While Loops

CSCI 1101B

Today’s Outline

● Iteration (review)

● When Should We Not Use For-Each Loops?

● While Loops

● Controlling Loops○ The Break Statement○ The Continue Statement

Example - First, Recall The Turtle Module

from turtle import *

joe = Turtle()joe.shape("turtle")

Example: Running a Race

joe.forward(100) # The 100 meter dash!done()

# ...but what if we wanted to take # things one step at a time?

# (e.g., perhaps there are # obstacles to look out for!)

Example: Running a Race

joe.forward(1)joe.forward(1)joe.forward(1)joe.forward(1)

# and so on…!

done()

Example: Running a Race

joe.forward(1)joe.forward(1)joe.forward(1)joe.forward(1)

# and so on…!

done()

# How could we make this better?

Example: Running a Race (For-Each Loop)

num_iterations = 4

for i in range(num_iterations): joe.forward(1)

done()

Example: Running a Race (For-Each Loop)

num_iterations = 4

for i in range(num_iterations): joe.forward(1)

done()

# Let’s learn another way...

Example: Running a Race (While Loop)

num_iterations = 4current_num = 0

while current_num < num_iterations: joe.forward(1) current_num += 1

done()

Example: Running a Race (While Loop)

num_iterations = 4current_num = 0 # initialization

while current_num < num_iterations: # test condition joe.forward(1) current_num += 1 # update

done()

Example: Running a Race

Why have while loops if for-each loops work just fine?

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?

While Loops

When we don’t know how long a loop will last, we can use a while loop.

while <<condition>>: <<block>>

Example: Running a Race (While Loop)

from random import randint # Lets us get a random int!

simon_says = True

while simon_says: joe.forward(1) if not randint(0, 2): # What’s this mean? simon_says = Falsedone()

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”

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?”)

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

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

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!”)

The Continue Statement

for num in range(10):

if num % 2 == 1:

continue

print(num, end=” ”)

The Continue Statement

for num in range(10):

if num % 2 == 1:

continue

print(num, end=” ”)

This code prints out: 0 2 4 6 8

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

Summary

When do we use the following?

● For each and while loops

● Break and continue statements

Be careful of infinite loops!