Loops

Last week’s highlights

• Lists

• Ranges

Agenda

Loops types of loops: while, for Other loop statements: break, continue The phonebook of Rick 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