [i**3 for i in range(10) if i%2 == 0] >>L = [int(x) for x in line_list] line = “3 12 5 82 1” # original data read from a file line_list = line.split() #split to get a list of strings, SHOULD CONVERT TO INT NEXT