Loops. Upsorn Praphamontripong. CS 1110/CS 1111 Introduction to Programming Spring 2018

Transcription

1 Loops Upsorn Praphamontripong CS 1110/CS 1111 Introduction to Programming Spring 2018

2 Walking and Shaking if Given a list of Python experts in this room Upsorn-bot wants to shake hands the Python experts who wear glasses 2

3 Repeated Process if Walking and shaking Given a list of Python experts in this room Write code that print shake if an expert wears glasses and print no if an expert does not wear glasses 100 experts = 100 if-else blocks?? if expert_1 == "wear": print("shake") else: print("no") if expert_2 == "wear": print("shake") else: print("no")... if expert_n == "wear": print("shake") else: print("no") 3

4 Looping What is looping? The process of doing something in the same order again and again Why do we do looping? To automated some repeated processes To shorten the code, less time/effort, less chance of errors To make code more readable and maintainable To save memory as few instances are created When should we do looping? How do we write loops? 4

5 Loops in Python while loop a condition-controlled loop for loop using the in operator with a list Today s focus for loop using the range operator a count-controlled loop Nested loops 5

6 while <condition>: <statements> While Loop <handler> to break the condition such that the loop terminates index = 1 Condition (Boolean expression) while index <= 10: print(index) index += 1 indented Try: tracing through code with 6

7 Practice Write a program, using a while loop, that prints the numbers 10, 8, 6, 4, 2 (exactly this order) 7

8 While Loop (2) while input("do you want to continue (Y/N)? ") == "Y": print("let's continue") done = "" while done!= "quit": print("let's continue -- not done yet") done = input("continue? (type quit to quit) ") 8

9 Practice Imagine you are writing a program to mimic the Simpson kids in the Simpsons ride scenario Write a program, using a while loop, that repeatedly asks Are we there yet? until the answer is yes the program then prints Yay!! 9

10 While Loop (3) cnt = 1 while infinite > -1: print(cnt) cnt += 1 Variable not found!! cnt = 1 while cnt > -1: print(cnt) cnt += 1 Infinite loop!!

11 While Loop (4) list_of_colors = [ green, blue, red, yellow ] index = 0 while index < len(list_of_colors): print(list_of_colors[index]) index += 1 11

12 Back to Walking and Shaking Given a list of Python experts in this room Upsorn-bot wants to shake hands the Python experts who wear glasses

13 Repeated Process while loop Walking and shaking Given a list of Python experts in this room while # there are more experts # if an expert wears glasses # print "shake" # otherwise # print "no" Write code that print shake if an expert wears glasses and print no if an expert does not wear glasses experts = ["wear", "no", "no", "wear", "wear", "no"] ctr = 0 while ctr < len(experts): expert = experts[ctr] if expert == "wear": print("shake") else: print("no") ctr += 1 13

14 Common mistake: infinite loop experts = ["wear", "no", "no", "wear", "wear", "no"] ctr = 0 while ctr < len(experts): if experts[ctr] == "wear": print("shake") else: print("no") # ctr = ctr + 1 shake shake shake shake... 14

15 Common mistake: index out of range experts = ["wear", "no", "no", "wear", "wear", "no"] ctr = 0 while ctr >= 0: if experts[ctr] == "wear": print("shake") else: print("no") ctr = ctr + 1 shake no no shake shake no Index out of range 15

16 Practice Write a function, using a while loop, that takes a list of animals and a list of sounds. Use the given animals and sounds and print the Old MacDonald had a farm song You may assume that the sizes of both lists are the same (that is, the number of animals and sounds given are the same) 16

17 For Loop (using in operator) for <iterate_var> in <collection>: <statements> animals = ['dog', 'cat', fish'] for animal in animals: print('current animal :', animal) for letter in 'Python': print('current Letter :', letter) 17

18 Example: for Loop (using in operator) Walking and shaking Given a list of Python experts in this room for # expert in list of experts # if an expert wears glasses # print "shake" # otherwise # print "no" Write code that print shake if an expert wears glasses and print no if an expert does not wear glasses for expert in experts: if expert == "wear": print("shake") else: print("no") experts = ["wear", "no", "no", "wear", "wear", "no"] 18

19 Practice Write a function, using a for loop, that takes a list of what-to-do. Use the given what-to-do and print the If you re happy and you know it song 19

20 For Loop (using range operator) for <iterate_var> in range(start, stop, step): <statements> default = 1 Iterate upto this but not include for num in range(1, 100, 5): print(num) for num in range(1, 100): print(num) 20

21 Practice Write a program, using a for loop, that prints the numbers 2, 4, 6,..., 20 Write a program, using a for loop, that prints the numbers 20, 18, 16,..., 2 21

22 Another Example for Loop (using range operator) Imagine the user enters a string and a substring. Remove the substring from the string without using any built-in functions/methods besides len() (i.e., you may not use.index()). The substring will be at most one character long result = "" string1 = "for loop, using range" size1 = len(string1) string2 = "o" position = 0 for position in range(0, size1): if string1[position]!= string2: result = result + string1[position] position += 1 print("result = " + result) 22

23 While Loop vs. For Loop While loop Use when you don t know the number of iterations Need to explicitly handle the termination For loop Use when you know the number of iterations Loop stops as it reaches the end of range, or the end of list i = 5 while i > 0: print(i * i * i) i -= 1 for i in range(5, 0, -1): print(i * i * i) 23