Monday 10/23

Reminders

• Readiness Quiz for PA 1 is posted.
• PA1 Part A is due 10/24, TOMORROW!
• PA1 Part B is due 10/31
• Quiz this week is on while loops and modules.

While Loop Patterns and break

A common pattern for while loops is to process input until some condition is met. For example, imagine we need to add up numbers until the user enters an empty line:

Option 1:

total = 0
user_entered = input("Enter a number: ")
while user_entered != "":
    num = int(user_entered)
    total += num
    user_entered = input("Enter a number: ")

Pros: Loop condition reveals the purpose of the loop.

Cons: We hate the code repetition.

Option 2:

total = 0
while True:
    user_entered = input("Enter a number: ")
    if user_entered == "":
        break
    total += int(user_entered)

Pros: Code repetition is gone.

Cons: Loop condition is meaningless. We need to dig through the body of the loop to understand the flow of control.

Warning

break statements should be used sparingly and with caution. They can make it more difficult for the reader to follow the control flow in your loop.

Option 3:

total = 0
user_entered = "0"
while user_entered != "":
    num = int(user_entered)
    total += num
    user_entered = input("Enter a number: ")

Cons: It can be awkward to find an initial value that doesn't terminate the loop and doesn't violate the loop logic.

Note: Many other languages include a do-while loop construct specifically to make logic of this sort less awkward.

continue

Let's assume we only want to include the even numbers in the total:

total = 0
while True:
    user_entered = input("Enter a number: ")
    if user_entered == "":
        break
    num = int(user_entered)
    if num % 2 == 1:  # odd
        continue  # Jump to the top and re-check loop condition

    total += int(user_entered)

Warning

continue statements should be used sparingly and with caution. They can make it more difficult for the reader to follow the control flow in your loop.

Exercise:

What will be printed by the following loop?

x = 10
while x < 1000:
    print(x)
    x *= 2
    if x < 50:
        continue
    if x % 2 == 0:
        x += 1
    if x == 163:
        break

local, global, build-in namespaces and scope

scope: The area of code where a name is visible. This is a concept, not a part of the Python language.

namespace: A mapping from names to objects. Namespaces are actual entities in a Python program:

GRAVITY = 9.81  # Global constants should in all-caps.

def cool_function(some_param):
    some_local = some_param * 10
    print(f"Locals in function:\n {locals()}\n\n")
    print(f"Globals in function:\n {globals()}\n\n")
    return some_local

result = cool_function(2)

print(f"Locals:\n {locals()}\n\n")
print(f"Globals:\n {globals()}\n\n")
print(f"Built-ins:\n{dir(__builtins__)}\n\n")

globals() and locals() are dictionaries. __builtins__ is a module.

Work Time for PA

• If you haven't taken the readiness quiz: Go ahead and make an attempt. You have unlimited submissions.
• If you haven't finished Part 1: Write roll_dice and test it against the tests from last monday.
• If you have already finished Part 1: work on tests for Part 2.
• If you have already finished Part 2: show me or the TA your tests for feedback.