CSCI 1101B. While Loops

Transcription

1 CSCI 1101B While Loops

2 Today s Outline Iteration (review) When Can We Not Use For-Each Loops? While Loops Controlling Loops The Break Statement The Continue Statement

3 Example - First, Recall The Turtle Module from turtle import * joe = Turtle() joe.shape("turtle")

4 Example: Running a Race # and so on! done()

5 Example: Running a Race # and so on! done() # How could we make this better?

6 Example: Running a Race (For-Each Loop) num_iterations = 4 for i in range(num_iterations): done()

7 Example: Running a Race (For-Each Loop) num_iterations = 4 for i in range(num_iterations): done() # Let s learn another way...

8 Example: Running a Race (While Loop) num_iterations = 4 current_num = 0 while current_num < num_iterations: current_num += 1 done()

9 Example: Running a Race (While Loop) num_iterations = 4 current_num = 0 # initialization while current_num < num_iterations: # test condition current_num += 1 # update done()

10 Example: Running a Race Why have while loops if for-each loops work just fine?

11 Example: Running a Race Why have while loops if for-each loops work just fine? What if we didn t know the number of iterations? What if we just had a stopping point that would happen at some point along the way?

12 While Loops When we don t know how long a loop will last, we can use a while loop. while <<condition>>: <<block>>

13 Example: Running a Race (While Loop) from random import randint # (...) simon_says = True while simon_says: if not randint(0, 100): simon_says = False done()

14 Summary: When are For Loops not the best? What if we don t know how long a loop should last? Stir until the mixture is boiling Keep the game going until the user quits Keep searching for a new answer until you find the right one

15 While Loops: Example 2 text = while text!= quit : text = input( Enter command or quit: ) if text == quit : print(...exiting program. ) elif text == sing : print( La la la, la li da. ) else: print( Come again? )

16 Caution: Infinite Loops If your test condition never evaluates to False, the instructions will endlessly loop. while True: print( This is the song that never ends ) Make sure you have a feasible terminating condition to avoid these infinite loops. (Remember, you can restart the shell if this happens accidentally!) with apologies to Norman Martin

17 Controlling Loops Python lets us control iteration in two main ways: 1. We can break out of a loop using the break keyword 2. We can skip ahead to the next iteration with the continue keyword

18 The Break Statement text = while True: text = input( Enter command or quit: ) if text == quit : print(...exiting program. ) break if text == sing : print( La la la, la li da. ) print( Hooray! )

19 The Continue Statement for num in range(10): if num % 2 == 1: continue print(num, end= )

20 The Continue Statement for num in range(10): if num % 2 == 1: continue print(num, end= ) This code prints out:

21 Good Practice: Don t use Break/Continue much What happens when we depend on breaks and continues? Loop termination is not clearly explicit Encourages lazy programming Makes it harder to keep code updated

22 Summary When do we use the following? For each and while loops Break and continue statements Be careful of infinite loops!