Lists and Ranges

Last weeks’ highlights

• Memory and variables

• Different variable types (int, float, str, bool)

• Different operations for different types

• Functions

def foo(param1, param2, ..):
    ...
    return res

• if-else statements

if expression: 
    statement1
else:
    statement2

Agenda

Lists Ranges Lists An ordered collection of elements Each element in a list can be of any type Basic operations # student: name, age, height, SAT student = ['Rick', 81, 1.83, 9000] print(student) ['Rick', 81, 1.83, 9000] student[3] 9000 len(student) 4 student[0:-1] ['Rick', 81, 1.83] student[::-1] [9000, 1.83, 81, 'Rick'] student[6:10:2] # What happens here? [] my_list = ['Rick','Morty', 'Beth', 'Jerry', 'Summer'] new_list = my_list[::2] print(my_list) print(new_list) ['Rick', 'Morty', 'Beth', 'Jerry', 'Summer'] ['Rick', 'Beth', 'Summer'] students = ['Rick','Morty', 'Beth', 'Jerry', 'Summer'] print(students) ['Rick', 'Morty', 'Beth', 'Jerry', 'Summer'] students[1:5]=["Tiny Rick!", "Rick c-135", "Rick c-135", "Rick c-135"] print(students) ['Rick', 'Tiny Rick!', 'Rick c-135', 'Rick c-135', 'Rick c-135'] students[2]+=" (a.k.a evil Rick)" print(students) ['Rick', 'Tiny Rick!', 'Rick c-135 (a.k.a evil Rick)', 'Rick c-135', 'Rick c-135'] Example 1: Minimal result Given two lists, st1 and lst2, compute for each the sum of the list divided by the middle value, and print the smaller of the two results. Plan Calculate sum divided by middle for lst1 Calculate sum divided by middle for lst2 Print the smaller one Pseudo code res1 \(\leftarrow\) sum_div_middle(lst1) res2 \(\leftarrow\) sum_div_middle(lst2) print min(res1, res2) Implement pseudo code: def sum_div_middle(lst): return sum(lst) / lst[len(lst)//2] res1=sum_div_middle([1,3,7]) res2=sum_div_middle([100,30,7]) print(min(res1,res2)) 3.6666666666666665 Nested lists/Matrix mat = [["m", "a", "t", "R"], ["i", "c", "k", "s"]] print(mat) [['m', 'a', 't', 'R'], ['i', 'c', 'k', 's']] print(len(mat)) 2 print(len(mat[1])) 4 print(mat[1][2]) k And what about mat[2][1]? Example 2: Element-wise product For a pair of 2x2 matrices, multiply each number in the first matrix with its corresponding number in the second matrix Corresponding number = the number that lies in the same position m=[[2,2], [1,3]] n=[[4,2], [9,2]] result=[[],[]] i=0; j=0 result[i].append(m[i][j]*n[i][j]) i=0; j=1 result[i].append(m[i][j]*n[i][j]) i=1; j=0 result[i].append(m[i][j]*n[i][j]) i=1; j=1 result[i].append(m[i][j]*n[i][j]) print(result) [[8, 4], [9, 6]] List’s methods Applied using the dot notation: variable_name.function_name() Might change the original object Strings: Immutable Lists: Mutable Partial list of methods method description lst.append(item) append an item to the end of the list lst.count(val) return the number of occurrences of val lst.extend(another_lst) extend list by appending items from another list lst.index(val) return first index of val lst.insert(index, item) insert an item before index lst.pop(index) remove the item at location index and return it (remove last element if index is omitted) lst.remove(val) remove first occurrence of a val lst.reverse() reverse the list (in place) lst.sort() sort the list (in place) *bolded does not change the list Other list’s function: function description len() returns the list’s length sum() returns a sum of all list elements min() returns the minimal element max() returns the maximal element sorted() returns a sorted copy of the list in returns True if element in list (e.g. 1 in lst) Sorting lists a = [3,7,1] b = sorted(a) print(a) print(b) [3, 7, 1] [1, 3, 7] a = [3,7,1] b = a.sort() print(a) print(b) [1, 3, 7] None Iterable Represent a sequence of some kind What kind of sequence did we see so far? Strings, Lists Range Another iterable range(from, to, step) returns: from, from+step, from+2*step,…, from+i*step until (and not including) to is reached. list(range(7)) [0, 1, 2, 3, 4, 5, 6] list(range(3,7)) [3, 4, 5, 6] list(range(7, 3, -1)) [7, 6, 5, 4] list(range(-3,7,1)) [-3, -2, -1, 0, 1, 2, 3, 4, 5, 6] list(range(1, 7, -3)) [] list(range(0, 137, 0)) --------------------------------------------------------------------------- ValueError Traceback (most recent call last) Cell In[26], line 1 ----> 1 list(range(0, 137, 0)) ValueError: range() arg 3 must not be zero