Lists, Ranges and Loops

Python Programming for Engineers

Tel-Aviv University / 0509-1820 / Fall 2025-2026

Last week’s 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 Loops types of loops: while, for Other loop statements: break, continue The phonebook of Rick 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 For loops Syntax for element in iterable: statement1 statement2 … rest of code… Print each element separately with a descriptive text: lst = ['rick', 'c', 137] for elem in lst: print("current element:", elem) current element: rick current element: c current element: 137 Slide 48 Iteration can also be done on a string name = "Rick" for letter in name: print("Give me", letter) print("What did we get?", name) Give me R Give me i Give me c Give me k What did we get? Rick Put zeros instead each even element # Modify the list lst = [1,2,3,4,7] for i in range(len(lst)): if lst[i] % 2 == 0: lst[i] = 0 print(lst) [1, 0, 3, 0, 7] # The This is NOT equivalent! Does not modify the list... lst = [1,2,3,4,7] for elem in lst: if elem % 2 == 0: elem = 0 print(lst) [1, 2, 3, 4, 7] Slide 55 Infinite for loop # Do not run this code x=[1] for a in x: x.append(1) print(x) Avoid changing the list on which you iterate! (may cause all kind of problem, including infinite loops) Example #3: concatenate all characters in a matrix into a single string mat = [["m", "a", "t", "R"], ["i", "c", "k", "s"]] # Write your code here mat = [["m", "a", "t", "R"], ["i", "c", "k", "s"]] my_string="" num_rows = len(mat) num_columns = len(mat[0]) for i in range(num_rows): for j in range(num_columns): my_string+=mat[i][j] print(my_string) matRicks Try this with Python Tutor Slide 56 while loops Syntax while expression: ## expression is either True or False statement1 statement2 … rest of code… Print numbers from 1 to 7 with a descriptive text: cur_number = 1 while cur_number<=7: print("current number:", cur_number) cur_number+=1 current number: 1 current number: 2 current number: 3 current number: 4 current number: 5 current number: 6 current number: 7 Infinite loop # Do not run this code i = 1 while i < 137: print(i) break and continue statements Interrupts the nearest enclosing loop Jointly used with an if statement break statement – breaking loops lst = [4, 2, -6, 3,-9] for elem in lst: if elem < 0: print("First negative number is", elem) break print("done!") First negative number is -6 done! x=0 while x<30: x+=1 if not x % 10: break print(x, end='\t') 1 2 3 4 5 6 7 8 9 continue statement – Skipping loops x=0 while x<30: x+=1 if not x % 10: continue print(x, end='\t') 1 2 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 19 21 22 23 24 25 26 27 28 29 Loops - for or while? In most cases it is more natural to use for In some cases it is better to use while for: Predefined number of iterations No need to initialize or advance the loop variable while: Unknown number of iterations Can specify a stop condition Self learning Exercise 1: List methods What will be printed? students=['Rick', 'Tiny Rick!', 'Beth', 'Jerry', 'Summer', 'Jessica', 'Rick', 'Rick'] x = students.pop(-1) if x in students[:len(students)//2] and x in students[len(students)//2:]: students.remove(x) students.remove(x) if x in students[:len(students)//2] or x in students[len(students)//2:]: students.remove(x) print("Rick" in students) False Exercise 2: Multiplication chart print the multiplicaion chart. hint: try this code first. for letter in ["a", "b", "c"]: print(letter, end="\t") Solution for i in range(1,11): for j in range(1,11): print(i*j, end ="\t") print() 1 2 3 4 5 6 7 8 9 10 2 4 6 8 10 12 14 16 18 20 3 6 9 12 15 18 21 24 27 30 4 8 12 16 20 24 28 32 36 40 5 10 15 20 25 30 35 40 45 50 6 12 18 24 30 36 42 48 54 60 7 14 21 28 35 42 49 56 63 70 8 16 24 32 40 48 56 64 72 80 9 18 27 36 45 54 63 72 81 90 10 20 30 40 50 60 70 80 90 100 Exercise 3: Unique list of numbers Write a function that receives a list of number, and returns a list where each number from the original list appears exactly once. [1,3,3,1,7,7,7,1,3] \(\rightarrow\) [1,3,7] lst = [1,3,3,1,7,7,7,1,3] ## You code here Solution lst = [1,3,3,1,7,7,7,1,3] uniques = [] for x in lst: if x in uniques: continue uniques.append(x) print(uniques) [1, 3, 7] Exercise 4: The phone book of Rick We want to develop an application for managing the Contacts of Ricks It will contain a database A list of contacts Each contact is a list in the format [name, phone] Functionality Find contact(s) by full name or prefix Add contact Remove contact phone_book = [['Morty','1111'], ['Beth', '2222'], ['rick c-138', '8888'], ['rick c-139', '9999']] Retrieve the number for a given person name = 'rick' ## Change the value here ### Write your code here if found: print("Name:", name , "Phone:", phone) else: print("Cannot find", name , "in contact list.") Solution name = 'rick' ## Change the value here found = False phone = "" for contact in phone_book: if contact[0] == name: found = True phone = contact[1] break if found: print("Name:", name , "Phone:", phone) else: print("Cannot find", name , "in contact list.") Cannot find rick in contact list. Add a contact name = 'rick'# Change the value here num = 1234 phone_book.append([name, num]) print(phone_book) [['Morty', '1111'], ['Beth', '2222'], ['rick c-138', '8888'], ['rick c-139', '9999'], ['rick', 1234]] Remove a contact name = 'rick'# Change the value here found = False for contact in phone_book: if contact[0] == name: ### Write your code here if not found: print("Cannot find the contact", "name") Solution name = 'rick'# Change the value here found = False for contact in phone_book: if contact[0] == name: phone_book.remove(contact) print(f'The contact {name} was deleted') found = True break if not found: print("Cannot find the contact", name) The contact rick was deleted Search for all contacts with given prefix. Ignore character case (aka, case insensitive). prefix = "Rick" ### Write your code here prefix = "Rick" prefix = prefix.lower() for contact in phone_book: if contact[0].lower().startswith(prefix): print("Name:", contact[0], "Phone:", contact[1]) Name: rick c-138 Phone: 8888 Name: rick c-139 Phone: 9999